Walk into any U.S. pharmacy, and you'll find a multicolored range of melatonin supplements marketed as sleep aids for adults and children alike.

The body-made hormone melatonin streams through our systems every evening, preparing us for rest. But while melatonin supplements claim to mimic this natural process, experts say bottles stamped with claims like "100% drug free" may be misleading.

Melatonin carries little risk when used correctly, but as a dietary supplement, it skirts the strong governmental regulation that's reserved for pharmaceuticals. This has led to inaccurate dosage labeling, incomplete safety information, and scant research into melatonin's long-term effects on kids, some scientists argue. Researchers worry that, given that many people perceive melatonin as harmless, some are misusing the supplement by taking unnecessarily high doses at improper times of the day.

"It's being promoted like there's absolutely no risk … [People say] 'It's all natural, and it just is a safe sleep aid,'" said Dr. Pieter Cohen, a Harvard Medical School researcher who studies supplements. "And that's where you can get into trouble."

The U.S. Food and Drug Administration (FDA) classifies melatonin as a supplement, while countries like the U.K. and Australia deem it a medication. This designation comes with a level of governmental oversight that U.S. supplements aren't subject to. Given melatonin's soaring popularity, some experts think the United States should follow suit and make melatonin an over-the-counter drug.

But if melatonin is already produced naturally by the body, why would the supplement version be riskier? Live Science spoke with experts to find out.

Supplements aren't well regulated

In the U.S., a 1994 federal law required that supplements be regulated as food, not drugs. The FDA doesn't assess the safety, efficacy or manufacturing quality of supplements before they're marketed but instead intervenes if harmful side effects are reported or if it's made aware of inaccurate labels. This largely leaves the supplement industry to self-regulate.

Cohen's research suggests that actual melatonin concentrations in products may range from 74% to 347% of what's reported on the label. Meanwhile, safety warnings and instructions on the best way to use melatonin aren't required on labels. Still, many popular brands adhere to voluntary melatonin labeling guidelines from the Council for Responsible Nutrition, a supplement trade organization. These guidelines include disclaimers like "may cause drowsiness" and "for occasional or intermittent use only."

Despite such disclaimers, nearly 1 in 5 school-aged children use it regularly as a sleep aid, a trend scientists are concerned by. Long-term safety data is lacking in kids, in part because melatonin hasn't gone through the safety trials required of new drugs. And melatonin isn't patented, so there's no strong incentive for drug companies to invest in safety studies, according to Debra Skene, a neuroendocrinologist at the University of Surrey in England. "There's no money in it," she said.

Many parents inaccurately view melatonin as akin to a "little glass of warm milk," Cohen said. But if parents give children high-dose supplements or multiple doses at once, "you're talking about side effects and getting sick and not helping sleep," he said.

Calls to U.S. poison control centers about melatonin poisoning in young children have shot up with the supplement’s popularity. This has been driven by accidental ingestions in children under 5, and in some cases, kids had serious symptoms, like respiratory failure or seizures. That said, some of those effects may be due to certain melatonin products' inclusion of serotonin, which can be dangerous if consumed in large quantities.

Evidence does suggest that melatonin can help children with attention-deficit/hyperactivity disorder and autism get to sleep, among whom sleep problems are common. But the potential benefits of melatonin for neurotypical kids are less clear.

One concern scientists have is that the regular use of melatonin may affect puberty. In various animal species, melatonin helps regulate reproduction, and supplemental melatonin can disrupt these effects. The few human studies that exist haven't shown clear effects on puberty, but there hasn't been an in-depth study tracking hormone levels in the blood, said Helen Burgess, who researches the circadian rhythm at the University of Michigan.

"I'm OK with intermittent short-term use," she said of melatonin supplementation in kids. But given the lack of data, she suggested that parents consult a pediatrician to look for another solution if their child needs melatonin regularly.

The lesser-known effect of melatonin

Supplements supercharge the body's supply of melatonin, said Margarita Dubocovich, who studies melatonin at the University at Buffalo. Supplement dosages generally range from 1 to 10 milligrams (mg), while adults produce about the equivalent of a 0.3-mg pill nightly, she said.

When taken orally, supplements and medications pass through the digestive tract before reaching systemic circulation, meaning the full dose of melatonin won't necessarily make it into the bloodstream. However, the amount that circulates will still be far higher than what's naturally produced in the body, said Skene.

Upping your melatonin dosage doesn't knock you out; the drowsiness triggered by the supplements is generally mild. The supplement also avoids the side effects of conventional sleeping pills: memory loss, gastrointestinal issues and next-day "hangovers."

However, melatonin has a lesser-known effect. Given its role in regulating the circadian rhythm — the 24-hour clock that regulates many bodily systems — it can scramble the body's sense of timing when regularly taken outside your bedtime, such as too early before bed or in the middle of the night.

That's why melatonin can help treat jet lag if taken for multiple days just before or after traveling. But if people take pills as they're tossing and turning throughout the night, this could throw off the circadian rhythm, Dubocovich said. This practice may induce a kind of jet lag (without the perks of international travel). In the short term, that can come with drowsiness during the daytime, irritability, upset stomach, brain fog and insomnia.

"I think that melatonin could be good if you take it in a controlled manner and the right dose," Dubocovich said, suggesting a few milligrams taken right at bedtime. "But how people are taking it [currently], they are intoxicating themselves."

Not all researchers agree that melatonin-induced jet lag poses a big risk, but taking more than 3 mg in the middle of the night still "isn't a good idea," said Burgess. This dose may linger in the body through the morning, she said, leading to next-day grogginess.

Burgess' research suggests that supplemental melatonin is most effective at shifting the biological clock forward in time, which happens when it's taken in the early evening. Still, she said the impact of melatonin on the circadian rhythm "just does not seem to be really recognized or understood by the public at all."

Should melatonin be a drug?

If melatonin were an over-the-counter medication, it would require clear labels with instructions and accurate dosing. "You should be able to know what dose you're getting, and it shouldn't be sold in whatever dose the manufacturer … decides to pour in on a given day," Cohen said.

Yet, reclassification could disrupt access to a relatively safe sleep aid that many consumers rely on, argued Jeff Ventura, a spokesman for the Council for Responsible Nutrition, the supplement industry trade organization that created the melatonin labeling guidelines. "Moving melatonin into the drug category would require formal FDA drug approval pathways, which could limit consumer access, reduce product variety, and significantly increase costs" for both manufacturers and consumers, he told Live Science in an email.

It's unlikely the supplement will be made an over-the-counter drug anytime soon. Cohen attributes the lack of a serious movement to the supplemental industry lobby, which includes the Council for Responsible Nutrition. "Any move to take such a great money winner like melatonin out of their hands would cause them to fight it tooth and nail," he said.

Burgess, who serves on the scientific advisory board for the melatonin brand Natrol, agrees that purity is an issue. However, she said she isn't too concerned about melatonin's availability, given its "pretty benign effects." Still, she suggests going with a big name brand with a US Pharmacopeia (USP) stamp verifying its listed ingredients are accurate; USP is a nongovernmental organization that sets quality standards for medicines, food ingredients, and dietary supplements.

After studying the hormone for 40 years, Dubocovich agrees that melatonin should be regulated as an over-the-counter medication.

"Melatonin can help support a good night’s sleep, especially in small doses that mirror the body’s natural rhythm," Dubocovich said. "But not all melatonin is created equal."

This article is for informational purposes only and is not meant to offer medical advice.

How much do you know about sleep and dreams? Find out with our sleep quiz!

Sleep could encode epileptic seizures in the brain by repurposing the processes used to solidify memories, potentially making seizures harder to treat or prevent, new research suggests. But the new study also suggests a possible way to counter the effect: using electrical stimulation to keep the brain from "memorizing" the seizure, the researchers say.

"It opens a whole new realm of therapeutic options tailored to each patient," said study co-author Vaclav Kremen, a neuroscientist at the Mayo Clinic. He added that electrical stimulation could be personalized to each individual’s unique seizure profile.

Sleep and seizures

People often struggle to store memories after epileptic seizures, and research in rats suggests this occurs because the brain’s memory storing system solidifies neuronal connections that trigger seizures in lieu of locking in memories. However, the link between epilepsy, memory and slumber hasn't been adequately assessed in humans because most of these studies involve measuring brain activity for only a few days and the research usually takes place in clinics, which don't lend themselves to a good night's sleep.

"Hospital stays can change sleep and seizure patterns because of medication adjustments, stress, noise, and disrupted routines," Dr. Erin Conrad, a neuroscientist at the University of Pennsylvania who was not involved with the work, told Live Science in an email.

In the new study, published March 4 in The Journal of Neuroscience, electrodes were implanted for months or years into participants who slept at home, allowing the researchers to collect data over a long period without disturbing sleep. "That gives a more realistic picture of how sleep changes after seizures in everyday conditions," Conrad said.

The team analyzed two groups of participants with drug-resistant epilepsy who participated between 2010 and 2011 at the University of Melbourne in Australia or between 2019 and 2023 at the Mayo Clinic in Minnesota. One group was implanted with deep brain stimulation devices that can detect and reduce seizure activity, while the other got an investigational seizure advisory system that records brain signals but does not try to interrupt seizures. The study was small, totaling 11 participants, so the findings may not be generalizable to all epilepsy patients, Kremen told Live Science. Nonetheless, the work provides clues as to how changing brain patterns during sleep could underlie the link between epilepsy and memory.

The team found that people slept approximately 24 minutes longer on nights following epileptic seizures, yet not all stages of sleep were prolonged.

Rapid-eye-movement (REM) sleep, which is important for emotion processing and dreaming, shortened by approximately 12 minutes on nights after an epileptic seizure compared with nights in between seizures. Dr. Laurent Sheybani, a neuroscientist at the University of Geneva who was not involved with the work, told Live Science in an email that "12 minutes can appear low indeed, but keep in mind that overall duration of REM sleep is not very long either" — typically about 1 hour and 40 minutes — so the drop is meaningful.

What replaces the missing minutes of REM sleep also matters, Conrad said. The team found an increase in the length and intensity of the deepest stage of sleep, called the slow-wave stage, which is key to storing memories. One hypothesis is that the brain uses memory-forming pathways to "remember" how to form seizures in the future, but the observations from this study alone can't show that's the case.

Forecasting seizures using sleep patterns

In future work, the team needs to determine whether lengthening the memory-forming stage of sleep actually reinforces epileptic seizures by "memorizing" the seizure pathways.

"Using brain devices that adjust stimulation based on both seizures and sleep patterns is an exciting possibility," Conrad said. Such devices use electrodes to record brain activity and deliver an electrical impulse to halt a seizure as soon as it’s detected. They use a closed-loop feedback system to improve detection over time as the system recognizes a specific person’s seizure patterns.

"This kind of approach could help test the study's main idea by seeing whether changing sleep-related brain activity after a seizure reduces the chance of future seizures," Conrad added. "If the theory holds up, these kinds of adaptive, closed-loop systems could become a new way to personalize treatment."

Kremen said the findings hint that electrical brain stimulation that interrupts this seizure memory formation could be an additional future treatment alongside medications for patients with drug-resistant epilepsy. "We are really invested in trying to find therapies for very sick people with epilepsy that don't have any hope with normal drug therapies," he said.

This article is for informational purposes only and is not meant to offer medical advice.

A night or two of poor sleep can do more than cause tiredness — it may also mess with stem cells in the gut, making the organ susceptible to inflammatory disorders, a mouse study finds.

The new research revealed that sleep deprivation disrupts the function of gut stem cells in ways that could raise the risk of inflammatory bowel disease (IBD), including ulcerative colitis and Crohn's disease.

The study, published Feb. 5 in the journal Cell Stem Cell, described a multicomponent pathway that relays aberrant signals from the sleep center in the brain to the intestinal cells. This haywire signaling reduces the regenerative capacity of the gut lining.

"We now have evidence showing that sleep is not just [important] for the brain but for overall health," Dragana Rogulja, a neuroscientist at Harvard Medical School who was not involved in the study, told Live Science.

Insomnia affects an estimated 10% of adults worldwide. In addition to wreaking havoc on people's daily function, chronic sleep disruptions are linked to an increase in the incidence of numerous chronic conditions, including IBD, diabetes, high blood pressure and major depressive disorder.

Among those with IBD, over 75% report experiencing sleep disturbances. In a study of over 1,200 people whose IBD was in remission, those with impaired sleep had double the risk of relapse, compared with well-rested individuals with the condition. However, most studies of sleep disturbances focus on the brain, so it was largely unknown how these disturbances affect other organs, such as the gut.

To uncover how the sleep-deprived brain influences the bowels, the team focused on intestinal stem cells, which are key players in maintaining gut health — namely, the integrity of the organ's lining. They deprived mice of sleep for two days and observed that their intestines showed signs of oxidative stress. These mice had nearly half the number of stem cells as the guts of well-rested mice, as well as a reduced capacity to regenerate after damage.

"This really showed how quickly and severely sleep disruption damages the gut," study co-author Zhengquan Yu, a molecular biologist at China Agricultural University, told Live Science.

A deeper exploration of the molecular changes taking place in the intestines revealed that sleep deprivation was tied to an increase in serotonin in the mice's guts. Serotonin is crucial for signaling the gut to release digestive fluids and for controlling the muscle contractions that move food through the system. However, prolonged exposure to high serotonin levels can contribute to issues such as diarrhea, IBD and the development of tumors. Thus, tight control of serotonin levels is essential for a healthy gut.

In the mice, a lack of sleep not only caused an excessive amount of serotonin to be released from intestinal cells but also reduced "reuptake" of the molecule — meaning more and more of the chemical messenger built up in the gut. When the team tried injecting serotonin into the guts of well-rested mice, they observed changes that resembled those caused by sleep deprivation.

But exactly how do the brain signals associated with poor sleep reach the gut? Yu and his colleagues speculated that the vagus nerve, which regulates gut-brain communication under stress, could bridge this gap.

To test their theory, the researchers examined the effects of sleep deprivation in mice that had their vagus nerve severed. These animals maintained normal levels of serotonin and a higher number of intestinal stem cells, compared with sleep-deprived mice with intact vagus nerves. Blocking vagus-nerve signals also protected the gut from the effects of disrupted sleep. The researchers also identified the chemical messenger acetylcholine as the main signaling molecule released from the vagus nerve that triggers serotonin release.

"Every component in this cascade is really important as a target for potential therapy," study co-author Maksim Plikus, a cell biologist at the University of California, Irvine, told Live Science. Next, the team wants to study the cascade in miniature models of the gut, called organoids.

"We have to move to human intestinal organoids to start testing conservation of the cell types and the circuitry," Plikus said.

Now, Yu and his team are investigating the relevance of this pathway in the context of chronic sleep disorders to determine if long-term activation of the vagus nerve could contribute to the development of cancer or IBD. Eventually, they aim to develop therapies targeting the vagus nerve or the molecular pathways involved, to treat gut dysfunction in patients with insomnia.

This article is for informational purposes only and is not meant to offer medical advice.

Sleep quiz: How much do you know about sleep and dreams?

Night owls are known to habitually stay up late and ride a wave of energy that can carry them into the wee hours of the morning before they start to nod off. Morning people, on the other hand, tend to be most alert and energized upon waking up in the a.m., and they tucker out earlier than night owls do.

These two rhythms of wakefulness and sleepiness, known as "chronotypes," are thought to be at least partially determined by people's genetics. But research suggests that being a natural night owl may come with some downsides.

For instance, a recent study linked being a night owl to a higher risk of poor heart health and cardiovascular conditions such as stroke and heart attack. Another study found that night owls had a higher risk of early death. However, in both studies, scientists cautioned that the chronotype itself doesn't appear to explain all of this risk. Rather, lifestyle factors may be the main culprits separating night owls' health from that of morning larks.

A higher rate of smoking among night owls seems to be one big piece of the puzzle, along with greater alcohol consumption, poorer diet and lower physical activity, on average. So, if you're a night owl who keeps those critical lifestyle factors in check, current data suggest that your risk of heart disease and early death may not be that different from an average morning person's risk. But more research is needed to know for sure.

Would you call yourself a night owl or a morning person? Or do you not really fit into either category? Let us know in the poll below.

This article is for informational purposes only and is not meant to offer medical advice.

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In the Hollywood blockbuster "Inception" (2010), a dedicated team of "dream extractors" is hired to alter a CEO's decision-making by manipulating his dreams. In the movie, this feat involves a private jet and liters of sedative gas — but a new study suggests they could have achieved a similar effect with only some steel-drum jingles and a comfy bed in a research lab.

The new work shows that audio cues played to sleeping volunteers during rapid-eye-movement (REM) sleep, the stage of sleep when most dreaming occurs, can manipulate dream content.

In some cases, this manipulation improved the volunteers' ability to solve puzzles they had first encountered the day before.

The work was published Feb. 5 in the journal Neuroscience of Consciousness.

'Sleep on it'

If you are stuck on a problem, people often advise you to "sleep on it." And there's some scientific evidence to back this up, said study co-author Ken Paller, a cognitive neuroscientist at Northwestern University. For example, in one 2012 study, volunteers asked to solve association-based problems performed better after sleeping than did another group that stayed awake.

But how sleep might achieve this was unclear.

"The motivation for this study was to see if dreaming has something to do with the benefits of sleep we get for problem solving," Paller told Live Science.

Paller's team recruited 20 participants who reported having a history of or interest in lucid dreaming, a dream state in which the sleeper becomes aware that they are dreaming and can manipulate their dream to some degree.

Before taking a snooze in the research lab, these participants were tasked with solving puzzles that tested creative cognition within a certain time limit. These included tasks in which volunteers had to alter matchstick diagrams to make certain shapes by moving a limited number of sticks.

As the volunteers considered each puzzle, a short soundtrack played; the tune was unique to each conundrum. These themes included guitar riffs, whistling tunes and steel-drum songs. The puzzles were difficult enough that each participant was left with several unsolved puzzles by the time the testing concluded.

Lead study author Karen Konkoly, who worked on the project while studying dreams in Paller's lab, also taught the volunteers specific eye movements, with the idea that, if the participants were experiencing lucid dreaming, they could attempt to communicate that to the researchers by moving their eyes.

Then, the researchers fitted the participants' scalps with electrodes to measure their brain activity and eye movements while they slept. The participants were allowed to watch "Inception" or "Waking Life" (2001), another film about lucid dreaming, while the electrodes were applied.

Hours later, as the volunteers entered REM sleep, the research team, led by Konkoly, began playing soundtracks associated with puzzles they had failed to solve. Immediately afterward, they woke up the participants to record any dreams they'd had in diaries. The participants recorded their dreams over the next two weeks and spent one more night in the lab solving puzzles.

Three-quarters of the volunteers reported having dreams that related to the unsolved puzzles, and the data suggested they were more likely to dream about the puzzles the researchers had cued up with audio. Six dreamers, upon hearing the puzzle soundtracks, signaled to Konkoly they were lucid by moving their eyes or changing their breathing in preset patterns.

The next day, all of the volunteers attempted the puzzles again. The results were mixed.

If certain unsolved puzzles had appeared in the volunteers' dreams, the volunteers were more likely to solve those puzzles the next day, compared with puzzles they had not dreamed about. The volunteers solved 42% of the puzzles they dreamed about and only 17% of those that didn't appear in their dreams.

Does lucid dreaming help or hinder?

This finding doesn't definitively prove that dreams help us solve puzzles, though. It's possible that the volunteers simply dreamed about the puzzles they were most curious about and were more likely to solve at baseline.

To the authors' surprise, the volunteers whose eye movements suggested they had lucidly dreamed were less likely to solve the puzzles than those who'd had non-lucid dreams about the puzzles. Paller said the study's small sample size may have produced this effect.

"I think we didn't have enough lucid dreams to really be sure about that," he said.

Emma Peters, a dream engineer at the University of Bern in Switzerland who was not involved in the study, said a major talking point in the field had been whether lucid dreaming might actually impair creative thinking, compared to non-lucid dreaming.

"The idea is, you can do creative problem-solving in dreams because your dreams are so bizarre," she said, "and they make associations that you would normally not do if you were consciously there."

For Paller, interpretations of dream research face another important limitation: the other parts of the sleep cycle in which dreams don't occur as often. At present, it's impossible to rule out the possibility that brain activity in these stages could be a driving force in creative thinking; the downstream results of that thinking may then emerge in remembered dreams.

But the field is gradually building a picture of what goes on in the sleeping brain. For Paller, these unresolved mysteries are what keeps dream science exciting.

"I think science is fun when there's still things you need to understand," he said, "and you haven't got there."

Small changes in exercise, sleep and dietary habits could do wonders for people's health and possibly extend their overall lifespan, a large U.K. study suggests.

The research, published Jan. 13 in the journal eClinicalMedicine, sought to find the smallest possible lifestyle improvements that could measurably lengthen people's lifespans. The researchers searched within data collected from almost 60,000 people in the UK Biobank cohort, a repository of medical and lifestyle data from hundreds of thousands of U.K. adults.

The team linked the participants' documented habits to their theoretical overall longevity and health, as calculated using statistical modelling. They found that people who slept as little as five extra minutes daily, engaged in just two extra minutes of moderate-to-vigorous physical activity per day, and added a half serving of vegetables per day tended to live significantly longer than the worst performers — meaning those whose sleep habits, exercise, and nutrition patterns put them in the bottom 5% of the overall cohort.

The former group had an extra year of overall lifespan compared with the latter, according to the statistical model.

That's not to say that adding a few minutes of exercise or sleep and making small changes to diet will guarantee an extra year of life, Stephen Burgess, a statistician at the University of Cambridge who was not involved in the study, told Live Science in an email.

"By itself, this study does not prove that these habits improve health," he cautioned. "They model what might happen to our life span if changing these factors does improve health."

Interestingly, the data suggested that improvements across several aspects of well-being are "greater than the sum of their parts," lead study author Nicholas Koemel, a dietitian and research fellow at The University of Sydney, told Live Science. For instance, to gain one additional year of lifespan through sleep alone, the study suggested a person would have to sleep an extra 25 minutes per night — a luxury many cannot afford. But very small improvements in sleep, exercise and diet may have a significant combined effect.

Koemel said the findings suggest that "healthy habits work better as a package."

"Each one of our behaviors has an interlinking impact on what we do," he said. "If we have a poor night's sleep, we oftentimes eat differently, we move differently. And we see that across all of these different behaviors."

Per the model, the study participants with the most optimal combination of these behaviors — at least 40 minutes of moderate-to-vigorous exercise, seven to eight hours of sleep per day, and an overall healthy diet — were predicted to live an extra nine years, overall, and live nine extra years in good health, compared with the poorest 3% of performers.

By their very design, studies like these, known as cohort studies, need to be taken with a grain of salt. They compare two groups retroactively rather than asking people to change their habits and seeing how that change affects their health. As such, they can never conclusively prove that one particular change caused an associated effect; they can only draw a link between the two.

For instance, sleep and exercise habits were measured for only up to a week in the UK Biobank cohort, and the data assume that people kept up those same habits in the long run. Similarly, diet was assessed only at the beginning of the study, rather than monitored over time. So it's possible that the participants changed their habits after these assessments were taken, which would weaken the possibility that their habits improved their lifespan.

All of this leaves room for the possibility that it was not these lifestyle differences, but rather some other factor that wasn't measured, that caused the improvement in lifespan.

It may be, for instance, that one group is wealthier, and it so happens that this makes it easier for that group to exercise, sleep well and eat well. But ultimately, that difference is partially explained by the wealth, not only the behaviors, Burgess explained. It is also possible wealthy people might live in a less polluted area than poorer people, which could contribute to the difference in lifespan. There's no way of telling from this study alone.

"The overall message that small changes in these factors are likely to be beneficial is probably correct," Burgess said. "But whether the exact numbers are accurate or not is less clear."

Koemel agreed, saying that more research is needed to confirm the findings. Still, the idea that even tiny changes in lifestyle could have large effects could offer an interesting alternative for those looking to improve their overall well-being, he suggested.

"New Year's resolutions oftentimes fail because we're pushing too hard," he argued. "We're trying to go to the gym every day. We're trying to be perfect." This research suggests there may be a "different path to get from A to Z," by making smaller changes across several areas of well-being that can build up to healthier habits while improving overall health.

This article is for informational purposes only and is not meant to offer medical advice.

You may assume that children have nightmares about monsters under the bed, while adults dream about stressful events like deadlines. But is there any scientific evidence showing that dreams change as we age?

Although there are plausible mechanisms for how aging could cause dreams to change over time, very little research has explored the topic.

"Dreaming depends on neural systems involved in imagination, memory and emotion, all of which develop and reorganize with age," Dr. Giulio Bernardi, head of the Sleep, Plasticity, and Conscious Experience (SPACE) research group at the IMT School for Advanced Studies Lucca in Italy, told Live Science. However, "surprisingly few studies have systematically examined how dreams change across the lifespan."

The studies that have investigated this idea have shown that people tend to report their dreams differently at various stages of their lives. While younger dreamers tend to see and feel more vividly, older people recall more complicated and less-emotional situations.

One of the simplest, and perhaps most common, explanations for the way our dreams adapt and evolve is called the "continuity hypothesis," first put forward in 1971.

According to that theory, our dreams tend to mirror what we're experiencing in our waking lives — if we're lounging on vacation, we may see sun and sand when we fall asleep, but if we're anxious about work, we may be transported to the office. But ultimately, the parallels between sleep and reality reveal very little about why our dreams might change as we grow up.

"Changes in dreaming across life reflect the complex interaction between brain development, sleep architecture, and cognitive-emotional maturation," Bernardi said. Everything from memory to sleep quality can influence how we experience both dreams and their aftermath: "These factors determine not only how vividly dreams are produced during sleep but also how likely they are to be remembered upon awakening."

How dreams change with age

The foundational research on how children dream was done by sleep researcher David Foulkes from the 1970s to the 1990s. According to his research, young people's dreams tend to be relatively straightforward — they feature animals, static objects and simple interactions. (However, research on children's dreams involves complications because the results depend on each child's ability to understand what dreams are and how to relay them to others.)

In adolescence, dreams tend to become more frequent and more vivid than they were in childhood, reflecting the many changes we're experiencing in our waking lives. While younger adolescents report dreaming about falling, being chased, and confronting monsters or animals, older teens reexperience the stresses of school and new relationships.

In adult life, dreams usually become a bit more mundane. One study found that adults and older adults dream about arriving somewhere late and "trying again and again to do something" more often than other age groups. Odd dreams and nightmares still occur, but the aggression of adolescence starts to fade and the complexity of dreams starts to reflect our waking life even more.

Studies show that in old age, people don't tend to report having as many dreams. Many also experience "white dreams," in which they recall having a dream but aren't quite sure what happened during it. While some of this can be explained by the lower quality of sleep that older adults tend to experience, much of this change — as is the case with dreams at all ages — has to do with the ability to accurately and descriptively remember what we saw in our sleep.

"Dreaming is defined as subjective experiences that occur during sleep," Michael Schredl, head of the sleep laboratory at the Central Institute of Mental Health in Germany, told Live Science. "What we get is only the dream or dream report, the recollection of those experiences that occurred during sleep."

At the end of life, and during the dying process, people often report seeing departed loved ones and recall visions of packing and preparing to go on a trip in their dreams. Studies of hospice patients have found that these dreams are often comforting and put people at ease, mirroring the reflection that often occurs at the end of life.

Sleep quiz: How much do you know about sleep and dreams?

We've all experienced the intense struggle to pay attention after a bad night's sleep — and a new study shows what happens in the brain as that feeling arises.

When you zone out after pulling an all-nighter, the brain flushes out cerebrospinal fluid (CSF), which surrounds the brain and spinal cord and is part of the brain's waste-disposal system. This CSF then floods back into the brain when you snap out of it, according to the study, published Oct. 29 in the journal Nature Neuroscience.

These fatigue-induced attention failures are also accompanied by massive changes in blood flow and pupil size, the study found. Until now, these physiological responses were not known to work in tandem when the brain spaced out from exhaustion.

"By measuring so many different kinds of information about the brain at the same time, we were able to see that these many different things that at first we thought were separate were actually kind of moving together," study co-author Laura Lewis, an associate professor of neuroscience at MIT, told Live Science.

Sleep is critical for maintaining a healthy brain, and experts recommend that adults get seven to nine hours of shut-eye a night. Not getting enough sleep takes a toll on a person's mental and physical health, and also impairs our ability to pay attention. "However, the neural basis of sleep deprivation-induced attentional failures is not yet well understood," the authors wrote in the study.

To investigate, they recruited 26 healthy volunteers ages 19 to 40 — of whom 19 were female. All the participants took part in two study conditions spaced roughly 10 days apart: well-rested and sleep-deprived. Half completed the well-rested session first, while the others started with sleep deprivation.

The rested individuals slept for 6.5 to 9 hours at home, while sleep-deprived individuals were kept awake all night in the laboratory. In the morning before each trial, the team strapped electroencephalogram (EEG) caps onto the participants to record their brain waves with electrodes. Simultaneously, the participants underwent functional MRI (fMRI) scans to reveal patterns in blood and CSF flow in the brain. Eye-trackers measured the participants' pupil size.

The participants then completed tasks that flexed their visual and auditory attention; they pressed a button as soon as they saw an image or heard a noise. Additionally, the researchers gathered data on baseline brain activity by having the participants rest for 25 minutes without doing any tasks.

As expected, participants took longer to notice the stimuli when tired, and missed the cues more often than they did when they were well rested. But the team was surprised to see giant pulses of CSF in the exhausted individuals, alongside patterns of slow brain waves — both of which are normally observed in non-REM sleep.

Specifically, the patterns resembled that seen as a person shifts from stage N1 into N2, the first two of three stages of non-REM sleep that people experience each slumber. "This was something that previously we'd only seen on this scale during sleep," Lewis said.

The CSF flow was closely linked to pupil size, with the large inward flow following pupil dilation and the outflow coming after pupil constriction. This connection was more pronounced in sleep-deprived individuals, which could suggest the body's circulatory system underpins this coupling, the authors wrote. The CSF flows also coincided with when individuals zoned out during tasks.

"When you have attention failures … you have this fluid being flushed out of your brain, and when you regain attention, when you begin to respond to stimuli again, this fluid is flowing back into the brain," study first author Zinong Yang, a computational neuroscientist at MIT, told Live Science.

"Those are things that we don't usually think about as being tightly locked in time — your ability to pay attention to the world and then basic fluid movement in the brain," Lewis added.

The researchers think that the brain patterns they're seeing might reflect the sleep-deprived brain shifting into a sleep-like state, but while it's still awake. The lapses in attention signal the start of these sleep-like brain processes, but they get interrupted before proper sleep sets in.

But for now, the functional reason behind the huge shifts in blood flow are not clear, Lewis noted. Future work could examine whether and how these patterns affect the clearance of toxic metabolic waste from the brain, the authors wrote.

Michael Chee, director of the Centre for Sleep and Cognition at the National University of Singapore, who was not involved in the study, said the research is "an impressive piece of physiology work." He thinks the autonomic nervous system, which controls unconscious bodily functions, is driving these signals.

"I think that the big lesson is that 'Hey, look; this humble autonomic system that we don't really pay attention to is actually orchestrating some of the biggest signal changes, pupil changes, EEG signal changes, BOLD signal [fMRI] changes,'" he told Live Science.

However, Chee emphasized that the participants experienced 24 hours of sleep deprivation, whereas most people lose only a few hours of sleep on a bad night. "That's a heavy manipulation," he said, so "they are outsized effects."

Studying these brain changes in people with sleep problems could point to new treatment targets, Chee added.

Sleep quiz: How much do you know about sleep and dreams?

It's that time of year again: The evening now arrives earlier than many people punch out of work. On Sunday, Nov. 2, the clocks fell back by an hour for residents of 48 U.S. states and Navajo Nation, marking the end of daylight saving time (DST). In Europe, DST ended on Sunday, Oct. 26.

Despite the American Academy of Sleep Medicine (AASM) being in favor of establishing permanent standard time, many individuals want to keep DST year-round. In a Live Science poll published Oct. 28, we asked our readers what they thought about the contentious issue, asking whether or not they would get rid of daylight saving time.

In what is now the largest Live Science poll to date, 3,280 readers responded by the end of the voting window on Nov. 6. The vast majority said they would eliminate DST, with 76% stating they would stick to standard time, permanently. Meanwhile, 18% said they would rather remain in DST year-round.

Only about 4% of respondents wanted to keep the status quo, while approximately 1% said they would prefer to continue flicking between DST and standard time, but just not in the same way as we do under the current system. The remaining 1% hadn't made up their minds.

"I would be up for scrapping it. DST just seems so pointless," said Devin Smith, from Detroit. Fellow Michiganders shared this view, with Rob Beare echoing a preference for standard time because "10 pm daylight in the summer isn't as important as 8 am sunlight in winter when kids are trying to safely make their way to school."

But others explained they would stay in DST permanently because they wanted lighter evenings. "More evening daylight means more useful time after work/school," "M'Lud" wrote. "Waking up in the dark feels normal. Arriving home in the dark can be depressing."

Dave M agreed, stating a preference for year-round DST to get rid of "that yearly November shock of suddenly driving home from work in the dark." Dave added that the annual fall back also limits the opportunities for recreational activities, with Jane B also noting that permanent DST would "make it easier to accomplish post-business hour tasks."

Others were led by the research on the negative health repercussions of a mismatch between our body clocks and the environment. "The science is clear and standard time is solar time," PazKe said. "It shouldn't even be a question."

What do you think? Share your view in the comments below.

We dream every night, but we often can't remember those dreams. Some mornings, we recall vivid details, and other days, we piece together hazy outlines. But what about color? Do most people dream in color or in black and white?

Some people may be surprised to learn that this has proved a divisive question. Modern research has indicated that television and films have a lot to do with how we experience dreaming and what we can remember when we wake up.

"Since we're used to colored media, we think dreams must be kind of like watching a movie or watching something on YouTube," Eric Schwitzgebel, a professor of philosophy at the University of California, Riverside, told Live Science. "Those things are colored, so we tend to assume that dreams are too."

But that hasn't always been the case. Up until the 1960s, researchers thought people largely dreamed in black and white, and surveys have backed this hypothesis. A small study of 277 people published in 1942 found that 70.7% of the college sophomores surveyed rarely or never saw colors in their dreams. Nearly 60 years later, Schwitzgebel asked a group of 124 college students the same questions — and the results had shifted drastically. In the more recent survey, less than 20% of the students surveyed reported rarely or never seeing colors in their dreams.

Other recent studies have produced similar results. Researchers have discovered a pattern: People born before the advent of color television and movies were much more likely to report having monochromatic dreams than people born after. This suggests that the way we interpret our dreams is affected by the types of media we consume.

Entertainment isn't the only factor. Much of what we take away from our nightly dreams has to do with how accurately we remember them and which details stick with us the most.

"Dreams are defined as subjective experiences during sleep, and the only way we can get to them is if the person is remembering them after waking up," Michael Schredl, head of the sleep laboratory at the Central Institute of Mental Health in Germany, told Live Science. "The main problem is, 'how good are you at recalling?'"

Just as in waking life, the colors of objects can be quite forgettable if they match what we expect to see. For example, a yellow banana in a dream probably wouldn't leave a lasting impression.

"You don't think about it, and it's difficult to remember," Schredl said. But if a neon-pink banana appears in a dream, it might make more of an impact.

What's more, if a particular color is significant to a person, they may be more likely to remember it.

"If the color has a specific meaning for the person in her or his waking life, then it might be that the color might point to something," Schredl said. "It's not about the color itself but how the color is affecting the person."

But Schwitzgebel argues that the question of whether we dream in black and white or color could be a bit misguided in the first place. When we imagine a scene in which the colors aren't important, our mental image may not be in black and white or in color; it may just be a fuzzy, "indeterminate" image. Or perhaps what we remember in the morning is slightly different from the mental image we had while dreaming, informed more by assumptions than memory. Dreams might be less of a visual, movie-like experience than we tend to assume, he said. Rather, it's possible our media consumption affects how we remember our dreams.

"A lot of people can't really quite get their minds around what it would mean for a dream experience to be neither colored nor black and white," he said.

Sleep quiz: How much do you know about sleep and dreams?

Many countries observe daylight saving time (DST), in which clocks get pushed ahead one hour in March to gain more hours of daylight in the spring and summer. In the U.S., all but two states, Hawaii and Arizona (except for Navajo Nation) observe DST and thus are preparing to make the switch back to standard time on Sunday Nov. 2.

Not everyone is happy with this routine change, however, with recent surveys showing that 54% of Americans and 42% of Britons want to scrap DST altogether.

The American Academy of Sleep Medicine (AASM) favors permanent standard time because it is better aligned with our circadian rhythm, the body's internal clock. Because light exposure regulates the circadian rhythm, a mismatch between our body clock and the environment can lead to problems such as reduced sleep, higher heart rate and blood pressure, and impaired immune function, AASM representatives wrote in a position statement.

Last month, we reported on a study predicting that abandoning DST and permanently staying on standard time could prevent over 2 million obesity cases and around 300,000 strokes a year in the U.S. by better aligning our circadian rhythm with the environment.

Yet others suggest there are benefits to keeping DST. Recent research, which has yet to be peer-reviewed, reviewed 157 studies from 36 countries on the links between DST and health outcomes and found evidence for fewer deaths and traffic accidents in the summer.

So, if it were up to you, would you abandon daylight saving time? Take our poll, and let us know what you think of each time policy in the comments below. If you choose "I'm undecided," what would persuade you either way?

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Scientists have identified five distinct "sleep profiles."

And based on their analysis, the researchers concluded that there's more to a good night's sleep than the length of sleep or whether it was fitful. Both genetics and emotional states such as feeling down, anxious, stressed or sad contribute to sleep quality, and each sleep profile was tied to how well people functioned in various ways, researchers reported in a study published Oct. 7 in the journal PLOS Biology.

"I really hope that clinicians will consider [that there is more to] sleep on their first clinical assessment," study lead author Aurore Perrault, a researcher at the Woolcock Institute of Medical Research in Sydney, told Live Science in an email. "Going beyond just, 'How is your sleep? Good or bad?' or 'How long are you sleeping?' but rather ask more questions."

Scientists know people who sleep poorly are at much higher risk of developing depression, anxiety, cardiovascular disease and cognitive impairment, and tend to have shorter life spans, the authors wrote.

But the reason and direction of correlation for those connections hasn't been clear, and most prior studies didn't really classify sleep at a fine-grained level.

To remedy that problem, researchers used data from the Human Connectome Project, which maps nerve fiber-level connections throughout the human brain, to try to get a more detailed picture of sleep quality and its relationship to well-being.

The dataset contains brain imaging and a wide range of self-reported details about each person's lifestyle, mental and physical health, as well as personality and sleep characteristics — for example, typical sleep duration, difficulty staying asleep and use of sleep aids.

People diagnosed with clinical depression were excluded from the study, but if people in the dataset had subclinical symptoms of anxiety or depression that didn't impede their daily function, they were included. The researchers focused on data from 770 people ages 22 to 36 whose sleep likely had not been impacted by the aging process, Perrault said.

Using unsupervised machine learning, which employs artificial intelligence to analyze data without predefined categories, the researchers found statistical links between certain lifestyle characteristics, brain imaging and sleep characteristics.

Participants were asked to rate their sleep over the past month using the Pittsburgh Sleep Quality Index. A score of 5 or less is considered a good sleeper, and the mean score among the people studied was 5.14 on a scale of 0 to 19, the paper noted.

While there were good sleepers, researchers focused on those with scores above 5. This produced five sleep profiles. For instance, "poor sleepers" are people who have trouble sleeping and experience mental health symptoms, such as feelings of anxiousness, and "disturbed sleepers" are people whose sleep is disrupted to the point that it affects their health and cognitive functioning.

Each profile is also linked to a "neural signature," or a brain response that contains information about what the person may have been experiencing at the time, such as body temperature, nightmares or hormonal fluctuations.

This suggests that sleep profiles are associated with both the brain's wiring and markers of health and well-being, Perrault said.

Dr. Henry Yaggi, a professor of internal medicine and director of the Yale Centers for Sleep Medicine, praised the methodology of the study. "It's a much more comprehensive assessment of sleep than I think we've had in the past," said Yaggi, who was not involved in the research. "This is not a one-size-fits-all,” he told Live Science. “There are profiles of sleep with connections to mental health."

Perrault thinks the five sleep profiles could potentially be used as biomarkers, or measurable warning signs of conditions that could come later down the line. Because the profiles were defined in healthy young adults, more research is needed to see if they could work as biomarkers, perhaps for spotting developing anxiety and depression, Yaggi said.

Both Perrault and Yaggi told Live Science that they see a potential for the five sleep profiles to be used to guide clinical interventions. For instance, perhaps certain treatments such as talk therapy, sleep apps and continuous positive airway pressure (CPAP) machines aimed at improving sleep are more useful to "short sleepers" than "sleep-aid users." Broadly, that line of research could also shed light on why about 40% of patients do not respond to cognitive behavioral therapy for insomnia (CBT-I), a type of talk and behavioral therapy, Perrault added.

This article is for informational purposes only and is not meant to offer medical advice.

Sleep quiz: How much do you know about sleep and dreams?

When you wake up in the morning, it might feel like your brain just switched on at the ring of an alarm, although you still might feel groggy for a while. But the actual process the brain goes through to wake up is a gradual, coordinated event. So exactly how does it happen?

First, let's define what it means to be awake. "Being awake means the brain is in a state that supports awareness, movement and thinking," Rachel Rowe, a professor of integrative physiology at the University of Colorado Boulder, told Live Science in an email. "Unlike sleep, where brain waves are slow and synchronized, wakefulness is marked by faster, more flexible activity that lets us respond to the world around us."

There isn't a single moment when the brain flips from asleep to awake, however, said Aurélie Stephan, a sleep researcher at the University of Lausanne in Switzerland. Research has shown that the subcortical regions of the brain — a group of neural formations located below the cerebral cortex — are responsible for waking us up. The reticular activating system (RAS) first acts like the "starter switch," Rowe explained, sending signals to activate the thalamus, a structure that relays sensory information to other parts of the brain, and then the cerebral cortex, the wrinkled outer layer of the brain.

In a 2025 study, Stephan and her colleagues also found that the brain executes a signature pattern of activity upon waking. When the study participants woke up from non-REM sleep, which comprises different stages of sleep ranging from light to deep sleep — their brain activity first showed a short burst in slower, sleep-like waves, followed by faster waves linked to wakefulness.

When they woke up from REM sleep — a sleep stage characterized by vivid dreams and rapid eye movements — their brain waves went straight to faster activity. Overall, regardless of which stage of sleep participants were in, their brain activity appeared to start from the front and central regions of the brain and move to the back of the brain as they woke up, the researchers found.

Why we feel so groggy in the morning

Once we're awake, our brains still need time to reach their full cognitive capacity. This period, called sleep inertia, can last anywhere from 15 to 30 minutes — sometimes even an hour, Stephan said. Researchers don't know why this morning grogginess happens, but the time we wake up can play an important role in how we feel. And ditching the alarm clock could help.

"When your brain [naturally] wakes up, it will send a signal in a moment that makes sense" to end your slumber, Stephan explained. There are many regions taking internal and external signals into account that are discussing with each other when to transition to different stages of sleep and also, ultimately, when to wake you up spontaneously, she explained.

Our arousal system is listening for these internal and external inputs and creating cycles in which we're more alert about every 50 seconds. Our level of alertness fluctuates within those 50 second periods, growing and diminishing over and over.

"During the buildup phase … it is harder to be woken up," Stephan said. But when the cycle wanes, "our sleep is more fragile and it is easier to wake up," she said. "So basically within this 50 seconds period we have a period of sleep continuity and a period of sleep fragility."

That's why Stephan advises her friends to always wake up at the same time, without the help of an alarm.

"Your brain will wait for the right 50-second moment, and you'll feel less sleepy upon awakening," she said. "If you have an alarm clock, it's kind of random. It could wake you up in the worst moment ever, and then you will have a strong sleep inertia."

Still, much of what we know about waking up remains a mystery. Scientists still aren't sure why the same amount of sleep feels refreshing one day but not the next. Some research has suggested that diet and length of sleep can impact morning alertness, or how the brain switches from awake to asleep.

"What pushes our brain to wake up spontaneously actually remains an open question," Stephan said.

Sleep quiz: How much do you know about sleep and dreams?

The amount of REM sleep you get may influence which details of your memories remain in storage, a new brain study suggests.

Previous research had found that sleep helps fortify our memories, but the question of how it shapes the contents of these memories has been harder to pin down. Now, a study published Oct. 1 in the journal Communications Biology hints that the time spent in different stages of sleep may influence this aspect of memory storage.

The sleep cycle is split into four stages: one stage of rapid eye movement (REM) and three non-REM stages, including "deep sleep," marked by slow brain waves. To test how these sleep stages impact our memories, the researchers asked 32 healthy young adults to learn 96 word-picture pairs — such as an action word linked to an image of an animal or plant — while their brain activity was recorded with an electroencephalogram (EEG), which monitors brain waves that wash over the surface of the brain.

The volunteers were then monitored with EEG as they slept overnight and had their recall tested the next morning. The researchers compared the before-and-after brain patterns using a technique called representational similarity analysis. These data enabled the scientists to focus both on detailed memories tied to specific images — like a photo of a beagle — and on broader, categorical memories, covering all the animal images, for instance.

"By using EEG, we could track how brain activity linked to memories changed from before to after sleep," first study author Jing Liu, a research assistant professor at The Hong Kong Polytechnic University, told Live Science in an email.

The team uncovered a pattern: Brainwaves linked to the individual images weakened after sleep, while the broader category signals remained stable.

The shift was stronger when REM made up more of an individual's total sleep time, compared to deep sleep. Liu explained that this pattern suggests REM sleep may help the brain link new memories with what it already knows, while slow-wave sleep helps keep those memories in their original, more-detailed form.

"Even when people remembered the same things after waking, the brain patterns behind those memories had shifted," she added. This suggests sleep not only strengthens memories but may reorganize how they're represented in the brain, with REM and slow-wave sleep contributing in different ways.

Together, these results add to evidence that memory consolidation — the brain's process of stabilizing and reorganizing new memories — involves both preservation and transformation. Rather than storing memories of experiences exactly as they happened, the brain may be subtly restructuring them during sleep, balancing accuracy with generalization. The distinction, the researchers noted, could help explain how knowledge networks in the brain evolve over time.

However, the pattern doesn't necessarily mean that deep sleep and REM sleep work in opposition to one another. Rather, the two phases support different facets of remembering, Dr. George Dragoi, professor of psychiatry and neuroscience at Yale University who was not involved in the study, told Live Science in an email.

"The results here point to a complementary role of REM and slow-wave sleep in different types of memory," he said, such as general knowledge and facts versus memories of specific experiences.

He added that keeping regular sleep schedules may help support these processes, since good sleep quality is broadly linked to healthy cognitive function. "Longer REM periods may promote the kind of memory transformation this study highlights," he suggested.

Liu, however, cautioned that the results show associations, not causation.

"[EEG] prevents us from precisely identifying the brain regions driving these changes," she said, adding that combining EEG with recordings taken directly from electrodes placed inside the skull could clarify the circuitry behind the effect. She also pointed to future studies that might try to reactivate specific memories during sleep — for instance, by replaying sounds or cues linked to earlier learning — or interrupt particular sleep stages to see whether that changes how flexibly people can use what they've learned.

Science of sleep quiz: How much do you know about sleep and dreams?

Abandoning the biannual switch to daylight saving time could prevent more than 300,000 strokes and slash over 2 million obesity cases a year, a new model using data from over 300 million Americans suggests.

Adopting permanent standard time (ST) takes less toll on our circadian rhythm — the body's rough 24-hour pacemaker — than daylight saving time (DST) or flicking between the time policies twice a year, the researchers found.

As a result of this reduced burden on our body clock, staying in ST year round could prevent almost 900,000 more cases of obesity and around 85,000 more strokes a year than remaining in DST year-round, the scientists concluded in a study published Sept. 15 in the journal PNAS.

"We can't propose public policy without data," study co-author Jamie Zeitzer, a professor of psychiatry and behavioral sciences at Stanford University, told Live Science. The data from this research marks "the beginning of a conversation," he said.

Time for a change?

DST, which makes clocks wind forward by an hour every spring, was first introduced in the U.S. in 1918 to save fuel during World War I. It was later made federal law by the Uniform Time Act of 1966.

Our circadian rhythm regulates and coordinates the functioning of each cell in the body. This internal pacemaker is highly sensitive to light exposure, and disruptions to it — for instance, when people must wake up or stay awake during dark hours — creates a "circadian burden" that has been associated with a measurable increase in negative health outcomes, including heart attacks, strokes and car accidents around the time of the switchover.

Insufficient sleep and a circadian burden are also metabolic stressors associated with reduced energy expenditure and excess food intake — risk factors for weight gain and obesity. As such, the position of the American Academy of Sleep Medicine is to scrap DST altogether and permanently stick to ST. Although the health risks of biannual switching are low for individuals, the effects are noticeable at the population level, Zeitzer said.

"You're really looking at lottery ticket kind of risk. But if 350 million people are doing it on the same day, someone's going to win the lottery," Zeitzer told Live Science. "[It's] just not one that you want to win."

To model the health impacts of a mismatch between circadian rhythm and the environment, Zeitzer and colleague Lara Weed, a bioengineer at Stanford University, calculated the circadian burden associated with each time policy: permanent ST, permanent DST and biannual shifting.

To do this, they simulated artificial light and sunlight exposure for the people in every U.S. county across a year under all three time policies. They assumed regular sleep routines (10 p.m. to 7 a.m.), and regular work schedules in a well-lit office (9 a.m. to 5 p.m., Monday to Friday).

The team found that biannually flicking between time policies created the largest disruptions to circadian clocks and therefore the largest circadian burden.

The researchers then used data on county-level prevalence of chronic diseases to determine how the different circadian burdens predicted eight health outcomes: arthritis, cancer, chronic obstructive pulmonary disease, coronary heart disease, depression, diabetes, obesity and stroke. They controlled for various socioeconomic and health factors that could also determine disease prevalence, such as high blood pressure, health insurance status and unemployment.

Sticking to ST would prevent around 2,602,866 cases of obesity and 306,988 strokes compared to a biannual shift. Permanent DST prevented obesity and strokes to a lesser degree, with 1,705,437 fewer predicted obesity cases and 220,092 fewer strokes on average, compared to the biannual switch.

Their model found neither policy led a statistically meaningful reduction in the six other health outcomes.

However, the research is based on simulations which deliberately included unrealistic assumptions, including universal regular light exposure and sleep, and it also did not consider seasonal variations in behavior. Moreover, the health outcomes dataset relied on self-reported information, such as for body mass index (BMI) and whether they'd had a past stroke or not.

Also, they did not include race in their model, despite there being persistent racial and ethnic disparities in sleep in the U.S. This exclusion "doesn't necessarily change the results of the study," said Karin Johnson, a professor of neurology at UMass Chan School of Medicine-Baystate and a spokesperson for the American Academy of Sleep Medicine, who was not involved in the research.

It does mean, however, that the increased risks of stroke and obesity are likely to be felt most keenly by people [namely, Black and Hispanic or Latino populations] that are "already at risk for sleep disparities from other causes," Johnson told Live Science in an email.

Sleep headphones are audio devices designed to help you fall asleep and stay asleep more easily. There are two main types: Tiny and squishy earbuds (such as our favorite Ozlo Sleepbuds), or bigger headbands with built-in speakers (like the excellent AcousticSheep SleepPhones V), for example. Both tend to prioritize comfort and effective noise cancellation, unlike 'regular' headphones that focus primarily on sound quality, battery life and smart features. Many sleep headphones also come with pre-loaded soundscapes, such as nature sounds, that can help your body relax.

As Sam Sadighi, a psychologist and certified sleep practitioner, told Live Science: "Sleep headphones aid uninterrupted sleep by masking disruptive external noises, such as snoring partners or city traffic, and when paired with calming soundscapes, white noise, or guided meditations, they can also support relaxation and make it easier to drift off. Sleep headphones can also provide a helpful distraction for individuals experiencing bedtime anxiety or an overactive mind." For more information on the pros and cons of sleep headphones, scroll down to the FAQs section.

Sleep headphones are notoriously difficult to assess objectively, which is why we tried to be as fair and balanced in our testing process as possible. We looked at their design, features, ease of use and overall value for money, and shared our personal experiences with the products. Below, you will find the six sleep headphones that impressed us the most — we hope they will work well for you, too. You can also pair them with one of the best sleep trackers for detailed insights into your bedtime routine. Give yourself the gift of peaceful slumber this spring.

The quick list

The best sleep headphones we recommend in 2026

Best sleep earbuds

The Ozlo Sleepbuds take the well-deserved top spot. They won us over with their secure, ultra-comfortable fit, great sound quality and a dedicated app filled with genuinely relaxing soundscapes. These sleep earbuds are relatively small and squishy, but they do a surprisingly good job of blocking out unwanted sounds, and they can last up to 10 hours on a single charge — longer than most similar products we tested. True, the Ozlo Sleepbuds cost a steep $349 and are rarely on sale, making them the most expensive option in this guide, but we still think they can provide enough value to outperform their competitors.

For starters, the Ozlo Sleepbuds made a tangible difference to the quality of our sleep, and that is not something we can confidently say about most other sleep earbuds. We suspect that it is partially down to the skillfully composed soundscapes available on the Ozlo app. Whenever we listened to them while we were falling asleep, we woke up the following morning feeling more refreshed than usual — and we have receipts to prove it. Both of our trusted fitness trackers, the Oura Ring Gen 4 and the Whoop MG, have detected fewer sleep disturbances and slightly longer bouts of deep sleep (the most restorative of sleep stages).

That said, we could not find any scientific papers that would help us explain this, even though Ozlo proudly proclaimed on its website that its "sleep technology is clinically proven to help you fall asleep faster and stay asleep longer." As such, we would advise you to take our experiences with a pinch of salt. Still, whether it was just a placebo effect or a genuine improvement to the quality of sleep, it does not change the fact that Ozlo Sleepbuds are an exceptionally well-made product that would suit most people. These sleep earbuds are soft enough for side sleepers, with enough tips and wing sizes to snugly fit most ears, and with a good degree of audio depth and richness to satisfy audiophiles.

There are some downsides to the Ozlo Sleepbuds, though. For example, you have to keep them within 10 feet of the charging case and 30 feet of your mobile device. That is because these earbuds rely on Bluetooth Low Energy, a technology that reduces power use and allows for their ultra-compact design, but vastly limits their connectivity range. This means that Ozlo Sleepbuds may be somewhat impractical to use during daytime activities. Moreover, these earbuds do not feature active noise cancellation, so they may not perform too well in noisy conditions, and as we mentioned already, they are relatively expensive.

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Best sleep earbuds for noisy conditions

Most sleep headphones can help you block external sounds to some degree, but only a handful of them are designed to perform in very noisy environments. The Soundcore Sleep A30 is one of those rare exceptions. Unlike most other sleep earbuds out there, they feature active noise cancellation and novel sound masking features that can block up to 30 dB of noise and help you get a more restful shuteye. Your snoring spouse, barking pets or traffic outside will no longer ruin your sleep, and we can attest to that.

The Soundcore Sleep A30 sleep earbuds are smart in the full sense of the word. They come with an exceptionally robust and advanced sleep app, with the biggest selection of sleep soundscapes and seemingly the most advanced sleep-tracking features of all the products mentioned in this guide. For example, the A30 are the only sleep earbuds we tested that can detect your partner's snoring, analyse its volume and frequency, and then generate appropriate noise-masking audio to block it out. Sounds like a gimmick? We thought so too, but it does seem to do the trick. While we can't say for sure if our partner did actually snore during testing, we did not hear anything at all, either. On the contrary, we enjoyed a genuinely peaceful sleep.

The design of these sleep buds, however, is a hit-and-miss. On the plus side, the Soundcore Sleep A30 features adjustable tap controls and four tip sizes, making them relatively easy to customize. We also found these sleep earbuds quite soft and comfortable to wear. That said, they did not feel as flexible, snuggly and secure-fitting as the Ozlo Sleepbuds. Some side sleepers and people with sensitive ears may still consider the Sleep A30 too bulky and rigid.

Battery life is another issue. Active noise cancellation tends to use up more energy than passive noise cancellation, and that is also the case with the Soundcore Sleep A30. While these sleep airbuds claim up to nine hours of continuous listening on a single charge, this only applies when you play app-based soundscapes at a 50% volume. If you stream your own audio, this drops to just 6 hours.

Lastly, the price. The Soundcore Sleep A30 costs roughly $229, making it one of the most expensive options in this guide.

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Best sleep headphones

Earbuds do a great job of blocking out unwanted sounds and lulling you to a peaceful slumber, but many people — for example, side sleepers and those prone to ear pain or ear infections — prefer sleep headphones. These snuggly headbands with built-in headphones tend to be easier on the ears and more comfortable to wear for prolonged periods, plus they can double as sleep eye masks. But, exactly which sleep headphones should you buy? It is difficult to pick that one product that would suit the widest range of individuals, but if we had to, we would give special recognition to the AcousticSheep SleepPhones V8.

Comfort and fit are the two factors that can make or break your experience with sleep headphones, and this is where SleepPhones seem to outshine other products we tested. Most brands make their sleepbands in one size only and use only one type of stretchy fabric. AcousticSheep, on the other hand, offers two different types of fabric — a warmer, plushier Fleece and a lighter, more summer-friendly Breeze — and three different sizes to choose from. You can also choose between five different color options.

We tested the SleepPhones with a Breeze fabric during the summer and were impressed by how snuggly and breathable they felt in warm August temperatures. Also, it is not just the headband that makes these sleep headphones so comfortable. We could barely feel the presence of its wafer-thin speakers on our ears, even when we were sleeping on our side. This sleepband makes you actually want to wear it, rather than just bear with it for the sake of music and podcasts.

The AcousticSheep Wireless SleepPhones V8 sound pretty well, too. Understandably, they are nowhere near as good as the Bose QuietComfort, for example, but their audio quality is still impressive. These sleep headphones will not exhaust your ears with harsh treble, uneven bassline or an imbalanced range of volume.

However, what makes the SleepPhones so great is also what fuels their downsides. While they are highly customizable, choosing the right headband size may feel tricky if your head measurements fall on a size cut-off, and their thin, barely-there speakers will not be able to block out much external noise or stay firmly in place if you toss and turn a lot in your sleep. Lastly, the price. The AcousticSheep Wireless SleepPhones V8 are indeed very well-made, but this also means they cost more than most other sleep headphones — $99.95 for the model with a regular USB charger, and $149.95 with an induction charger.

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Best budget sleep headphones

We gave the AcousticSheep SleepPhones V8 a well-deserved title of the best sleep headphones, but it is hard to deny that it is a premium product that may not suit everyone's budget. If you do not fancy spending nearly a hundred dollars (or more), consider these Musicozy sleep headphones instead — they cost roughly a quarter of what you would pay for the SleepPhones, but their quality is not too far off.

Their biggest advantage is sound quality. This humble-looking headband delivers a surprisingly rich and well-balanced sound and a more extensive volume range than most other sleep headphones we tested. The speakers in Musicozy headphones may not be particularly effective for blocking sounds — they do not have active noise cancellation and are too small to cover the entire ear — but that is what helps make them comfortable to wear. These headphones did not cause us ear soreness, even after side-sleeping for several hours, and the light and breathable headband easily withstood hot summer temperatures.

The built-in microphone is another feature that makes the Musicozy headphones stand out in the budget section. Its audio quality may not match professional headsets, but it is good enough for taking phone calls. If you work jobs that require you to be on a standby during the night, this could be a useful feature for you.

That said, we also had a few major dislikes here. The Musicozy sleep headphones come in one size only, which will inevitably exclude some people. Secondly, the headband can be a real hassle to wash — moving the speakers and control unit in and out of the headband requires a good degree of patience and manual dexterity. Lastly, the fabric can be a hit-and-miss. While it is stretchy and sweat-wicking, and therefore suitable for light exercise and outdoor activities, it lacks the coziness of, for example, the fabric in our SleepPhones.

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Best sleep headphones for napping on a plane

Sleeping on a plane is a completely different ball game. Whether you travel in an economy or a first-class seat, the whirring noises of the engine and other flight apparatus will always serve you as a constant reminder that you are trapped in a metal tin thousands of feet above the ground. No wonder then that so many of us love traveling with noise-cancelling headphones. They help us pass the time and take some uninterrupted naps, and unlike the smaller earbuds, bulky headphones will not randomly fall out of your ear into your co-passenger's lap or get lost under the seat.

Our writers' go-to option for sleeping on a plane is the Bose QuietComfort headphones. As their name clearly indicates, they focus on the two aspects that are critical for comfortable air travel: snuggly fit and effective noise cancellation. We have worn our Bose QuietComfort on multiple flights, both short and long, and we have never experienced ear irritation, pressure headaches or 'listener fatigue' (a state of mental exhaustion caused by listening to something for prolonged periods). On the contrary, these headphones always felt pleasant and comfortable to wear even after a long day of traveling. The Bose QuietComfort headphones also do a fabulous job of blocking out low-frequency sounds. The aforementioned whirring of a jet engine, for example, can be effectively wiped out by its top-class active noise cancellation features.

We also appreciate its great sound quality. True, the Bose QuietComfort headphones have a slight tendency to emphasize bass, and their ANC mode can sometimes introduce a faint background noise, but this is more of a niggle than a serious complaint. Besides, you can always customize your audio settings in the designated Bose app.

We only have two gripes with these headphones: the $359 price tag and their patchy app connectivity. They are just a bit too expensive for what they have to offer, and connecting them to the Bose app can be an uphill battle for no apparent reason.

Looking for something more budget-friendly? Consider the Soundcore Space 2 instead (scroll down to read more.)

Second-best

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Best earbuds for podcasts and audiobooks

Some people love dozing off to chillout music and sounds of nature, while others prefer falling asleep to podcasts and audiobooks. If you are in the latter category, forget all the headphones we have mentioned so far and consider the Final ZE500 instead. These tiny earbuds are designed specifically for ASMR sounds and voice-focused audio, and while they may not be advanced or rich-sounding enough to satisfy the needs of music lovers, they do indeed excel at creating immersive, whisper-like soundscapes.

The Final ZE500 earbuds are also exceptionally comfortable to wear. They are tiny, lightweight and secure-fitting, and do not tend to fatigue your eardrums or cause excessive pressure in your ear canal. You can comfortably wear them to bed without worrying about losing them halfway through your nap.

That said, the Final ZE500 may not be particularly well-suited for side sleepers. These earbuds are small, true, but they are also relatively rigid. While we did not find this design to be an issue, people with sensitive ears may feel some discomfort when they press these earbuds into the pillow.

Short battery life is another downside. The Final ZE500 last 4.5 hours on a single charge — the shortest of all the models mentioned in this guide. While this can be extended to 18 hours with a charging case, you will not be able to listen to these earbuds nonstop throughout the whole night.

The Final ZE500 are not particularly feature-packed, either. The accompanying app has some innovative ASMR-specific settings that can help to fine-tune your experience, but it has no ANC, sound equalizers, or any other features typical of everyday earbuds. They are also not waterproof, meaning they are not suitable for sweat-heavy workouts.

However, these are just minor infractions. If you stick to podcasts and audiobooks, the Final ZE500 are hard to fault. The best part? They cost just $74.99, making them one of the most affordable options in this guide.

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Best alternative to sleep headphones

Headphones and earbuds do a great job of cancelling noise and sending you to a peaceful sleep, but that does not work for everyone. For example, parents with small children, hospital visitors or people travelling on sleeper trains may want to get some shuteye while still staying aware of their surroundings. Individuals with very sensitive ears or suffering from tension headaches may also be against using sleep headphones. So, what should you do if you are not keen on wearing a headset every night? We would recommend the Jabees Peace Pillow Speaker.

This little device uses bone conduction technology, meaning it does not rely on air to transmit the sounds like the 'regular' speakers do. Instead, it uses the vibrations in your skull to transmit the sound waves directly to your inner ear. Sounds scary? Not at all. Bone conduction is a safe technology, widely used in hearing aids and open-ear headphones used by outdoor runners. True, the sound quality on bone conduction devices is never too impressive, but it is still good enough to send you off to sleep with a podcast, audiobook or basic soundscapes.

What we liked most about the Jabees Peace is its functionality. For example, this speaker is exceptionally travel-friendly. It weighs only 1.59 oz (45g) and measures just 3.39 inches in length and 2.28 inches in width (8.6 and 5.78 cm, respectively). You can take it almost anywhere, and it lasts up to 10 hours on a single charge. Plus, you can barely feel it under the pillow. Lastly, the Jabees Peace features an optional SD card with eight pre-loaded soundscapes, so it can also be used as a white noise machine.

However, pillow speakers have one major disadvantage — they can easily shift around. They may not be the best idea for you if you toss and turn a lot in your sleep, or if you share your bed. Little devices like that can inadvertently wake up or disturb your partner if they accidentally move onto their side.

Second-best alternative to sleep headphones

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Best sleep headphones: comparison

Contributing experts & product testers

Best sleep headphones: Frequently Asked Questions

How to choose the best sleep headphones for you

Our experts agree — choosing the right sleep headphones should largely depend on your needs, sleeping habits and ear sensitivities, and less so on the sound quality or the looks of the device. "Sleep headphones do not need to match the audio standards of premium music headphones. What matters more is comfort, battery life, and the clarity of the audio used to support sleep," Sadighi said.

The first step is to decide between earbuds and over-ear headphones. Each type has its pros and cons, and many people use them interchangeably depending on the circumstances and the intended effect. "It is really personal preference," Cushing said, "but if I had to choose one, I would go with the over-ear headphones because they are less likely to cause soreness in your ears or earwax buildup." O'Brien agreed and added, "I have seen more complaints on sleep earbuds due to ear infections and discomfort than in almost any other headphone category."

Headphones also tend to be more comfortable for side sleepers. "Earbuds can work well if they are soft and low-profile, but they are more likely to cause discomfort or fall out," Timper said. Another perk of sleep headphones is that they can also serve as sleeping masks.

On the other hand, over-ear headphones do not tend to stay in one place during the night. "The headbands are a good idea, but only when the speakers are wafer-thin and sit flush. If the headband is too warm, you sweat through your pillowcase. If the headband is too loose, it slips," O'Brien said. To make matters worse, many headbands are available in one size only. Moreover, the vast majority of sleep headphones can only be recharged with a plugged cable, whereas most earbuds can charge up their battery while sitting in portable cases.

Whichever type you choose, make sure your sleep headphones do not have any wires. "A wired headphone set that gets tangled while you toss and turn in your sleep can pose a safety risk," O'Brien said. Not to mention, it is simply unpleasant to sleep with cables rolling around your head and neck.

So, which sleep headphones should you choose? "Ultimately, it comes down to your sleep position and what feels best for your body," Timper said.

Latest updates

How we tested these sleep headphones

How we test the best sleep headphones

Sleep headphones are difficult to test objectively, and not just because they come in all shapes and sizes. While some models can work wonders for one's sleep, it is not guaranteed that they will have the same effect on other people. Different individuals have different ear shapes, hearing sensitivities and ideas as to what constitutes good, restful sleep. Therefore, we went to great lengths to ensure that all sleep headphones in this guide were tested to the exact same standard and judged in a fair and balanced manner. After spending at least three nights with each device, we ranked it across the following categories:

Design. We look into the shape, weight and fit of the headphones, assess how comfortable they feel in different sleep positions, and give our verdict on their ease of use (particularly during the night) and durability.

Functionality: Simply put, we check what this product can actually do. We want to know how extensive its volume settings and sound modes are, whether it comes with any dedicated apps and/or sleep-focused features, and, if applicable, if it can be effectively used for activities other than sleeping (for example, studying or working out).

Performance: We assess the product's battery life, sound quality and app connectivity (if applicable) and note whether the headphones appear to have a specific effect on our sleep duration and/or sleep quality.

We then weigh these three factors against the price of the sleep headphones in question, and give a final score out of five.

Both Salvador Dalí and Thomas Edison are said to have used dreams to stoke their creative process. To spark new ideas, the men would let themselves drift off to sleep for just a few moments, and then wake up and note the dreamlike visions they had on the brink of unconsciousness.

But you may have heard that dreams happen only in a stage called rapid-eye-movement (REM) sleep. So is it possible to dream during other stages of sleep, as Dali and Edison supposedly did??

Although dreams in different sleep stages can take on different forms, REM sleep is definitely not the only time we dream, experts told Live Science.

"The belief that dreams are only associated with REM sleep comes from people who are not familiar with the subject in detail, or who have somewhat outdated knowledge," Isabelle Arnulf, a sleep neurologist at Sorbonne University in Paris, told Live Science in an email.

When REM sleep was first described in the 1950s, it had a couple of easily observable characteristics that prompted scientists to link the sleep stage to dreams. REM sleep, which makes up about 25% of our total time asleep, is characterized by brain waves that resemble electrical activity during wakefulness, fast eye movements and full-body paralysis. Scientists theorized that these rapid eye movements reflected people "watching" their dreams and that the body was paralyzed to prevent them from acting out those dreams in real life. While these theories about REM sleep still hold, scientists now know that these outward physical characteristics aren't necessarily required for dreams to take place.

It is true that when people are awoken during REM sleep, they often report remembering vivid dreams with a strong narrative thread. But since the discovery of REM sleep, further research has shown that people dream during non-REM (NREM) sleep, too.

Related: What happens in your brain while you sleep?

In one study, Arnulf used a drug to suppress REM sleep in a small group of participants and then regularly woke them up throughout the night to see if they'd had any dreams. Many participants reported remembering dream-like mental content upon waking up, even though they had never entered REM sleep. Based on their reports, the structure and content of those dreams was a little different, though.

"NREM dreams are less frequent, shorter, less vivid, more conceptual or thought-like, and often lack a clear narrative," Francesca Siclari, a sleep researcher at the Netherlands Institute for Neuroscience, told Live Science in an email. "In contrast, REM dreams are almost always story-like, vivid, and rich in sensory detail. That said, there's a lot of variability. Sometimes NREM dreams are very minimal in content, and other times they can be remarkably similar to REM dreams."

Siclari has used electroencephalography (EEG) — a technique that measures electrical activity in the brain — to learn more about dreams during different sleep stages. She's found that both REM and non-REM dreams share some similarities in EEG signatures, even though REM and non-REM sleep stages generally look quite different on an EEG scan. She also discovered that when people's brain activity is slower, they're less likely to report a dream when woken up.

Although these results shed light on the mechanics behind dreaming in REM and non-REM sleep, many questions remain. Scientists still don't understand exactly how the brain generates dreams, why we have such a hard time remembering them, and why we even dream at all. They hope if they can learn more about dreams — and the different sleep stages in which they occur — it could reveal deeper truths about how the human brain works.

"[Dreaming is] much more frequent and varied than most people realize — we typically remember only a tiny fraction of our dreams," Siclari said. "Studying them gives us a unique perspective on how the mind works when it’s decoupled from the outside world."

Sleep quiz: How much do you know about sleep and dreams?

What makes the best sleep trackers? Sleep trackers should offer detailed sleep data and present this information in an easy-to-digest format. Preferably being small and comfortable to wear, you shouldn't be aware you're wearing one to bed, and they definitely shouldn't interfere with your sleep.

However, sleep trackers do come in all shapes and sizes. Because of this, we've included suitable smartwatches, ring-style trackers and trackers without screens in this guide, as not one size fits all. Screens may be important for the best fitness trackers, but for monitoring sleep, screens are not necessary. Although, we've included some with screens for those looking to also track their workouts.

Sleep is important for a healthy body and mind. Our personal trainer expert, Alasdair Nicoll from The Fitness Group, told Live Science: "Your body doesn’t get stronger during your workouts — it gets stronger afterwards, during rest and recovery. That’s where sleep comes in, and why tracking it can be so valuable."

"Sleep trackers give you insight into both how long you’re sleeping and, more importantly, the quality of that sleep. Many people assume they’re getting enough just because they’re in bed for seven or eight hours, but it’s the depth and consistency of your sleep that really makes the difference in how well your body recovers, repairs muscle tissue and prepares for your next session."

If you want to find out more about the best sleep trackers, our FAQ section may answer some of your burning questions (especially now, when so many models get discounted in the January sales). All of the products in this guide have been thoroughly tested and reviewed by our expert reviewers. Our How We Test section contains more information on this process.

The quick list

The best sleep trackers we recommend in 2026: Fully tested by our experts

The best sleep tracker overall

In our review of the Oura Ring Gen 4, we were impressed by its ability to accurately track our sleep, capturing when we fell asleep and woke up precisely. Not only this, but the Oura Ring learns about you and your lifestyle over time, which leads to even more accurate data, as time goes on. The accuracy of the data matters as this helps to provide personalized sleep advice to help you improve your health and wellbeing. On that front, we liked the Oura app and felt the metrics were detailed and provided enough depth. We found it a genuinely helpful addition in monitoring our sleep-wake cycle.

However, the design of the Oura Ring 4 means that the ring, although highly durable, is rather chunky. This isn't ideal when having to wear it to bed to gain the sleep data. A good alternative that's a little less bulky is the Ultrahuman Ring AIR.

One aspect that also excels is the battery life. We tested the Oura Ring 4 for four months and we never had to charge it more than once a week. This is good news for anyone who is likely to want to wear this every day and every night.

The wonderful thing about investing in the Oura Ring 4 is that you are getting build quality alongside tracking accuracy and personalized, tailored sleep data. Yes, it's an expensive option and yes, the expense continues with the monthly subscription that will set you back $5.99 per month, but its sleep-tracking features are some of the best we've seen and tested. So, if sleep-tracking accuracy is important to you, we won't hesitate to recommend the Oura Ring 4.

Read more: Oura Ring 4 review

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The best sleep tracker for beginners

The Amazfit Balance is a good option for those new to fitness tracking, and in particular, sleep tracking. It has extensive sleep tracking features available without it feeling overwhelming.

The watch pairs with the Zepp app and there are extra features available, such as an AI-powered sleep coach called Zepp Aura. However, these extra features do require a subscription for access. Although this is slightly annoying, we think these features may be worth the extra cost for beginners, as the insights provided by Zepp Aura may be useful to those new to tracking their sleep.

Overall, the Amazfit Balance performed well in terms of accurately capturing our sleep data. However, it was sometimes too good at recognising when we'd slowed down, logging resting periods, such as when our reviewer was watching TV, as sleep. If you're looking for higher accuracy, this watch may not suit you. You might be better off with the Oura Ring 3 whose accuracy was spot-on. This is a ring-style tracker, though, so if you're looking for a fitness watch style, you may want to consider the Garmin Venu 3, which offers detailed sleep data and has Sleep Coach included as standard.

One huge positive of this tracker is that, in our review of the Amazfit Balance, our reviewer noted how they forgot they were wearing it, with the strap being super-comfortable and described as "buttery soft." This is ideal for a sleep tracker as you don't want it to feel uncomfortable or pinch your skin, waking you during the night. It's also lightweight and compact, meaning it shouldn't interrupt your sleep from feeling bulky on your wrist. Furthermore, the outstanding 14-day battery life is another bonus as you won't have to worry about it running out of juice every other day, like some trackers we've tested.

The watch works by monitoring your heart rate, heart rate variability, blood oxygen, breathing rate, body temperature and body composition. From these measures, it can then work out the quality of your sleep. Every morning, you receive a readiness score (you must wear the watch overnight to get this) that tells you how ready you are for the day ahead.

The score is out of 100, with a lower score indicating your energy levels are low. If the watch detects a low score, or if the score changes, it will recommend ways you can improve your energy levels and even suggest possible causes as to why they're low. This could be very useful for beginners, as it makes the data it captures accessible to those who do not want to be swamped with information.

Read more: Amazfit Balance review

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The best subscription sleep tracker

Unique in that it has no screen, the Whoop MG is a reliable sleep tracker that is not only comfortable to wear but also reduces the temptation to look at another screen while in bed. The lack of a screen may be an issue for exercise, where you might want to check your mid-session stats, but if you want to use the Whoop MG mainly for sleep tracking, we think the lack of a screen is an added bonus.

Furthermore, there are no controls or buttons that you may accidentally press before getting into bed, making this a fuss-free tracker, where all of the data gets sent to your phone. You are in charge of when you look at that data — the Whoop MG simply captures it for perusing whenever the time is right.

The Whoop MG is also quite unique in that it requires subscription — the minimum amount of time you must hold a subscription for is 12 months, so this is something to be mindful of when purchasing.

In our review of the Whoop MG, our reviewer noted that the strap was comfortable to wear and didn't cause any irritation. This is important in any fitness tracker but especially one that you are going to wear to bed. The last thing you want is to be woken up because you feel uncomfortable or your skin gets too irritated.

The Whoop MG captures a wealth of sleep data including the quantity and quality of sleep as well as the stages of your sleep. Knowing and understanding the stages of sleep you enter and when can help you understand more about how your body and mind function when you're resting. This knowledge can lead to better sleep habits including knowing what time to go to bed and what time to wake up. Worried you won't figure this out? The Whoop MG even suggests a good bedtime based on your data and performance during the day.

Your sleep data is also used to calculate something called your recovery score. The tracker uses the quantity and quality of your sleep, among other things, to work out how ready you are to take on exercise. This is helpful to those looking to understand how sleep can impact performance and fitness levels.

One downside for those looking for a fitness tracker that tracks exercise and sleep is that the Whoop MG doesn't have GPS. This isn't a problem when sleeping (as you're not going anywhere… unless you end up sleep exercising, if that's a thing!?) but this can be an issue for those looking for a fitness tracker to capture walks and runs, for example.

While we think this is the best subscription sleep tracker, if you aren't keen to pay a monthly fee you may prefer the Garmin Venu 3 in this guide, as the Sleep Coach is included as standard.

Read more: Whoop MG review

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The best sleep tracker for Fitbit fans

In our review of the Fitbit Luxe, we noted it was comfortable to wear day and night, making it an ideal purchase as a sleep tracker. It's slimline design means you won't be bothered by it during the night, but this doesn't mean you'll be missing out on features.

The Fitbit Luxe can monitor how well you slept and give you a breakdown of the various stages of sleep you were in during the night. It can also sense changes in your skin temperature, which can sometimes be a sign of an infection. That's quite a lot for such a small device.

While you don't need a subscription to Fitbit Premium to use the Fitbit Luxe, a subscription does give you more detailed insights which might prove useful, depending on what you want from your sleep tracker. It's important to note that it's the subscription to Fitbit Premium that give access to the Daily Readiness Score. This score is calculated on your exercise stats, sleep data and more to help you pace yourself throughout the day, helping to reduce fatigue. This information can be useful so it's worth bearing in mind you will need to pay for a further subscription to access this.

While we liked the Fitbit Luxe and its sleep tracking capabilities, we did find the lack of GPS and offline audio annoying for workouts. If you're primarily looking for a sleep tracker, these issues will not matter, but if you were hoping to use it for tracking your fitness and exercise too, you may prefer the Garmin Venu 3 or, if you're on a tighter budget, the Redmi Watch 5.

Read more: Fitbit Luxe review

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The best sleep tracker for Garmin fans

While some Garmin watches can be bulky, in our review of the Garmin Venu 3, we noted this watch was less bulky than some other Garmin watches — making it an ideal Garmin watch for sleep tracking. It's also lightweight so ideal for being inconspicuous overnight.

Now on to the sleep tracking of the Venu 3. It offers a Sleep Coach which, unlike some other trackers in this guide, comes as part of the standard functionality. That means no extra subscription to pay. You also get the standard sleep tracking which can help improve your sleep patterns. From this sleep data, you receive your Morning Report which is a summary of your sleep as well as notes for the day ahead, such as a weather report. This feature can be disabled if you don't enjoy it but for those looking to analyse their sleep, this can be useful to see each morning.

A nice addition on the Venu 3 is that you can load and listen to music or podcasts via the watch with wireless headphones. If you're someone who tends to do this in order to fall asleep, this may appeal to you.

We noted that this watch is rather plain-looking but this isn't necessarily a bad thing. It's a Garmin watch that isn't so military in style — it's a little more calm and sophisticated. Plus, if you're using it mainly for sleep tracking, a plain-looking watch won't matter.

While the Venu 3 is designed to be used for fitness tracking, our reviewer noted it had limited workout features compared to other Garmin watches. However, if you're using this tracker mainly for sleep tracking, this limitation isn't an issue. It might only become an issue if you're specifically looking for a fitness watch to track your workouts and sleep.

Read more: Garmin Venu 3 review

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The best budget sleep tracker

The Redmi Watch 5 made it as one of our best budget fitness trackers. It was a great smartwatch to test and had an amazing battery life, while in typical usage mode, of around 24 days. It was also incredibly intuitive to use, so even if you're new to fitness and sleep tracking, you will be able to get your head around its layout in no time.

The sleep tracking on this budget device was outstanding when comparing it to more premium brands. You can quickly review your sleep data on the device itself in the morning, or go to the accompanying Mi Fitness App where you will see more detailed sleep data.

Your sleep data is monitored over a week to allow you to then discover your "Sleep Animal." Your Sleep Animal basically gives you an idea of what kind of sleeper you are and how this might relate to the natural world, in the form of animals' sleeping patterns. The device also logs your sleep cycles which can be viewed on the app.

And there is more — the app also gives an interpretation of your sleep, which is basically a summary of your sleep the night before while the suggestions gives you a bit of science for you to understand why this sleep pattern may be good or bad for you. These suggestions suggest what you can do to alter it, if you wish to. Finally, your average BPM is captured during sleep, which is a useful tool as it can sometimes highlight any underlying health issues we may have. This is because it can capture your BPM without any external distractions such as stressors, giving a clearer picture of your overall health.

While it may have all of this sleep tracking available, it's important the Redmi Watch 5 is comfortable to wear overnight. Luckily, in our review of the Redmi Watch 5, we noted it was comfortable, with us barely noticing we were wearing it day or night. However, the crown did tend to catch on clothes so this is something to be mindful of if you wear long-sleeved pyjamas. We also found the strap pinched our skin when putting the watch on after charging (although this may only happen once a month due to its great battery life).

Read more: Redmi Watch 5 review

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The best sleep tracker for comfort

If watch-style fitness trackers don't appeal to you, then the Ultrahuman Ring AIR might. We've placed it as the best for comfort as it is featherlight, weighing only a few grams. It is also discreet and unobtrusive, meaning you won't notice it when you're in bed, the same as you may not notice a normal ring while sleeping. Additionally, our reviewer noted, in the Ultrahuman Ring AIR review, that it was comfortable to wear, with user reviews saying it was more comfortable than the Oura 3.

Cleverly, this device uses various sensors to capture data around movement, heart rate and blood oxygen and then uses this to work out your sleep duration, sleep stages, resting heart rate and, a new one to the table, your circadian rhythm. Other trackers in this guide have not offered insights into circadian rhythm so this makes the Ultrahuman Ring AIR stand out, in our eyes.

Once you have had these various elements measured, the app can then offer personalized advice on how to improve your energy levels. Our reviewer found the personalized notifications helpful, allowing them to learn a lot about their circadian rhythm during the testing period. The best thing about all of this? No subscription is needed.

And while the device can capture all of this information, it also displays a wealth of data insights on the accompanying app. The app is essential to access this data — and there is a lot of it! But every data point is explained so beginners need not fear.

Sadly, there were a few things that let the Ultrahuman Ring AIR down. The app connectivity was intermittent which was frustrating when you rely on the app to access your data and the ring scratched easily.

There are also only limited workout features, so it's not ideal for those looking for a fitness tracker that can do it all. A good alternative that tackles some, but not all, of these problems is the Oura Ring 3.

Read more: Ultrahuman Ring AIR review

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The best sleep tracker for style

We gave the Withings ScanWatch Horizon four out of five stars in our review, seeing it as a hybrid between a traditional watch and a smartwatch. With a traditional watch face design to tell the time and a small screen at the top that displays fitness data, the Withing ScanWatch Horizon is a great choice for those less keen on the smartwatch style.

To see your data in full, you need to pair the device with the Withings Health Mate app. This is also where you access your sleep data. The watch monitors your heart rate while asleep and the Coach function offers handy science-backed health wisdom about your data. It can also capture your sleep cycles.

From this data, you are given a score out of 100 based on your overall sleep time, sleep depth, regularity of your bedtime and the number of interruptions you may experience during the night. This score is a simple way to understand what your sleep quality is like.

Furthermore, a nice feature of the Withings ScanWatch Horizon is that, rather than having a full color display blind you in the morning upon waking, only the hour markers glow in the dark. This watch is much less focused on being a "smart" watch and more about tradition. This glow-in-the-dark feature is useful in the mornings on waking up but may not be enjoyed as much during the night.

Sadly, the Withings ScanWatch Horizon is a little on the heavier side than other trackers we've tested. Our reviewer noted they were aware of it on their wrist so this may put sensitive sleepers off, as they may find the weight irritating.

We particularly liked the lack of a touchscreen as this makes it good for those who are less tech-savvy or less interested in smartwatches and gadgets but would still like to track their sleep. The watch works by simply twisting the crown to access fitness stats on the top small screen. This movement is familiar to anyone who wears a traditional watch.

One huge positive was that we experienced no technical hiccups over the testing period. However, this watch lends itself to overall well-being rather than fitness. For example, in our review of the Withings ScanWatch Horizon, we noted it was better for those looking to get moving rather than regular exercisers.

With such a small screen for fitness data, it might not suit those who want to view detailed stats on the watch face itself. It also doesn't have lots of detailed fitness stats, so hardcode fitness fanatics or data junkies may not be too impressed.

Read more: Withings ScanWatch Horizon review

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The best sleep trackers: comparison

Our expert consultants

Best sleep trackers: Frequently Asked Questions

How to choose a sleep tracker

There is no one-size-fits-all solution here. "Sleep is a very personal experience and so is tracking of one’s sleep," Dzierzewski said. "The choice of a sleep tracker should be based on individual factors. What form factor do you find most comfortable? What data are you hoping to see? What device would be easiest for you to use? What price point are you comfortable with? These and other questions can help guide decision-making about sleep trackers," Dzierzewski said.

Accuracy is another important factor to consider. The vast majority of commercial wearables do not measure your sleep directly — they assess it based on other metrics that change while you are asleep, such as your movement, skin temperature, heart rate and breathing rate. "Sleep trackers vary in terms of how well they measure each of these aspects, so these are key features to investigate. If you’re keen to get into the details, then a more premium tracker may be a better idea for you," Cann said.

However, that does not mean you have to spend hundreds of dollars to get a good shuteye. Most people can experience a substantial improvement in their sleep quality by focusing on a handful of key metrics, namely sleep duration, sleep timings, and time spent awake during the night, Cann said. Many budget options can track those three metrics reasonably well.

It is worth noting that the accuracy of a given sleep tracker may vary depending on who wears it. "For example, our sleep changes with age, so check that your sleep tracker has been tested on your age group. Otherwise, you may get inaccurate readings," Cann said. For example, older people tend to be more restless at night, and this may affect the accuracy of optical sensors that measure heart rate.

How we tested these best sleep trackers

How we test the best sleep trackers

Length of time tested for: All of the trackers in this guide were tested for a minimum of one month, with some being tested for a few months. Our expert reviewers wore the devices day and night to see how well they coped with continued use, to see how reliable their advertised battery lives were, and to also test the accuracy of the data captured.

What tests we do: Our expert reviewers used these trackers for various exercises such as gym workouts, running, walking, yoga and swimming. Our reviewers wore the devices overnight to see how well they calculated their sleep quality and to find out what suggestions were made on how to improve their sleep.

Latest updates

August 28, 2025: We swapped the Whoop 4.0 for the Whoop MG fitness tracker.

By some estimates, the average person spends about one-third of their lives sleeping (or trying to sleep). Sleep is a resting state for the brain and body that occurs at regular intervals, and getting enough sleep is essential for maintaining good health.

When a person is asleep, their core body temperature drops. Heart rate and breathing also slow down and overall metabolic activity decreases by about 10%. A sleeping body may seem inactive at first glance, with the brain relatively unresponsive to external stimuli. But in reality, a sleeping brain is a busy brain. Sleep enables the brain to perform essential repairs and maintenance tasks, building connections between cells that optimize the brain's performance when awake. Dreaming during sleep is thought to help the brain organize and stabilize memories, as well as process and store information.

Sleep also enables the body to repair and replenish cellular material that is damaged or depleted. Important bodily functions such as the production of hormones, tissue growth and muscle repair, for example, occur mostly while we are asleep, and a system that flushes toxins and waste from the brain is most active in sleep.

Everything you need to know about sleep

Sleep glossary

• Circadian rhythm – The internal clock that your body follows during a 24-hour period, moving between alertness and rest in response to changes in light. Circadian rhythms regulate eating habits and other behaviors and bodily functions, in addition to the cycle of sleeping and waking.
• Parasomnia – Disruptive events or behaviors that interfere with restful sleep. These may include sleepwalking, talking during sleep, sleep-related eating disorders, sleep paralysis or intense and vivid nightmares called night terrors.
• Microsleep – A temporary episode of sleep lasting for a few seconds, followed by a swift return to wakefulness. This is sometimes referred to as "nodding off," and is most likely to occur after sleep deprivation. Microsleeping can be dangerous if it takes place during certain activities, such as driving or operating heavy machinery.
• Sleep debt – The difference between the amount of quality sleep that one requires in order to feel well-rested, and the amount of sleep one actually gets. Accumulated, chronic sleep debt can have lasting health impacts, even after the person "catches up" on their missed sleep.

Sleep pictures

Discover more about sleep

• Why am I always tired if I get enough sleep?
• Early study reveals why sleeping pills may not supply the best-quality snooze
• Sleep technique used by Salvador Dalí really works

This article is for informational purposes only and is not meant to offer medical advice.

Science of sleep quiz: How much do you know about sleep and dreams?

Some lucky people have rare genetic mutations that enable them to feel well-rested after just four hours of sleep, while the rest of us need around eight hours to function.

Now, researchers have identified one of these mutations, named SIK3-N783Y, in a human super-sleeper. The team then studied the mutation in genetically modified mice and found that the mice carrying this mutation also got less shut-eye, according to a new study.

The newly identified mutation is one of several that researchers have linked to shorter sleep patterns. Scientists hope that by understanding the genetics of natural short sleepers, who seem to thrive on less sleep, they can develop better treatments for sleep disorders.

"Our bodies continue to work when we go to bed," detoxifying themselves and repairing damage, study co-author Ying-Hui Fu, a neuroscientist and geneticist at the University of California, San Francisco, told Nature. "These people [natural short sleepers], all these functions our bodies are doing while we are sleeping, they can just perform at a higher level than we can."

The researchers published their findings Monday (May 5) in the journal PNAS.

Related: Why do we get a 'second wind' of energy at the end of the day?

There are a variety of negative effects associated with not getting enough sleep, from feeling sluggish and being more forgetful to an increased risk of heart problems. The amount of sleep we need varies as we age, but most adults require around seven to nine hours each night to be at their best. Those with the natural short sleep trait, however, seem to function fine with less.

A natural short sleeper requires around four to six hours of sleep per night. Not only do they thrive on less sleep than the rest of the human population, but they also tend to feel worse if they sleep for longer than their normal hours, according to the study.

Previous studies have identified four genes associated with short sleep and five relevant mutations within those genes. The newly identified mutation affects a fifth gene, Sik3, which has previously been linked to sleepiness. Researchers tested the mutation by giving it to lab mice. They found that mice with the mutation slept around 31 minutes less than those without it, and 54 minutes less following a period of sleep deprivation, which the researchers induced by gently handling the mice, according to the study.

Mice normally sleep for around 12 hours per day, much longer than humans, so this new mutation-linked sleep reduction of up to 54 minutes is less than is seen in human subjects with naturally short sleep patterns. The researchers noted that this could be due to mice usually experiencing more fragmented sleep than humans, or the result of their mouse strain being inbred.

Researchers still have much to learn about the genetics of natural short sleepers and their nighttime superpower. The new findings highlight that Sik3 is a promising therapeutic target for researchers to explore as they attempt to improve sleep efficiency and satisfaction, according to the study.

Science of sleep quiz: How much do you know about sleep and dreams?

Seeing or hearing someone yawn can make you yawn. This phenomenon is not limited to humans; some animals experience contagious yawning, too.

But why is yawning contagious?

Brain cells called mirror neurons may play a role in contagious yawning, Dr. Charles Sweet, a board-certified psychiatrist and medical adviser at Linear Health, told Live Science in an email. Mirror neurons respond to actions that we observe in others.

"When you see someone yawn, those neurons fire up," Sweet said. This neurological mechanism may explain why yawning spreads so easily within social groups.

Research has also shown that people and animals are more likely to yawn in response to yawns from familiar individuals compared with acquaintances and strangers. For example, in a 2013 study, dogs yawned more frequently when watching their owner yawn than when watching a stranger yawn. This pattern, known as the "familiarity bias," is likely driven by attentional biases, said Andrew Gallup, a teaching professor of behavioral biology at Johns Hopkins University, as individuals naturally pay closer attention to those within their social circles.

Related: Is it possible to avoid unwanted thoughts?

One hypothesis is that contagious yawning evolved to boost threat detection within groups, Gallup told Live Science in an email.

In a 2007 study published in the journal Evolutionary Psychology, Gallup and colleagues found that yawning helps cool the brain. This cooling effect may improve alertness and mental processing efficiency in the yawner, Gallup hypothesized. If yawning spreads through a group, it could help enhance the group's threat awareness, the team proposed.

More recent research by Gallup in humans has shown that simply seeing others yawn can improve an individual's ability to detect threats, further supporting the idea that contagious yawning increases vigilance levels in a group.

Another hypothesis is that contagious yawning evolved to keep groups in sync. Yawns follow a natural circadian rhythm and often signal transitions between activities. So, when yawning spreads within a group, it may serve to align activity patterns and behavior, Gallup explained.

Support for this hypothesis comes from a recent study on wild African lions (Panthera leo). The researchers observed yawns from 19 lions in two social groups and tracked the connection between yawn contagion and motor synchrony — when individuals show the same changes in behavior. The results were striking: Lions that "caught" a yawn from another lion were 11 times more likely to mirror the movements of the lion that yawned first, compared with lions who hadn't caught the yawn.

Not everyone is equally susceptible to contagious yawning. In controlled studies, approximately 40% to 60% of typical volunteers yawned in response to watching another person yawn in a video.

A key question in research is whether contagious yawning is linked to empathy. There have been several studies, but findings have been mixed. "Some studies have found predicted connections, while others fail to demonstrate such a relationship," Gallup said.

Although early research suggested that children with autism were less likely to yawn contagiously than nonautistic children were, a follow-up study found that when participants were explicitly instructed to focus on the yawning stimulus, this difference disappeared, Gallup explained, underlining that attention also plays a key role in yawn contagion.

One of the most consistent discoveries is the negative association between contagious yawning and psychopathy. "Individuals who score higher on measures of psychopathic traits tend to be less susceptible to contagious yawning," Gallup said. (Psychopathic traits include selfishness, manipulation and callousness.)

"At the end of the day, contagious yawning is less about being tired and more about connection," Sweet said. "It's your brain's quiet way of syncing up with the people (and sometimes pets) around you."

After a long day, you'd expect to feel exhausted by bedtime. But sometimes, the opposite happens: Instead of winding down, you get a sudden burst of energy, making sleep feel frustratingly out of reach.

But what's behind this second wind?

Part of the answer comes down to circadian rhythms, roughly 24-hour cycles largely controlled by part of the brain called the hypothalamus. For instance, a circadian rhythm helps regulate when we feel alert or drowsy. As part of this sleep-wake cycle, there's a period after sunset — known as the "wake maintenance zone" or "evening second wind" — when energy levels surge.

This happens because the body's wakefulness signals remain relatively high, while sleep-promoting signals, such as the sleep-promoting hormone melatonin, haven't fully kicked in yet, Roxanne Prichard, a professor of psychology with a background in neuroscience at the University of St. Thomas in St. Paul, Minnesota, told Live Science in an email.

From an evolutionary perspective, this burst of energy may have helped our ancestors complete essential tasks, "like making an evening meal and making sure there's a safe place to sleep," Prichard explained.

Related: What happens in your brain while you sleep?

Typically, the wake maintenance zone lasts for a few hours, but its exact length varies from person to person, Prichard said. For night owls, this wake maintenance zone can be longer than those of "morning larks," who may experience a shorter second wind and find it comparatively easier to fall asleep early.

Although a pre-bedtime energy surge is part of a normal circadian rhythm, mental health and lifestyle factors can make it more pronounced.

"Anxiety, late-night work, social media use, or engaging in stimulating activities can trigger the release of stress hormones like cortisol and epinephrine, increasing alertness," Dr. Chris Allen, a board-certified sleep medicine physician and sleep science adviser at Aeroflow Sleep, a sleep-apnea equipment company, told Live Science in an email.

Being on screens before bed can further disrupt sleep by delaying drowsiness. Research suggests that blue light emitted by electronic devices suppresses the secretion of the hormone melatonin. This essentially tricks the brain into thinking it's still daytime, Allen explained.

Another common culprit behind pre-bedtime energy surges is "revenge bedtime procrastination" — in her master's thesis on the subject, Lena Müller defined this concept as "the attempt to purposefully eke out some leisure time in a day full of obligations by postponing one's sleep." People often engage in mentally stimulating activities during this period, which often increases their alertness, Allen added.

For those experiencing a second wind, Prichard suggested embracing the burst of energy rather than resisting it. Anecdotally, "getting things done might actually help sleep later in the evening with the stress reduction that comes with light to moderate exercise or handling of stressors head-on," she said.

However, she advised keeping lighting minimal during these activities. Two small studies published in 2012 and 2019 suggested that warm, red-toned light may improve melatonin levels and sleep quality and help your internal circadian clock stay in sync with external time cues, compared to bluer light. That said, larger studies are necessary to confirm this theory. Some experts argue that red light disrupts sleep less than other types of light, but doesn't necessarily promote it.

Alternatively, to minimize your second wind rather than riding it out, there are several strategies you can try:

First, stick to a consistent sleep schedule. "Going to bed and waking up at the same time daily helps regulate the body's internal clock and reduces second-wind episodes," Allen said. This is also backed up by research. And again, limiting light exposure close to bedtime is key, he added.

Creating a relaxing pre-bedtime routine can also help signal to the brain that it's time to wind down; that's because the bodies' sleep-wake cycle is not only directed by light exposure but also by external, behavioral cues.

If pre-bedtime energy bursts consistently interfere with your sleep, though, it may be worth consulting a sleep specialist to rule out sleep disorders, Prichard recommended. In some cases, doctors may recommend patients to seek out therapy aimed at easing insomnia, she added.

This article is for informational purposes only and is not meant to offer medical advice.

Sleep quiz: How much do you know about sleep and dreams?

We spend a large portion of our lives sleeping. That might seem like an evolutionary disadvantage, given sleep leaves us immobile and potentially vulnerable to attacks for hours on end — but evidence suggests we have many key physiological functions tied to sleep, and disruptions in sleep are linked to a wide range of health problems, including a higher risk of death.

What's more, as we sleep, we often dream. In this way, sleep unlocks an experience that we can't access during our waking hours.

In the quiz below, test your knowledge of the science of sleeping, as well as the dreams that unfold during sleep. Remember to log in to put your name on the leaderboard; hints are available if you click the yellow button, and let us know your score in the comments.

More science quizzes

—Brain quiz: Test your knowledge of the most complex organ in the body

—What do you know about psychology's most infamous experiments?

—Human evolution quiz: What do you know about Homo sapiens?

When bats aren't flying, they often dangle from cave ceilings or the undersides of bridges. But why do bats sleep upside down?

This topsy-turvy behavior may result from the evolutionary journey bats took toward flight, said Tara Hohoff, a bat biologist and the coordinator of the Illinois Bat Conservation Program. "As bats evolved from land-bound mammals to taking flight, they started with gliding like flying squirrels," Hohoff told Live Science.

The ancestors of modern bats might have climbed tall trees and dropped to glide between the trunks, likely evolving strong limbs for these ascents, Alexander Lewis, a bat researcher at California State Polytechnic University, Humboldt, told Live Science. The powerful arms of these gliding creatures evolved into wings over time, he explained.

However, because bats do not have hollow bones like birds do, they do not have as much capacity for lift during flight. As such, bats still generally "hang upside down to drop into flight," Hohoff said.

Related: Which animals are evolving fastest?

Most people would find it challenging to hang from a ledge or any other surface, whether upside down or right side up. However, bats can hang upside down much more easily than humans can because of the way bats' muscles, tendons and talons have evolved.

"When a bat finds a place to roost, it contracts muscles attached to its talons, which opens them," Daniel Pavuk, a zoologist and the chair of biology at Bowling Green State University in Ohio, told Live Science. "As the talons touch the roosting surface, the bat relaxes its body. This allows the weight of its body to pull on the tendons connected to the talons."

The result is that the talons grip the roosting surface. "The joints of the talons lock, and the weight of the bat's body keeps the talons closed," Pavuk said. In other words, bats spend minimal energy to hang upside down; their bodies are relaxed, and gravity does the work for them.

Unlike humans, bats can hang upside down for long periods, according to the Iowa Department of Natural Resources. Whereas an upturned pose would lead blood to pool in a person's head, resulting in health problems over time, a bat's compact size lets its heart easily pump blood throughout its body.

After hanging upside down became the favored method of rest for bats, this new lifestyle supported the evolution of several other traits, Hohoff said. For instance, "the skeletons of bats evolved to be lightweight for flight," Pavuk said. "Because of this, their leg bones do not do well supporting their weight for extended periods of time." By hanging upside down, bats do not have to rest their weight on their weak legs.

Roosting upside down also protects bats from some of their potential predators, Pavuk said. Hanging from hard-to-reach locations, such as cave ceilings, can help them avoid their enemies, such as owls, hawks and snakes.

Bats that are on the ground can still take off from a standing position, Pavuk said. But, "it is more difficult than initiating flight from a hanging-down position," he explained.

However, some bats do not exclusively sleep upside down. For instance, "disk-winged bats in Central and South America have special suction cups on their thumbs that they use to adhere to the undersides of leaves, and do so at all sorts of angles," Lewis said.

Additional research into the evolution of bat flight and the way most bats hang upside down "would be really interesting, since they are the only flying mammals," Hohoff said. "There are some species that seem to have an easier time taking off from the ground, so studying what morphologically is different would be helpful to understand."

Sleeping pills may interfere with the brain's overnight cleaning system, new research in mice suggests.

Like cells everywhere in the body, cells in the brain produce a plethora of waste products that need to be regularly removed to prevent them from accumulating and causing damage. These wastes include toxic molecules associated with neurodegenerative disorders, such as Alzheimer's disease.

Normally, this housekeeping is conducted by the glymphatic system, a network of tunnels that surround the blood vessels in the brain. These tunnels contain a colorless liquid called cerebrospinal fluid, which collects "trash" from cells and then flushes it out of the brain. From there, the trash is processed by the body's larger waste-deposal unit, the lymphatic system.

The glymphatic system operates all the time, but evidence suggests that most of the hard work happens when we sleep. However, experts were unsure exactly what drove this vital process.

Related: 'Love hormone' oxytocin may be missing link between sleep apnea and high blood pressure

It turns out that, at least in mice, glymphatic clearance is driven by a chemical called norepinephrine, also called noradrenaline. This chemical is released by the brain stem and is best known for its roles in the fight-or-flight system.

Norepinephrine's role in brain cleaning was revealed in a new study published Wednesday (Jan. 8) in the journal Cell.

In the study, brain scans revealed that during deep sleep — a type of non-REM sleep that occurs when we transition from being awake to being asleep — norepinephrine is released in tiny waves about every 50 seconds.

These peaks of norepinephrine cause blood vessels in the brain to constrict, reducing blood flow through them. This creates more space for cerebrospinal fluid to flow through the tunnels of the surrounding glymphatic system and gather up cellular waste products. As norepinephrine levels fall off again, the system resets itself, and then the cycle repeats seconds later.

That's what glymphatic clearance looks like in mice that fall asleep of their own accord. However, the researchers found that sleeping pills may disrupt this whole process.

When the researchers gave mice a common sleeping pill called zolpidem, which is often sold under the brand name Ambien, the drug inhibited the release of norepinephrine by 50% compared with mice that fell asleep without medication. The drug's effects resulted in a more than 30% reduction in fluid transport through the glymphatic system to the brain.

These findings have yet to be confirmed in humans. However, previous studies have shown that our brains show similar fluctuations in the flow of blood and cerebrospinal fluid during deep sleep that may be triggered by norepinephrine. So it's thought that our glymphatic networks work similarly to those of mice.

The research hints that, while taking sleeping pills like zolpidem can help people to fall asleep faster, these drugs may inadvertently reduce a person's quality of sleep and potentially increase their risk of disease.

"Sleep medication in general has been associated with higher all-cause mortality [deaths from any cause] and with cognitive deficits — so there's already some clues that sleep medication is probably not giving you the best sleep," study lead author Natalie Hauglund, a neuroscientist at the University of Oxford in the U.K., told Live Science.

Going forward, the researchers plan to investigate if sleeping pills have any long-term impacts on the brain's overnight cleaning ritual. If they do, scientists may need to develop new sleeping pills that don't derail the glymphatic system, Hauglund said.

"We just want to look at the brain and say, 'Okay, the brain goes to sleep, then everything is fine,'" she said.

This article is for informational purposes only and is not meant to offer medical advice.

Although blind people cannot see, they often still possess the brain regions that enable vision. Does this mean that blind people can dream in visual images?

In some cases, they can. A 2014 study found that people who were not born blind but had lost their vision later in life sometimes reported visual experiences in their dreams. These dreams likely drew from memories from when they still had sight. However, the earlier a person had lost their vision in life, and the longer they were blind, the less likely they were to see images in dreams, the study noted.

"A person blind at age 7 or before who has been blind for, say, 20 years, loses virtually all visual experiences in their dreams," study co-author Maurice Ptito, a visual neuroscientist at the University of Montreal, told Live Science.

When it comes to people who are congenitally blind — those born without vision — "dreams tend to rely more heavily on other senses such as hearing, touch, taste and smell," Monica Gori, a cognitive neuroscientist at the Italian Institute of Technology in Genoa, told Live Science.

Most studies suggest visual experiences are generally absent in the dreams of those who are blind from birth. "However, there are a few dream studies that have challenged this notion," Ivana Rosenzweig, head of the Sleep and Brain Plasticity Centre at King's College London, told Live Science.

Related: Why can't we remember our dreams?

For instance, a 2003 study in the journal Trends in Cognitive Sciences reported that people with congenital blindness who were scanned with electrodes on their scalps as they slept had evidence of brain waves associated with vision. A 2023 study from Rosenzweig and her colleagues, which analyzed 180 dreams of seven congenitally blind people, also found reports of vision-like descriptions.

"What we find most surprising and exciting about the possibility that congenitally blind individuals might dream in visual images is the idea that dreams could provide access to experiences that these individuals have never encountered in their waking lives," Helene Vitali, a cognitive neuroscientist at the Italian Institute of Technology, told Live Science.

However, although activity may occur in the visual cortex of blind people's brains as they dream, that does not necessarily mean they see images as they sleep, Ptito said.

"In the congenitally blind, the visual cortex has been recruited for other functions," Ptito said. For instance, "with blind people who can read Braille, if you stimulate their visual cortex, they feel sensation in their fingers. The visual cortex does not stop working in blind people — it gets reassigned."

Ptito noted that "we've had blind people in fMRI [functional magnetic resonance imaging] scanners and sent smells to their noses while taking pictures of their brains, and found this information goes to their visual cortex." The same held true with auditory and touch sensations, he added.

Although Ptito argued that activity in the visual cortex does not mean that blind people see pictures during dreams, Vitali suggested other possibilities. One is that the brain's visual system may form abstract concepts from a blind person's senses that might be represented in a visual way for them. Another is that "REM sleep acts like a virtual reality simulator, helping the brain develop and maintain complex abilities like perception and motor skills," she said. This could mean blind people's brains could generate images within dreams even if they could never have such experiences when they are awake.

One key problem with investigating whether congenitally blind people dream in images is that, even if they do see pictures, they have no experience relating them to what sighted people describe as vision, Christopher Baird, an associate professor of physics at West Texas A&M University, explained in a blog post.

To help address this problem, Gori and her colleagues are now exploring how blind people construct and experience dream images. "The results are currently being finalized and will be available soon," she said. Ptito also noted that artificial intelligence may one day analyze brain scans of blind people as they dream to help see how similar their brain activity is to what sighted individuals experience.

One possible application of this research is the potential to harness dreams as a rehabilitative tool, Vitali said.

"There is evidence that dreaming about certain tasks or experiences can enhance memory and learning," Vitali said. This suggests "targeting the dream state through sensory stimulation during sleep could help reinforce specific abilities, such as spatial awareness." This may provide a new way to help people with blindness, as "blind individuals often face challenges in developing spatial maps and performing complex spatial tasks," she noted.

Exposure to light is crucial for our physical and mental health, as this and future articles in the series will show.

But the timing of that light exposure is also crucial. This tells our body to wake up in the morning, when to poo and the time of day to best focus or be alert. When we're exposed to light also controls our body temperature, blood pressure and even chemical reactions in our body.

But how does our body know when it's time to do all this? And what's light got to do with it?

What is the body clock, actually?

One of the key roles of light is to re-set our body clock, also known as the circadian clock. This works like an internal oscillator, similar to an actual clock, ticking away as you read this article.

But rather than ticking you can hear, the body clock is a network of genes and proteins that regulate each other. This network sends signals to organs via hormones and the nervous system. These complex loops of interactions and communications have a rhythm of about 24 hours.

In fact, we don't have one clock, we have trillions of body clocks throughout the body. The central clock is in the hypothalamus region of the brain, and each cell in every organ has its own. These clocks work in concert to help us adapt to the daily cycle of light and dark, aligning our body's functions with the time of day.

However, our body clock is not precise and works to a rhythm of about 24 hours (24 hours 30 minutes on average). So every morning, the central clock needs to be reset, signalling the start of a new day. This is why light is so important.

Related: Blood test could tell you what time it is in your body

The central clock is directly connected to light-sensing cells in our retinas (the back of the eye). This daily re-setting of the body clock with morning light is essential for ensuring our body works well, in sync with our environment.

In parallel, when we eat food also plays a role in re-setting the body clock, but this time the clock in organs other than the brain, such as the liver, kidneys or the gut.

So it's easy to see how our daily routines are closely linked with our body clocks. And in turn, our body clocks shape how our body works at set times of the day.

What time of day?

Let's take a closer look at sleep

The naturally occurring brain hormone melatonin is linked to our central clock and makes us feel sleepy at certain times of day. When it's light, our body stops making melatonin (its production is inhibited) and we are alert. Closer to bedtime, the hormone is made, then secreted, making us feel drowsy.

Our sleep is also partly controlled by our genes, which are part of our central clock. These genes influence our chronotype — whether we are a "lark" (early riser), "night owl" (late sleeper) or a "dove" (somewhere in between).

But exposure to light at night when we are supposed to be sleeping can have harmful effects. Even dim light from light pollution can impair our heart rate and how we metabolise sugar (glucose), may lead to psychiatric disorders such as depression, anxiety and bipolar disorder, and increases the overall risk of premature death.

The main reason for these harmful effects is that light "at the wrong time" disturbs the body clock, and these effects are more pronounced for "night owls".

This "misaligned" exposure to light is also connected to the detrimental health effects we often see in people who work night shifts, such as an increased risk of cancer, diabetes and heart disease.

How about the gut?

Digestion also follows a circadian rhythm. Muscles in the colon that help move waste are more active during the day and slow down at night.

The most significant increase in colon movement starts at 6.30am. This is one of the reasons why most people feel the urge to poo in the early morning rather than at night.

The gut's day-night rhythm is a direct result of the action of the gut's own clock and the central clock (which synchronizes the gut with the rest of the body). It's also influenced by when we eat.

How about focusing?

Our body clock also helps control our attention and alertness levels by changing how our brain functions at certain times of day. Attention and alertness levels improve in the afternoon and evening but dip during the night and early morning.

Those fluctuations impact performance and can lead to decreased productivity and an increased risk of errors and accidents during the less-alert hours.

So it's important to perform certain tasks that require our attention at certain times of day. That includes driving. In fact, disruption of the circadian clock at the start of daylight savings — when our body hasn't had a chance to adapt to the clocks changing — increases the risk of a car accident, particularly in the morning.

What else does our body clock control?

Our body clock influences many other aspects of our biology, including:

• physical performance by controlling the activity of our muscles
• blood pressure by controlling the system of hormones involved in regulating our blood volume and blood vessels
• body temperature by controlling our metabolism and our level of physical activity
• how our body handles drugs and toxins by controlling enzymes involved in how the liver and kidneys eliminate these substances from the body.

Morning light is important

But what does this all mean for us? Exposure to light, especially in the morning, is crucial for synchronizing our circadian clock and bodily functions.

As well as setting us up for a good night's sleep, increased morning light exposure benefits our mental health and reduces the risk of obesity. So boosting our exposure to morning light — for example, by going for a walk, or having breakfast outside — can directly benefit our mental and metabolic health.

However, there are other aspects about which we have less control, including the genes that control our body clock.

This edited article is republished from The Conversation under a Creative Commons license. Read the original article.

Disease name: Fatal familial insomnia (FFI)

Affected populations: The disease affects an estimated 1 to 2 people per million every year, according to the National Organization of Rare Disorders. FFI is passed from parent to child, and between 50 and 70 families worldwide are believed to carry the genetic mutation that causes FFI. Males and females are equally likely to develop the condition.

Causes: FFI is a neurodegenerative prion disease that is caused by a mutation in a gene called PRNP, which produces a so-called prion protein. Prions are misfolded versions of normal proteins, and their abnormal shape is toxic to cells in the body, particularly neurons in the brain. One of the tissues that is primarily damaged in patients with FFI is the thalamus, a region of the brain that regulates an array of body functions including sleep, body temperature and appetite.

Children need to inherit only one copy of the mutant PRNP gene from a parent to develop the condition. In rare instances, patients may spontaneously develop mutations in the PRNP gene, despite having no family history of FFI. They can then pass this mutation on to their children in the regular way.

Related: Not all insomnia is the same — in fact, there may be 5 types

Symptoms: The hallmark symptom of FFI is insomnia, or the inability to fall or stay asleep, which progressively worsens over time to the point where patients cannot sleep at all.

Patients with FFI also commonly experience memory loss, high blood pressure, hallucinations and involuntary jerking of their muscles. They may sweat profusely and lose their coordination.

Symptoms usually begin around age 40, but can develop as early as age 20 or as late as age 70. Patients eventually enter a coma-like state and typically die within nine to 30 months after their symptoms emerge.

Treatments: There is currently no cure for FFI. As the disease is so rare, there is also no standard way of treating it. Instead, patients may be given advice on how to best manage their symptoms and live as comfortable a life as possible. For instance, taking the drug clonazepam can reduce muscle jerking.

A medical case report from 2006 showed that trying to induce sleep — for instance, by engaging in rigorous exercise and taking narcoleptic drugs — extended and enhanced the life of 52-year-old man with FFI by about a year, but did not prevent his death.

In 2015, a clinical trial of a drug that aims to prevent the onset of FFI was launched. Over 10 years, 10 people who carry the FFI mutation will be given the antibiotic doxycycline and their prognosis and survival after disease onset will be compared to patients who previously died of FFI. Doxycycline has also been shown to prevent the formation of misfolded proteins in another prion disease, called Creutzfeldt-Jakob disease, helping patients live twice as long as those who didn't receive the treatment.

This article is for informational purposes only and is not meant to offer medical advice.

Ever wonder why some people build muscle more easily than others or why freckles come out in the sun? Send us your questions about how the human body works to community@livescience.com with the subject line "Health Desk Q," and you may see your question answered on the website!

Sleep apnea often comes with high blood pressure, which, in turn, contributes to the heart-health risks tied to both conditions. Now, scientists have pinpointed two brain chemicals that play a role in this chain reaction and could pave the way for new therapies.

In a study of lab rats published in May in The Journal of Physiology, the scientists zeroed in on two brain-made chemicals known to affect blood pressure: oxytocin, also famous for its roles in attachment and social bonding, and corticotropin-releasing hormone (CRH). They wanted to see how these two "neurohormones" influence the brainstem, a structure at the bottom of the brain tasked with controlling many involuntary functions, including blood pressure.

People with sleep apnea temporarily stop breathing while they are sleeping, briefly depriving the body of oxygen. This puts it in a hypoxic, or low-oxygen, state.

"When the body becomes low in oxygen, a state called hypoxia, this causes a reflex for us to want to increase our breathing that's going to bring our oxygen level back," said Dr. David Kline, a University of Missouri College of Veterinary Medicine professor who oversaw the study. "It also causes a reflex for our blood pressure to go up to get that oxygenated blood to go where it needs to go," Kline told Live Science.

Related: 'Love hormone' oxytocin may help mend broken hearts (literally), lab study suggests

However, while it's known that both oxytocin and CRH can alter blood pressure, their effects after these brief, repetitive bursts of hypoxia were not fully understood.

The researchers ran their experiment with lab rats that were split into two groups: One group was kept in normal oxygen levels, while the second group was intermittently placed under low-oxygen conditions to mimic aspects of sleep apnea episodes.

The experiment ran for 10 days, after which the scientists collected samples of the rats' brainstems to analyze their neuron activity using various techniques. Additional samples of brain tissue were taken to examine the activity of oxytocin and CRH using a microscope, and a count of specific brain cells that respond to the two chemicals was done manually.

Both oxytocin and CRH are made by cells in a structure called the paraventricular nucleus (PVN). These PVN cells plug into a major sensory center in the brainstem that receives signals from the body that instruct it on how to regulate the cardiovascular system, including blood pressure. Oxytocin and CRH have a hand in sending such signals — but the experiment showed that hypoxia enhances their influence.

The two chemicals had a greater effect on the brainstem activity of the hypoxic rats than it did on the rats kept at normal oxygen levels. After bouts of low oxygen, there was an increase in the release of the chemicals from the PVN, as well as an increase in the number of receptors they plug into in the brainstem. In turn, there was an uptick in the number of signals fired off by the brainstem's sensory center.

Based on these findings, Kline said sleep apnea may be exaggerating the effects of oxytocin and CRH on the brainstem, which may then result in increased blood pressure.

In other words, the release of the chemicals after hypoxic episodes causes blood pressure to become higher and higher each time, Kline said. Over the course of weeks, if this occurs too often, blood pressure then remains high because the brain regions responsible for controlling blood pressure have been altered, he hypothesized.

However, this study did not explicitly look into the mechanisms behind this; the group is now working on studies that could shed light on these unknowns.

After more chemicals involved in the mechanism are identified, specific drugs can be developed to target them and lower blood pressure in sleep apnea patients, Kline said.

Blanket drugs that affect the whole brain may not be the best option, he noted, because the effects of oxytocin and CRH depend on which brain regions they're interacting with. Both chemicals can actually decrease blood pressure if they target different parts of the brainstem than the team studied, for example.

However, in the zone the researchers focused on, both chemicals instead had an elevating effect, Procopio Gama de Barcellos Filho, a postdoctoral researcher in Kline's lab who led the study, told Live Science.

"I think all this basic research is really going to take us to new avenues that can be picked up by the clinicians and the drug companies," Kline said. However, he cautioned that there's still a long way to go to combine their findings into a therapeutic approach for human patients.

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The gut microbiome, a lively community of microbes that resides in the digestive tract, seems to run on a 24-hour clock. That may be really important for our health — but scientists are just beginning to learn why. Early research hints that the bugs play a part in myriad bodily functions, from regulating sleep to breaking down drugs. 

Evidence suggests that, in adults, the abundance of certain microbes in the gut fluctuates daily. In other words, this flux follows a circadian rhythm, similar to the bodily processes that dictate when we sleep and wake up. 

Recently, a study revealed that bacteria living in the guts of babies as young as 2 weeks old also have a circadian rhythm. The report, published in April in the journal Cell Host and Microbe, showed that this rhythmicity increases with age. 

The researchers found that the microbes maintain these day-night rhythms even when they're extracted from the body and grown in the lab, suggesting that their rhythm is intrinsically regulated and thus not dictated solely by factors in the gut itself. 

Related: Scientists may have found the missing link between heart disease and sleep problems

It's unknown why gut microbes behave this way, but their cyclical behavior may somehow help them colonize the human intestine, the study authors theorize. 

"Everything in biology has a reason," which often relates to whether a given trait would help an organism survive, Dirk Haller, co-senior study author and a professor of nutrition and immunology at the Technical University of Munich, told Live Science. In regard to the gut microbiome, the microbial community has evolved alongside the human body, becoming a core feature of our physiology.

The microbiome is beneficial to humans in many ways. For example, it helps protect us against infections by regulating the immune system and metabolizes parts of our food that we can't readily digest. The gut, in turn, provides microbes with a safe, warm place to live. There is therefore "extremely strong competition" between microbes for space in the gut, Haller said, and this competition drives the microbes to evolve. 

Following this logic, microbes within the gut likely have a circadian rhythm because there was an evolutionary benefit to doing so, both for the microbes and for their human hosts. Haller and his colleagues' recent study sheds some light on when these bacterial rhythms might start to appear in a given person's body — but how might that circadian rhythm affect our health?

"This is the most open question," Vincent Cassone, a professor of biology at the University of Kentucky who was not involved in the new study, told Live Science. 

Sleep, disease and drugs

Some past research has probed the circadian clocks of human gut bacteria, such as Klebsiella aerogenes and Bacillus subtilis. However, so far, most work in this field has focused on rodents' gut microbiomes. About 20% of microbes in the mouse gut are known to have a circadian rhythm, showing consistent fluctuations in their abundance at various points in the day. These microbes include bacteria in the orders Clostridiales, Lactobacillales and Bacteroidales, which are all also found in the human gut. 

These bacteria are called "oscillators," said Dr. Eugene Chang, a professor of medicine at the University of Chicago who was not involved in the new research. Most produce byproduct molecules, called metabolites, that are absorbed into the bloodstream and are taken to other organs, including the brain, he told Live Science. 

Once released into the blood, the metabolites can feed into the host's circadian networks and alter their function — for instance, they might influence the pattern and strength of activation of so-called circadian clock genes, Chang said. Scientists have tried disrupting this circadian cross talk in lab mice and have found that it affects physiological processes, including metabolism and inflammation. 

Some findings in mice hint that byproducts made by bacteria in our own guts may influence our sleep-wake cycles. One called butyric acid is produced when bacteria digest dietary fiber and has been shown to promote sleep in mice. However, more evidence is needed to see if the findings actually translate to humans.  

Related: 8 common sleep myths debunked

In a 2020 study of more than 4,000 people, Haller and his colleagues discovered that individuals with type 2 diabetes and obesity don't have the same daily variations in their gut microbiome as those without the conditions. This implies that there is a potential link between circadian rhythms and the microbiome in metabolic disease, but the reason why is still unknown, Haller said. 

The internal clocks that govern gut microbes, as well as those of the host's cells, all interact in a complex manner and are also affected by external factors, such as when we eat, said Dr. Garret A. FitzGerald, a professor in translational medicine and therapeutics at the University of Pennsylvania who was not involved in the research. Theoretically, this "complex interplay" might be disturbed by diseases that affect the gut, he told Live Science in an email. 

For instance, if cells that line the gut are damaged for some reason, this could influence how much of the breakdown products of drug metabolism end up in the bloodstream and affect other organs, such as the lungs, liver or brain, FitzGerald said. Our gut bugs play a role in how drugs are broken down and thus have a hand in this chain reaction.

There's an emerging scientific discipline called chronopharmacology in which scientists are investigating whether the time you take a drug can affect its safety and effectiveness. In addition to influencing people's sleep-wake cycles, bacterial circadian rhythms might partly dictate how the body reacts to a given drug. 

Furthermore, bacterial circadian rhythms may affect the bugs' resistance to antibiotics. For example, research has shown that the concentration of antibiotics needed to kill certain bacteria, such as Staphylococcus aureus, changes at specific times of the day. 

"Everybody tells us that we should take our drugs at certain times a day, Cassone said. "Maybe the gut microbiome and the human host have already known this — that certain times of the day are the best at which to get your vitamin B12, or your butyric acid for sleep or the like," he said. 

Whether this is true or not is "probably one of the great mysteries that I think will be solved pretty soon," Cassone said. 

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We spend one-quarter to one-third of our lives sleeping, but while we rest, the brain remains hard at work, performing regular maintenance. Sleep is known to be important for learning and memory creation, and you may have heard that, during sleep, the brain rids itself of waste built up while we are awake. 

But how does the brain flush these toxins out of its system, and why does this happen only during sleep?

The brain's waste-disposal apparatus is known as the glymphatic system, which includes a network of tunnels that surround blood vessels in the brain. This system is an analog of the waste clearance system found in the rest of the body, called the lymphatic system.

The tunnels in the glymphatic network contain a clear, watery substance called cerebrospinal fluid (CSF), which contains nutrients and physically cushions the brain. CSF from these tunnels into the spaces of the brain, where it mixes with another fluid found between active brain cells.

Related: How does the brain store memories?

Here, the CSF picks up metabolic waste from those cells; this includes amyloid-beta proteins, which build up in the brain in Alzheimer's disease. The "dirty" fluid then gets flushed out of the space and exits the brain through lymphatic vessels that funnel the CSF into the lymphatic system, where it's cleared away.

Aside from clearing away this waste, the glymphatic system helps transport fats, sugars and chemical messengers in the nervous system and may play a role in distributing drugs within the brain.

The system was first discovered in mice, but brain scans have shown that this system also exists in the human brain.

"Glymphatic connection is the plumbing of the brain," Jonathan Kipnis, an immunologist at Washington University in St. Louis, told Live Science in an email. 

The system depends on star-shaped cells called astrocytes, which form connections with blood vessels. Water channels in the astrocytes help the CSF flow from one place to another.

Dr. Maiken Nedergaard, whose team at the University of Rochester Medical Center discovered the glymphatic system in 2012, said the involvement of astrocytes in this important process surprised her. 

"Neuroscience has always been very neuron-centric. Astrocytes were regarded as the housekeeping cells," she told Live Science. "And here, we are saying that astrocytes can tell the brain what to do."

The team uncovered the glymphatic system by putting fluorescent molecules in the CSF of mice and tracing its flow. In similar experiments, they found that this flow was tied to sleep — it decreased by nearly 95% in awake mice compared with sleeping mice. 

Nedergaard theorizes that, because our brain focuses on integrating all the information we get when we are awake, it can't really clean itself at the same time, she said. "This definitely explains our biological need for sleep," she said. 

A person's state of sleep also seems to affect how CSF navigates the maze of billions of neurons to remove waste. Sleep synchronizes neurons, sending high-energy waves of charged particles through the fluid between cells. This helps move CSF into these spaces.

That said, recently, there have been some contradictory results about whether sleep drives the flow of waste-carrying CSF out of the brain. Some scientists found that the process can happen independent of whether an animal is asleep or awake. 

However, Kipnis told Live Science that an experimental method used in that study may itself affect glymphatic flow, muddying the results. In addition, he said many scientists agree that this waste clearance is most efficient during sleep. 

Nedergaard added that there isn't enough data in the study about how waste is actually removed from the brain. "[The data looks at] moving the garbage from one room to another, not getting rid of it."

The glymphatic system is thought to accumulate wear and tear with age, and this could contribute to a buildup of waste and abnormal proteins, potentially leading to diseases like Alzheimer's. "I think that every disease that is affected by accumulation of debris would be dependent on glymphatic function," Kipnis said.

Nedergaard agreed. In theory, therapies aimed at improving sleep quality could benefit people with slow glymphatic flow and potentially slow the progression of neurodegenerative diseases, she noted.

Editor's note: This story was updated on June 24, 2024, to correct the description of the findings of the cited 2024 Nature Neuroscience study. The story was first posted June 19.

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Anyone who has ever been awakened by someone else's snoring knows how loud it can be — especially when the snorer remains blissfully sound asleep.

But can people hear themselves snoring? Or do they have some sort of ability to tune out their own cacophony of snorts and gasps?

Interestingly, both are true.

"Some people wake to the sound of their own snoring," Dr. Anita Shelgikar, a clinical professor of sleep medicine and neurology at the University of Michigan, told Live Science in an email. "Other people are unaware of their snoring, regardless of the intensity or frequency of their snoring."

That's because everyone has a different arousal threshold, which is the propensity to wake up from sleep, Shelgikar said. Depending on the individual, certain noises can be more disruptive than others. 

Related: Why do we breathe so loudly when we sleep?

And whether a given person awakens to their own snores may also vary from night to night.

"The effect of noise on sleep is likely multifactorial, depending on factors such as noise volume, type of noise, the individual's arousal threshold and other environmental factors that may affect sleep," Shelgikar said. For some people, noises like whispering or soft music can be disruptive to sleep, she added.

But the role of noise in waking people up isn't straightforward, especially for snorers.

"Interestingly, recent studies have described an approach called 'phase-locked acoustic stimulation,' which is a specific protocol of sound delivery, to increase the amount of slow-wave sleep (otherwise known as deep sleep)," she said. 

Another factor is whether the snorer is in REM ("rapid eye movement") sleep, which is the phase of sleep when most dreams occur. 

"Some data indicate that the arousal threshold is lower during REM sleep than it is during NREM [non-REM] sleep," she said. "For many people, snoring is worse in REM sleep. This combination may lead to some people being more likely to hear their own snoring during REM sleep."

Snoring occurs when air can't properly flow through the nose or mouth. While loud and annoying, occasional snoring is nothing to be concerned about. However, if someone snores because of a condition such as sleep apnea, that can increase their risk of stroke or heart attack, according to the Cleveland Clinic.

Sleeping with your dog in the same room could be negatively affecting your sleep quality, according to my team's recently published research in Scientific Reports.

We recruited a nationally representative sample of more than 1,500 American adults who completed questionnaires assessing their sleep habits. Overall, about half of the participants reported co-sleeping with pets — defined in our study as sleeping in the same room with your pet for at least part of the night.

Next, our research team compared the sleep habits of people who did and didn't co-sleep with pets. Our analyses revealed that participants who co-slept with pets had poorer sleep quality and more insomnia symptoms than those who did not. These findings persisted even after accounting for demographic differences between these groups. When considering pet type, we found evidence for a negative effect on sleep when co-sleeping with dogs but no evidence for a negative effect on sleep when co-sleeping with cats.

Surprisingly, 93% of people in our study who co-slept with their pets believed that their pets had either a positive or neutral overall effect on their sleep. Although more research is needed, these findings could suggest that most people are unaware of the potential negative effects their pets may have on their sleep.

Why it matters

Most pet owners report that their pets have a generally positive effect on their mental health. Pets can improve their owners' health in numerous ways during the day, such as by encouraging physical activity, promoting a daily routine and providing love and companionship.

However, our study fills an important knowledge gap by indicating that co-sleeping with pets can affect sleep quality. Good sleep is a pillar of health and wellness. Even though pets may have an overall positive effect on mental health, it is possible that some of this benefit may be undermined if they are also causing you to lose sleep at night.

Although some people report that co-sleeping with their pets can provide them with a sense of comfort or intimacy, it is important for people sharing a bedroom with their pets to be aware of their potential to serve as a source of nighttime noise, heat or movement that can disrupt your ability to fall or stay asleep.

What still isn't known

Survey-based studies like ours are unable to prove that co-sleeping with pets causes disrupted sleep, although there is some evidence suggesting that this could be the case.

One important factor that our study did not assess was whether participants were also co-sleeping with other people like a spouse or child. Previous research suggests that sharing a bed with other people can also affect our sleep and that the mental health benefits of pet ownership could be stronger for people with a romantic partner.

What's next

It probably isn’t realistic for most people to just stop co-sleeping with their pets. So, what should someone do to improve their sleep if they already share a bed with their pets?

Some expert tips include choosing a mattress that is large enough for you and your pets, washing and changing your bedding regularly, and establishing and maintaining a consistent bedtime routine with your pets. Further research is needed to identify more specific habits and routines that pet owners can adopt to ensure a good night's sleep when sharing the bedroom with their pets.

This edited article is republished from The Conversation under a Creative Commons license. Read the original article.

As people in the U.S. prepare to set their clocks ahead one hour on Sunday, March 10, 2024, I find myself bracing for the annual ritual of media stories about the disruptions to daily routines caused by switching from standard time to daylight saving time.

About one-third of Americans say they don't look forward to these twice-yearly time changes. And nearly two-thirds would like to eliminate them completely, compared with 17% who aren't sure and 21% who would like to keep moving their clocks back and forth.

But the effects go beyond simple inconvenience. Researchers are discovering that "springing ahead" each March is connected with serious negative health effects, including an uptick in heart attacks and teen sleep deprivation. In contrast, the fall transition back to standard time is not associated with these health effects, as my co-authors and I explained in a 2020 commentary.

I've studied the pros and cons of these twice-annual rituals for more than five years as a professor of neurology and pediatrics and the director of Vanderbilt University Medical Center's sleep division. It's become clear to me and many of my colleagues that the transition to daylight saving time each spring affects health immediately after the clock change and also for the nearly eight months that Americans remain on daylight saving time.

Why our bodies are thrown off by DST

Americans are split on whether they prefer permanent daylight saving time or permanent standard time. But a growing number of states are supporting permanent standard time.

However, the two time shifts — jolting as they may be — are not equal. Standard time most closely approximates natural light, with the sun directly overhead at or near noon. In contrast, during daylight saving time from March until November, the clock change causes natural light to be present one hour later in the morning and one hour later in the evening according to clock time.

Morning light is essential for helping to set the body's natural rhythms: It wakes us up and improves alertness. Morning light also boosts mood — light boxes simulating natural light are prescribed for morning use to treat seasonal affective disorder.

Although the exact reasons why light activates us and benefits our mood are not yet known, this may be due to light's effects on increasing levels of cortisol, a hormone that modulates the stress response, or the effect of light on the amygdala, a part of the brain involved in emotions.

Adolescents also may be chronically sleep-deprived due to school, sports and social activities. For instance, many children start school around 8 a.m. or earlier, which means that during daylight saving time they get up and travel to school in pitch darkness.

The body of evidence makes a good case for adopting permanent standard time nationwide, as I testified at a March 2022 Congressional hearing and argued in a recent position statement for the Sleep Research Society. The American Medical Association recently called for permanent standard time. And in late 2022, Mexico adopted permanent standard time, citing benefits to health, productivity and energy savings.

The biggest advantage of daylight saving time is that it provides an extra hour of light in the late afternoon or evening, depending on time of year, for sports, shopping or eating outside. However, exposure to light later into the evening for almost eight months during daylight saving time comes at a price. This extended evening light delays the brain's release of melatonin, the hormone that promotes drowsiness, which in turn interferes with sleep and causes us to sleep less overall.

Because puberty also causes melatonin to be released later at night, meaning that teenagers have a delay in the natural signal that helps them fall asleep, adolescents are particularly susceptible to sleep problems from the extended evening light. This shift in melatonin during puberty lasts into our 20s.

The 'western edge' effect

Geography can also make a difference in how daylight saving time affects people. One study showed that people living on the western edge of a time zone, who get light later in the morning and later in the evening, got less sleep than their counterparts on the eastern edge of a time zone.

This study found that western-edge residents had higher rates of obesity, diabetes, heart disease and breast cancer, as well as lower per capita income and higher health care costs. Other research has found that rates of certain other cancers are higher on the western edge of a time zone.

Scientists believe that these health problems may result from a combination of chronic sleep deprivation and "circadian misalignment." Circadian misalignment refers to a mismatch in timing between our biological rhythms and the outside world. In other words, the timing of daily work, school or sleep routines is based on the clock, rather than on the sun's rise and set.

A brief history of daylight saving time

Congress instituted year-round daylight saving time during World War I and World War II, and once again during the energy crisis of the early 1970s. But the popularity of year-round daylight saving time fell from 79% to 42% after it went into effect in January 1974, largely due to safety concerns about children going to school in the dark.

The idea at that time was that having extra light later into the afternoon would save energy by decreasing the need for electric lighting. This idea has since been proved largely inaccurate, as heating needs may increase in the morning in the winter, while air conditioning needs can also increase in the late afternoon in the summer.

After World War II, designating the start and end dates for daylight saving time fell to state governments. Because this created many railroad scheduling and safety problems, however, Congress passed the Uniform Time Act in 1966. This law set the nationwide dates of daylight saving time from the last Sunday in April until the last Sunday in October. In 2007, Congress amended the act to expand the period in which daylight saving time is in effect from the second Sunday in March to the first Sunday in November – dates that remain in effect today.

But the Uniform Time Act does allow states and territories to opt out of daylight saving time. Arizona and Hawaii are on permanent standard time, along with Puerto Rico, the U.S. Virgin Islands, Northern Mariana Islands, Guam and American Samoa.

A shifting landscape

As of March 2024, 17 states have passed laws to adopt year-round daylight saving time. But federal law requires that they need to wait for Congress, and in some cases also neighboring states, to act. More than two dozen states introduced legislation related to the clock change in 2023, but no laws were passed.

Meanwhile, proposed legislation and resolutions for permanent standard time have increased from 15% in 2021 to 37% in 2024.

In March 2022, the U.S. Senate passed the Sunshine Protection Act  in a bid to make daylight saving time permanent. But the House did not move forward with this legislation. Florida Sen. Marco Rubio reintroduced the bill on March 1, 2023, but this bill has not progressed.

The spike in activity among states seeking to break from these twice-yearly changes reflects how more people are recognizing the downsides of this practice. Now, it's in the hands of legislators to decide whether to end the time shift altogether and to either choose a full year of having clocks more aligned with the sun or to artificially extend the day for more than half the year, for better or for worse.

This edited article is republished from The Conversation under a Creative Commons license. Read the original article.

If marine mammals want to sleep, they can't just close their eyes and drift away for the night, as they need to intermediately surface for air. Nor can they float at the water's surface and slumber, because that exposes them to predators and heat loss. So how do marine mammals get any shut-eye without putting themselves at risk?

One solution is to shut down one half of their brain at a time. Called unihemispheric sleep, it's one way marine mammals, such as dolphins, can get some rest while in open water.

"Unihemispheric sleep is really valuable to these animals because it allows them to maintain a low level of activity while still sleeping half of their brain at a time," Patrick Miller, a biologist at the University of St Andrews in the U.K., told Live Science.

Dolphins are the best-studied marine mammals capable of this style of sleeping. Brain scans on captive dolphins show that while one hemisphere is in slow-wave, deep sleep, the other hemisphere is alert, allowing the animals to literally sleep with one eye open. This style of sleeping is common in cetaceans — the group of mammals that includes dolphins, whales and porpoises — but it's not unique to them. Many bird species are known to use unihemispheric sleep, often allowing them to doze while flying.

But according to Miller, birds and dolphins use their half-brain sleeping technique for different purposes. For instance, in a flock of birds, many of the ones on the outside of the group keep the eye facing away from the flock open, to watch for predators. But dolphins do the opposite: While sleeping, they tend to keep the eye open that faces the rest of the pod, likely to avoid becoming separated.

Related: Why do horses sleep standing up?

Not all cetaceans seem to be capable of unihemispheric sleep. Some use bihemispheric sleep, in which both hemispheres of the brain fall asleep, just like humans and most other mammals do.

"It's really difficult to measure brain activities for animals out at sea that you're not able to capture, like a sperm whale, blue whale or a humpback whale," Miller said. "In that case, behavioral records are our best indicator of sleep behavior."

In those cases, researchers can affix tags to the animals to monitor their behavior. A 2008 study led by Miller used suction tags attached to sperm whales (Physeter macrocephalus) to show that they sleep out in the open ocean for short bouts. The whales make shallow dives beneath the surface, slow their swim speed to a stop, and pitch lazily upward. Their upward pitch is likely due to the buoyant oil, known as spermaceti, in their heads.

While sleeping, whole pods of whales can be seen nose-up, just below the ocean's surface. During this time, the animals are completely unresponsive, which suggests they are in a form of deep sleep. However, the animals can rest only about 20 minutes underwater before they need to come back up for air. Once the whale has caught its breath, it slinks below the surface for more rest, and may continue this behavior for up to 3.5 hours.

Northern elephant seals (Mirounga angustirostris) are bihemispheric sleepers as well and snooze in similar short bouts. A 2023 study led by Jessica Kendall-Bar, a postdoctoral researcher at the Scripps Institution of Oceanography, a department of the University of California, San Diego, was able to, for the first time, monitor brain activity in a sleeping mammal at sea. Kendall-Bar and colleagues found that the seals dive to a depth of around 1,000 feet (300 meters), at which point their brains slow down and enter rapid eye movement (REM) sleep. Once in REM sleep, the seals flip upside down and spin in a slow circle while continuing to sleep.

"Seals sleeping in the ocean were upside down 100% of REM sleep time, suggesting that just like us, they become paralyzed during REM," Kendall-Bar said in a YouTube video describing the research.

Likely due to predation risk, elephant seals limit their total sleep time at sea to just around two hours per day, "rivaling the record for the least sleep among all mammals, which is currently held by the African elephant (about 2 hours per day)," the researchers wrote in the study.

Male antechinuses live through just a single, extremely intense mating season. So to maximize their chances at sex, these little carnivorous marsupials waste as little time on sleep as possible, new research has found.

During a three-week breeding period, males belonging to two species — dusky (Antechinus swainsonii) and agile (A. agilis) antechinus — lost an average of three hours of sleep every night, or around 20% of their total sleep. "This level of sleep loss would make humans perform very poorly," lead author Erika Zaid, a graduate researcher in sleep eco-physiology at La Trobe University in Australia, told Live Science in an email. "Yet, antechinus endured doing so for three weeks."

Males sacrificed sleep to mate with as many females as possible and father a maximum number of offspring before dying, Zaid said. 

In the wild, male antechinuses drop dead in a single "programmed" event following the mating period. Males often survive for longer in captivity, Zaid said, but they become sterile. In the new study, published Thursday (Jan. 25) in the journal Current Biology, 2 captive males out of 10 died at the end of the breeding season. "They died in the same night, only [a] few hours apart," Zaid said.

Recent research suggested male marsupials of the dasyurid family — which live and reproduce for only one year and include antechinus — died due to extreme sleep deprivation, based on the observation that severely sleep-deprived laboratory rats eventually die of exhaustion.

However, Zaid thinks this may not be the case with antechinuses. "After the breeding season, antechinus males do develop skin lesions and fur loss that resemble those observed in sleep deprived rats," Zaid said. "However, we don't think sleep loss is the cause of the male die-off," because the two males that died weren't the most sleep-deprived.

Related: 'They mated like mad': Low-flying helicopter sparks massive crocodile orgy in Australia

Another theory is that male antechinuses die from a surge in testosterone and corticosteroid levels during the mating period, which can induce organ failure and leave males more susceptible to infections and parasite infestation, according to the study. But no such surge has been detected in males northern quolls (Dasyurus hallucatus) — another dasyurid species — prompting the researchers to think another, unknown trigger causes male antechinuses to die off. 

Unlike the males, female antechinuses survive for up to two years and can give birth to two litters. They might even benefit from the males dying en masse, as the corpses may provide a rich and abundant source of food to fuel their pregnant or lactating bodies, according to a study published Jan. 18 in the journal Australian Mammalogy.

The study described a sighting of a mainland dusky antechinus (A. mimetes) eating a dead member of its species in New England National Park, New South Wales, suggesting these critters indulge in cannibalism.

While cannibalism has been documented in some dasyurids, it is very rarely seen in the wild, Andrew Baker, an author of the Australian Mammalogy paper and an associate professor specializing in Australian mammals at the Queensland University of Technology in Australia, said in a statement. 

Zaid and her colleagues housed the antechinuses used for their study in individual enclosures and therefore didn't observe any cannibalism. "I guess cannibalism is a good strategy they might adopt to have easily accessible energy," Zaid said. But Baker and colleagues could not identify the sex of the cannibalistic antechinus, meaning it could have been a male eating another male, she said.

Much remains to be learned about antechinuses — including the details of their cannibalistic tendencies and how males cope with extreme lack of sleep during the mating season.

It may be that antechinuses feel the negative effects but simply get on with it, or that these critters are resilient to sleep deprivation, the researchers said in a statement shared with Live Science.

A sleep disorder that scientists previously believed was rare might be more common than originally thought. The condition is marked by feeling sleepy during the day despite having a good night's rest.

In a new study, scientists analyzed sleep data from nearly 800 people and found that 1.5% of them likely had idiopathic hypersomnia, a debilitating neurological condition that causes people to feel an excessive need to sleep and leads them to sleep a lot yet still feel sleepy during the day.

That 1.5% amounts to 12 people in the study — a rate significantly higher than previous studies suggested. For instance, an estimated 37 people per 100,000 were diagnosed with the condition in the U.S. in 2021 — or 0.037% of the population. However, scientists think the condition is likely underrecognized due to a lack of awareness and because "expensive" and "time-consuming" sleep testing is needed to make a diagnosis, Dr. David Plante, co-author of the new study and an associate professor of psychiatry at the University of Wisconsin-Madison, said in a statement.

These new estimates, published Wednesday (Dec. 13) in the journal Neurology, may help raise awareness of the disorder.

Related: Scientists may have found the missing link between heart disease and sleep problems

"Our results demonstrate that idiopathic hypersomnia is relatively common, more prevalent than generally assumed, so there is likely a sizable difference between the number of people with this disorder and those who seek treatment," Plante said in the statement.

"Further efforts to identify, diagnose and treat those impaired by idiopathic hypersomnia are needed," he added.

Plante and colleagues analyzed data on sleep patterns in 792 people from the ongoing Wisconsin Sleep Cohort study, which is investigating sleep disorders in 1,500 Wisconsin state employees. They looked at data from two types of lab test that doctors use to diagnose idiopathic hypersomnia: polysomnography, in which bodily functions such as brain activity and heart rate are measured during sleep, and the multiple sleep latency test, which assesses how quickly someone falls asleep during daytime naps.

The researchers also asked the participants questions, such as how tired or fatigued they felt during the day, how long they spent napping and how long they'd normally sleep at night, both when they had work the next day and when they didn't.

Based on these data and using current diagnostic criteria for idiopathic hypersomnia, the team concluded that 12 people likely had the condition. These individuals generally experienced more severe sleepiness during the day, even when they slept for the same or longer amounts of time than others.

On average, these people fell asleep around nine minutes quicker at night and six minutes quicker in the day compared with those deemed unlikely to have the condition. They also scored higher on a sleepiness survey, which included questions on their likelihood of dozing off while sitting or talking. On average, the people who researchers thought likely had idiopathic hypersomnia scored around 14 points on the survey, compared with 9 for those without the condition. A score below 10 usually amounts to having an average amount of sleepiness during the day.

The researchers had data on daytime sleepiness from 10 of the 12 people with probable idiopathic hypersomnia. They found that most of the 10 had experienced excessive daytime sleepiness for around 12 years, meaning their symptoms were chronic.

However, the team discovered that this sleepiness eventually went away for four of the 10 people, which suggests that the condition can sometimes remit, Plante said. More research is needed to understand what leads to remission in these cases.

Larger studies are needed to confirm these initial findings about the potential prevalence of idiopathic hypersomnia, as well as to investigate sleep patterns in a wider range of people. For instance, the study only looked at people who were employed, but those with idiopathic hypersomnia can sometimes struggle to find or keep a job.

It's therefore possible that the condition is even more common than these new estimates suggest, the authors wrote in the paper.

"Additional research may also clarify the causes of idiopathic hypersomnia and lead to new treatments," Plante said. Current treatments alleviate only the symptoms of the condition, rather than treating its underlying cause; for example, stimulants might be prescribed to help patients to stay awake during the day.

This article is for informational purposes only and is not meant to offer medical advice.

Ever wonder why some people build muscle more easily than others or why freckles come out in the sun? Send us your questions about how the human body works to community@livescience.com with the subject line "Health Desk Q," and you may see your question answered on the website!

Chinstrap penguins are one of nature's most extreme nappers, snatching more than 10,000 microsleeps lasting up to four seconds long every day, according to a new study.

The penguins, which live in breeding colonies each containing tens of thousands of individuals, must remain constantly vigilant against threats to their nests from predatory skua birds and hostile colony neighbors. 

The result is a sleeping schedule warped beyond all recognition — placing the penguins in a state somewhere between wakefulness and sleep that culminates in 11 hours of snoozing time per day. The researchers behind the study published their findings Dec. 30 in the journal Science.

Related: Scientists may be able to put Mars-bound astronauts into 'suspended animation' using sound waves, mouse study suggests

"It shows nicely at what point sleep is constrained by natural selection processes," Paul-Antoine Libourel, a researcher at the Lyon Center for Research in Neuroscience in France, told Live Science. "Animals face a clear tradeoff between sleeping and its benefits while not being vigilant, and being awake but with the physiological cost of not sleeping. These penguins found a way to gain the advantage of sleeping while also remaining vigilant to protect their eggs."

Named after the thin black strip that runs under their beaks, chinstrap penguins (Pygoscelis antarcticus) live on islands all the way around the South Pole. During the nesting season, males squat on eggs to provide protection and warmth while females leave on long hunts. In the chaotic and noisy environment of the colony, any sleep is welcome — but it comes at a cost of decreased watchfulness.

To study how the penguins managed the careful balancing act, the researchers studied the sleep patterns of 14 penguins out of a colony of 2,700 breeding pairs on King George's Island off the coast of Antarctica. After surgically implanting electrodes into the penguins' brains and connecting them to data loggers placed on their backs, the researchers released them back onto the island. 

After weeks of watching the penguins nodding off, the scientists had conclusive evidence of one of nature's most bizarre zonk strategies: the nesting penguins take 600 microsleeps an hour, each lasting an average of four seconds. By monitoring the brain activity of the drowsy birds as they microdozed, the researchers also found that sometimes they only slept with half of their brain, while the other hemisphere remained alert. 

And, to the researchers' surprise, all of these tiny sessions added up, providing a restorative function to the brains of the penguins across the entire day. 

Microsleep has been spotted in other birds and marine mammals, including albatrosses, dolphins, ducks and elephant seals — an adaptation that enables them to rest while on the move — but never for such extended periods of time, Libourel said.

"There are reports of drowsy states in other species, looking like microsleeps," he said. "However, those studies do not report any animal always sleeping in this way."

The scientists say that the chinstrap penguins' unusual dozing challenges many dominant assumptions about sleep across the animal kingdom. Their next steps will be to look for more animals with weird sleeping patterns.

"The data reported by Libourel et al. could be one of the most extreme examples of the incremental nature by which the benefits of sleep can accrue," Vladyslav Vyazovskiy, a professor of sleep physiology, and Christian Harding, a sleep researcher, both at the University of Oxford, wrote in a related commentary article on the discovery.

"Proving that sleeping in this way comes at no cost to the penguin would challenge the current interpretation of fragmentation as inherently detrimental to sleep quality," they wrote.

Nearly 1 in 5 elementary school-age children and preteens take melatonin, a recent Colorado study has found.

In addition, some parents regularly give melatonin to their preschool-age kids, according to the research, which was published in a letter in the journal JAMA Pediatrics on Monday (Nov. 13).

These findings raise concerns because there's a lack of long-term data on the safety and effectiveness of melatonin in children, the study authors wrote. The study also hints that the underlying causes of sleep problems in children taking melatonin are often going unaddressed.

Health care providers sometimes recommend melatonin as a short-term sleep aid in children with autism and severe sleep problems, because it's specifically been studied in that population, study co-author Julie Boergers, a psychologist and pediatric sleep specialist at Rhode Island Hospital and the Alpert Medical School of Brown University, said in a statement.

"But it is almost never a first-line treatment," she said. Families are typically advised to try addressing a child's sleep issues with behavioral strategies first. "Although [melatonin is] typically well-tolerated, whenever we're using any kind of medication or supplement in a young, developing body we want to exercise caution," she said.

Related: Eating just 2 cannabis gummies can put small kids at risk of toxic effects

Melatonin gummies, capsules and liquids contain a synthetic version of a sleep-promoting hormone produced by the body. For the new research, the scientists asked parents of more than 990 children ages 1 to 13 about their kids' melatonin consumption. The survey was conducted between January and April 2023 and included parents in the area around Boulder and Denver, Colorado.

About 5.6% of the preschool-age kids had consumed melatonin in the preceding month, compared with 18.5% of elementary school-age kids and 19.4% of preteens. The typical melatonin doses increased with age, from about 0.5 milligrams in the youngest group to 2 mg in the oldest, per the parents' reports.

The number of days a week that kids took melatonin varied, but most took the supplement either one day a week or seven days a week. Many of the kids who had taken melatonin in the preceding month had been consistently taking the supplement for much longer. For preschoolers, the median duration of melatonin use was 12 months, compared with 18 months for elementary schoolers and 21 months for preteens.

The study included a relatively small sample of people who were all from one area, and it was conducted via surveys, so there's uncertainty about whether the findings apply elsewhere. However, it does still raise some red flags.

Because they're considered "dietary supplements," melatonin supplements are not as tightly regulated by the Food and Drug Administration as prescription and over-the-counter drugs are. And many melatonin gummies sold in the U.S. are inaccurately labeled, containing 74% to 347% the amount of melatonin listed, past research has found. Some products actually contain no melatonin at all, while some contain CBD, the non-psychoactive ingredient in cannabis.

And research has shown that, as melatonin supplements have grown in popularity, there's been an uptick in calls to poison control centers about children — particularly those age 5 and younger — ingesting large amounts of melatonin, with a small percentage of them experiencing severe toxic effects, such as respiratory failure or seizures.

In addition, there's little data on how or whether long-term melatonin use might negatively affect children at different stages of development, and scientists have raised questions about whether the practice could impact kids' puberty-related hormones.

"We are not saying that melatonin is necessarily harmful to children," lead study author Lauren Hartstein, a postdoctoral fellow in the Sleep and Development Lab at Colorado University Boulder, said in the statement. "But much more research needs to be done before we can state with confidence that it is safe for kids to be taking long-term."

Ever wonder why some people build muscle more easily than others or why freckles come out in the sun? Send us your questions about how the human body works to community@livescience.com with the subject line "Health Desk Q," and you may see your question answered on the website!

Timing is everything. For early risers and late-nighters alike, listening to your internal clock may be the key to success. From the classroom to the courtroom and beyond, people perform best on challenging tasks at a time of day that aligns with their circadian rhythm.

Circadian rhythms are powerful internal timekeepers that drive a person's physiological and intellectual functioning throughout the day. Peaks in these circadian rhythms vary across individuals. Some people, known as larks or morning chronotypes, peak early and feel at their best in the morning. Others, known as owls or evening chronotypes, peak later in the day and perform best in the late afternoon or evening. And some people show neither morning nor evening preferences and are considered neutral chronotypes.

As a researcher seeking ways to improve cognitive function, I've explored whether your chronotype affects your mental performance. Understanding the kinds of mental processes that vary — or remain stable — over the course of a day may help people schedule their tasks in a way that optimizes performance.

Related: Can a night owl turn into an early bird?

Why your chronotype matters

Chronotype can be measured with a simple questionnaire that assesses things like your perceived alertness, preferred rising and retiring times and performance throughout the day. Even without a questionnaire, most people have a sense of whether they are a lark or an owl or fall somewhere in between. Do you wake up early, without an alarm, feeling sharp? Are you mentally drained and ready for pj's by nine? If so, you are likely a morning type. Do you sleep late and wake feeling sluggish and foggy? Are you more energized late at night? If so, you are likely an evening type.

People perform best on many challenging mental tasks — from paying attention and learning to solving problems and making complex decisions — when these actions are synchronized with their personal circadian peaks. This is known as the synchrony effect. Whether you are an air traffic controller scanning the radar, a CFO reviewing an earnings report or a high school student learning chemistry, synchrony can affect how well you perform.

Much of the evidence for synchrony effects comes from lab studies that test both larks and owls early in the morning and late in the day. People with strong chronotypes are more vigilant and better able to sustain attention at their peak relative to off-peak times. Their memories are sharper, with better list recall and more success in remembering "to-do" tasks like taking medication.

People are also less prone to mind wandering and less distracted at their optimal time. For example, a study I conducted gave participants three weakly related cue words (such as "ship," "outer" and "crawl"). They were tasked to find another word that linked all three (such as "space"). When my team and I presented misleading words alongside the cue words (such as "ocean" for ship, "inner" for outer and "baby" for crawl), those who were tested at synchronous times were better at ignoring the misleading words and finding the target solution than those who weren't.

Synchrony also affects high-level cognitive functions like persuasion, reasoning and decision-making. Studies on consumers have found that people are more discerning, skeptical and analytical at their peak times. They invest more time and effort in assigned tasks and are more likely to search for important information. Consequently, people make better investment decisions, are less prone to bias and are more likely to detect scams.

At off-peak times it takes people longer to solve problems, and they tend to be less careful and more reliant on mental shortcuts, leaving them vulnerable to flashy marketing schemes. Even ethical behavior can be compromised at non-optimal times, as people are more likely to cheat at their off-peak times.

Related: Night owls and morning larks, make room for 'afternoon people' and 'nappers'

In the classroom and the clinic

The basic mental abilities that are affected by synchrony — including attention, memory and analytical thinking — are all skills that contribute to academic success. This connection is especially significant for teens, who tend to be night owls but typically start school early.

One study randomly assigned over 700 adolescents to exam times in the early morning, late morning or afternoon. Owls had lower scores relative to larks in both morning sessions, but this disadvantage disappeared for owls taking the exam in the afternoon. Early start times may put student owls a step behind larks.

Time of day may also be a consideration when conducting assessments for cognitive disorders like attention-deficit disorder or Alzheimer's disease. Scheduling time may be particularly significant for older adults, who tend to be larks and often show larger synchrony effects than young adults. Performance is better at peak times on several key neuropsychological measures used to assess these conditions. Failing to consider synchrony may affect the accuracy of diagnoses and subsequently have consequences for clinical trial eligibility and data on treatment effectiveness.

Of course, synchrony doesn't affect performance on all tasks or for all people. Simple, easy tasks — like recognizing familiar faces or places, dialing a close friend's phone number or making a favorite recipe — are unlikely to change over the day. Furthermore, young adults who are neither larks nor owls show less variability in performance over the day.

For those who are true early birds or night owls, tackling the toughest mental tasks at times that align with their personal circadian peaks could improve their outcomes. When small improvements in performance offer an essential edge, synchrony may be one secret to success.

This edited article is republished from The Conversation under a Creative Commons license. Read the original article.

Do you usually hit the "snooze" button when your alarm blares in the morning? Good news: You're not alone, and those extra Zs could boost your cognitive performance when you first get up, new research suggests.

In a study of more than 1,700 surveyed adults worldwide, published Wednesday (Oct. 18) in the Journal of Sleep Research, 69% said they hit the snooze button or set multiple alarms at least "sometimes."

Then, in an experiment with 31 habitual "snoozers," the study authors found that hitting snooze improved some of these individuals' performance on cognitive tests. However, it didn't appear to impact their mood, how sleepy they felt in the morning or the normal increase in the stress hormone cortisol that happens within the first hour of waking and helps prepare the body for the day ahead.

"Snoozing for 30 minutes in the morning does not have any major negative effects on the sleep that night or how tired one feels when waking up," Tina Sundelin, lead study author and psychologist at Stockholm University in Sweden, told Live Science in an email. "For those who are drowsy in the morning, such as evening people, and find that snoozing helps them wake up, the study shows that they may actually be more objectively alert [meaning that they performed better on the cognitive tests] after snoozing — even if they don't feel more or less sleepy," she said.

Related: Scientists may have found the missing link between heart disease and sleep problems

Getting a good night's sleep is important for our general health and well-being, but many people regularly don't get enough sleep. About one-third of U.S. adults report sleeping less than the recommended seven or more hours a night. Some people hit snooze to ease the sting of waking up abruptly or to get a few extra minutes of sleep, but little is known about the health impacts of this habit.

In the new study, 1,732 adults were asked to describe their waking habits. On average, the people who reported at least occasional snoozing spent around 22 minutes sleeping after their first alarm went off. These individuals were around six years younger and almost four times more likely to be night owls than those who reported that they never hit snooze, the authors found. They were also more likely to sleep for a shorter period of time during the night and to experience drowsiness in the morning than non-snoozers.

For the 31 regular snoozers in the second leg of the study, snoozing for 30 minutes either improved or didn't affect their performance on cognitive tests they completed when they woke up, compared with when they were not allowed to snooze. These tests included basic arithmetic questions and having to memorize a list of words and later recognize them amongst new words.

When allowed to snooze, the participants were less likely to wake up from the deepest stage of sleep — called slow-wave sleep — than they were when they had to wake up right away. Waking during slow-wave can lead to sleep inertia, meaning the groggy feeling you get as the body transitions from sleep to waking.

The study was limited in that the experiment was small and the survey portion relied on people self-reporting their sleeping and waking habits. However, the team hopes the findings will open doors for future research into people's waking habits, such as the different ways in which snoozers wake up.

"Perhaps something like a light alarm (the kind that slowly makes the room brighter) would work in a similar way but without having several abrupt wakeups," Sundelin said. The team would also like to explore the potential long-term effects of snoozing and to study "extreme snoozers," who snooze for up to two hours each morning, she said.

This article is for informational purposes only and is not meant to offer medical advice.

Napping during the day is an ancient custom that is practiced worldwide.

While some people view napping as a luxurious indulgence, others see it as a way to maintain alertness and well-being. But napping can come with drawbacks as well as benefits.

As an orofacial pain specialist, I have extensive education in sleep medicine and how sleep impacts wellness, due mostly to the relationship between sleep and painful conditions such as headaches and facial pain. My training involved all aspects of sleep, especially sleep breathing disorders, insomnia and sleep-related movement disorders.

As such, I'm aware of the complex nature of napping, and why a short nap — that is, a nap during the daytime that lasts from 20 to 30 minutes — may be beneficial in myriad ways.

Related: What happens in your brain while you sleep?

An abundance of health benefits

Research shows that there are many benefits to napping. Short naps can boost mental functioning and memory, as well as improve alertness, attention and reaction time.

Short naps are also linked to increased productivity and creativity. Because napping seems to improve creative thinking, some companies have attempted to harness this by introducing napping rooms into the workplace.

What's more, it appears the brain uses nap time to process information gathered throughout the day, which appears to enhance problem-solving abilities. One small study revealed that people who took short naps were less frustrated and impulsive, which resulted in better focus and efficiency when performing work-related tasks. Napping may even lead to an improved ability to learn new motor skills, such as a golf swing or the playing of a musical instrument. This is because these memories or skills become consolidated in the brain during sleep, whether at night or while napping.

Napping can also reduce stress. One study found that naps of approximately 20 minutes improved the overall mood of participants. However, longer naps lasting more than 30 minutes are not typically associated with improved mood and increased feelings of well-being.

Short naps may also be associated with a reduced risk of cardiovascular diseases. If we are awake more than we should be, we tend to have a buildup of the "fight or flight" chemicals in our bodies. Studies show that more consistent sleep will help lower these chemicals, resulting in a normalization of blood pressure and heart rates. Napping appears to help this process for some people.

But just as in nighttime sleep, some people may have trouble drifting off for a nap, especially when they have limited time. Progressive muscle relaxation techniques have been shown to be beneficial for both nighttime sleep and napping. Other nonspecific relaxation techniques, like listening to relaxing music, appear to also be beneficial for falling asleep. Interestingly, many people overestimate their time awake when trying to sleep and underestimate the time they actually spent sleeping.

Related: Human brain looks years 'older' after just one night without sleep, small study shows

Napping can have drawbacks

One condition associated with napping longer than 30 minutes is sleep inertia — the grogginess and disorientation that people sometime experience after waking from a longer nap.

Usually, the longer the nap, the more sleep inertia there is to overcome. This can impair cognitive function from several minutes up to half an hour. In many cases, these effects can be minimized by consuming caffeine directly after the nap.

But it is important to note that caffeine is not a substitute for sleep. Caffeine acts to temporarily block the action of a chemical known as adenosine, a sleep-promoting agent that builds up during waking hours. If you are habitually dependent on caffeine consumption to keep you awake and alert, it may suggest that there is an underlying sleep disorder such as insomnia or sleep apnea, in which a person temporarily stops breathing during sleep.

Long or late afternoon naps can also interfere with nighttime sleep, either by leading to difficulties falling asleep or staying asleep during the night. This disruption of the regular sleep-wake cycle can result in overall sleep deprivation, which can have numerous negative health effects.

What's more, for those age 60 and up, longer naps — beyond 30 minutes — may increase the risk for cardiovascular problems. Researchers found that older adults taking naps for more than one hour per day have a higher incidence of increased blood pressure, high blood sugar, excess body fat around the waist, and abnormal cholesterol or triglyceride levels, sometimes known as metabolic syndrome.

The reason for this phenomenon is mostly unknown. Older individuals tend to nap more frequently than younger adults partly due to more disturbed sleep during the night. This could be related to more pain or other health factors that will interfere with sleep, sleep altering medications and altered sleep rhythms seen with aging.

Best practices

So, to maximize benefits while reducing risks, here are some tips: Keep naps short to avoid sleep inertia and nighttime sleep disruptions. Nap in the early afternoon, as that aligns with a decrease in energy levels after lunch and with the body's natural circadian dip, which is an increase in sleepiness similar to what occurs at dusk. Avoid late afternoon naps, finish naps at least four to six hours before bedtime, and create the right environment by napping in a quiet, comfortable and dimly lit space.

If you're struggling with daytime sleepiness, it's best to address the root cause rather than relying solely on napping. Reducing caffeine consumption, maintaining a regular sleep schedule and getting adequate nighttime sleep are essential steps to reduce daytime sleepiness.

Ultimately, napping should complement a healthy sleep routine, not serve as a substitute for sufficient nighttime rest. A balanced approach to napping can contribute to a more energized, focused and resilient life.

This edited article is republished from The Conversation under a Creative Commons license. Read the original article.

Extreme weather events and mass extinctions are some of the most serious effects of climate change, but a warming world is having many other— and less obvious — effects on our planet. From bumpier plane rides to sex-changing lizards, here are 15 of the most unexpected effects of a warming world.

Baseball home runs increase

Home runs in Major League Baseball seem to be increasing as temperatures rise. In a study published April 7 in the journal Bulletin of the American Meteorological Society, researchers said that as warmer air is less dense, baseballs fly farther after being batted away. 

They found that, between 2010 and 2019, more than 500 homers — accounting for 1% of home runs — could be put down to unusually hot days as a result of global warming.  They also predicted home runs could be 10% more common by 2100, compared with averages between 2010 and 2019. Playing more games at night when temperatures are milder could help counter the impact on the sport, the scientists suggested. 

Shrinking frogs croak in higher pitches 

As the world warms, male coquí frogs (Eleutherodactylus coqui) in Puerto Rico are croaking in higher pitches. In a study presented on May 8 at the 184th Meeting of the Acoustical Society of America, researchers revealed how rising temperatures are causing these amphibians to shrink, raising the pitch of their croaks. 

Male coquí frogs call to mark their territory and warn off rivals. The scientists found that frogs living nearer the base of a mountain, where it is warmer, croaked in higher pitches than larger frogs living at higher altitudes, where it is cooler. When researchers returned to the same slopes two decades later — when global temperatures had increased — they found that frogs were calling in higher pitches no matter where they were on the mountain.

Airplane turbulence getting worse 

Climate change is making plane rides bumpier as air streams shift. In a study published June 8 in the journal Geophysical Research Letters, scientists compared climate data from 1979 to 2020 with air turbulence data in the North Atlantic. They found that severe turbulence — which is caused by colliding streams of air that are traveling at different speeds — jumped by 55% from 17.7 hours in 1979 to 27.4 hours in 2020. Moderate levels of turbulence rose by 37% over the same time period. Climate change is likely behind this increase because hotter air means wind speeds and directions are changing more strongly, the researchers wrote in the study.

Sleep loss

By 2010, people had already lost around 44 hours of sleep each year because of hot nights associated with global warming. This could rise to 58 hours of lost sleep per year by 2100 under a high carbon emissions scenario, researchers estimated. In a study published in May 2022 in the journal One Earth, scientists compared sleep data collected using sleep-tracking wristbands from 48,000 people in 68 countries, and found that people sleep later and wake earlier on hotter nights.

Rise in dog bites

Humans are known to commit more violent crimes when the heat ramps up. Now, a study published June 15 in the journal Scientific Reports indicates that canine aggression also rises with the temperatures, with more dog bites recorded on hotter days. By analyzing data on 69,525 dog bites in eight U.S. cities — Dallas, Houston, Baltimore, Baton Rouge, Chicago, Louisville, Los Angeles and New York City — the team found an 11% increase in dog bites on days with high UV and a 4% rise when temperatures were high.

More wildfire lightning 

Global warming will alter lightning patterns around the world — and this may result in more wildfires. In a study published Feb. 10 in the journal Nature Communications, researchers examined "long-continuing current lightning" — a type of lightning known to be the main cause of lightning-induced wildfires. They estimated these strikes will  become 10% more common with each 1.8 degree Fahrenheit (1 degree Celsius) temperature rise. This could amount to a 40% increase in these strikes by the end of the century under a worst-case-scenario climate change projection. 

Related: Yellowstone's geysers at risk of extinction from climate change, tree skeletons reveal

Fewer babies

Hotter days have been linked to a dip in birth rates. In a study published 2018 in the journal Demography, researchers found that days with an average temperature above 80 F (26.7 C) were linked to a 0.4% decrease in birth rates roughly nine months later — the average length of pregnancy —  compared with days with temperatures of between 60 F (15.6 C) and 70 F (21.1 C). Rather than causing a dip in sex drive, the scientists think high temperatures could lower fertility — some studies have found that hot conditions can impair how well sperm swim.

Lizard sex change 

Rising temperatures mean central bearded dragons (Pogona vitticeps) in Australia are changing sex. For some reptiles, sex is influenced by the temperatures their eggs are exposed to as they develop, with higher temperatures linked to more females. In a 2015 study published  in the journal Nature, scientists described how 11 of 131 lizards captured in the wild had male sex chromosomes but warm incubation conditions caused them to develop a female anatomys, with sex-reversed lizards able to lay eggs.

Worse allergies

Rising temperatures are causing earlier and longer springs, which are propelling more pollen into the air, making life that little bit harder for people with allergies like hay fever. In a study published in 2021 in the journal Proceedings of the National Academy of Sciences, scientists found that the sneezing season is getting longer in North America, having increased by 20 days between 1990 and 2018. They also found pollen levels increased by 21% in the same time period. The changes are probably due to global warming and are already worsening allergies, the team wrote. 

Dimmer Earth 

In a study published in the journal Geophysical Research Letters in 2021, scientists analyzed the amount of sunlight reflected from Earth onto the moon between 1998 and 2017 and found our planet appears to be getting dimmer. Their findings revealed that hotter seas have reduced the amount of reflective low-lying clouds above the eastern Pacific Ocean, causing less sunlight to bounce off our planet. As this traps more light energy on the Earth, it could further increase global warming.

Erupting volcanoes 

Many of Earth's volcanoes are covered in ice, but as more of this ice melts due to global heating, the released water could mix with the hot rock and magma beneath to cause more frequent violent explosions. Melting ice reduces pressure on the magma, enabling more bubbles to form inside it. This bubbling magma then puts more pressure on the Earth’s crust above, until the molten liquid explodes through cracks in the crust. This idea was backed up by a study published 2017 in the journal Geology, in which researchers found that between about 4,500 and 5,500 years ago, the number of volcanic eruptions in Iceland dropped significantly when the climate cooled, compared with hotter periods.

Less coffee  

Climate change could halve the amount of coffee-growing land by 2050, a study published Jan. 26 in the journal PLOS One suggested. By modeling three different climate scenarios that limited global warming to either 2.7 F (1.5 C), 4.3 F (2.4 C) or to 7.2 F (4 C), scientists found that the number of regions highly suited for growing coffee — such as areas of Brazil, Vietnam, Indonesia and Colombia — could decline by 50%. 

Shrinking goats

Goats in the Italian Alps are getting smaller as our planet gets hotter. In a study published in the journal Frontiers in Zoology in 2014, researchers analyzed the body mass of juvenile Alpine chamois (Rupicapra rupicapra) living on the mountains between 1979 and 2010 and found they had shrunk by around 25%. Over the study period, the region warmed by 5.4 F to 7.2 F (3 C to 4 C). The scientists said the animals' reduced size appears to be down to the chamois spending more time resting and less time feeding during hot periods.

Deteriorating mummies

After remaining well-preserved for more than 7,000 years in the Atacama Desert, some of the world's oldest mummies have started to degrade while being kept in a museum. These mummies were prepared by hunter gatherers called the Chinchirro. Rising humidity levels amid climate change are thought to have spurred the growth of bacteria that digest the mummies, causing some to turn into a black ooze, according to a report by the Harvard John A. Paulson School for Engineering and Applied Sciences. By testing mummy samples in the lab, scientists identified humidity levels in the museum that would best preserve the historic objects.

Faster tree growth 

Increasing carbon dioxide levels in the atmosphere have spurred trees to grow faster in Central Europe. In a study published in the journal Nature Communications in 2014, scientists found that Norway spruce (Picea abies) and European beech (Fagus sylvatica) trees grew more than twice as fast in 2010 compared with 1960, amid rising levels of carbon dioxide. This greenhouse gas is a key ingredient for photosynthesis, the process by which plants use light energy to produce sugar and oxygen. This sugar can then be combined with oxygen in the process of respiration to release energy for growth.

People with heart disease often develop dreadful sleep problems, and now, scientists have identified a direct link between these conditions for the first time in a new study in mice and human tissues. 

Published Thursday (July 20) in the journal Science, the research shows that heart disease may derail the production of the sleep hormone melatonin in the brain due to damage to a group of nerves that innervate, or plug into, both organs — the superior cervical ganglion (SCG).

Found in the neck, these nerves are part of the autonomic nervous system, which regulates involuntary processes in the body, such as breathing and heart rate. Because nerves originating from the SCG connect to both the heart and the pineal gland — the tiny brain structure responsible for melatonin production — issues with the heart could explain why the body's melatonin-maker falls off track. 

"Imagine the ganglion as an electrical switchbox," senior author Stefan Engelhardt, a professor of pharmacology and toxicology at the Technical University of Munich, said in a statement. "In a patient suffering from sleep disturbances following a heart disease, you can think of a problem with one wire causing a fire to break out in the switchbox and then spreading to another wire."

Related: Irregular sleep may increase your risk of dying from cancer and heart disease 

The research is "important and timely," Brooke Aggarwal, an assistant professor of medical sciences at Columbia University who was not involved in the study, told Live Science in an email, noting that it "suggests a novel mechanism that may help to explain why those with heart disease are more prone to sleep disturbances." 

She went on to caution, though, that "future prospective studies need to be conducted, as well as clinical trials of any potential treatments stemming from this mechanism."

Struggling to sleep is a common side effect of heart disease — for example, up to 73% of people with heart failure experience symptoms of insomnia. Past studies have shown that melatonin levels are reduced in people with heart disease, but scientists didn't know why. 

In the new study, researchers analyzed human brain tissue samples from deceased heart disease patients and from people without heart disease. This postmortem analysis revealed a reduced number of nerve fiber, or axons in the SCG of people who had heart disease compared with the "heart-healthy" control group. The SCG of the individuals with heart disease were also markedly scarred and enlarged.

In supporting mouse experiments, the team found that immune cells called macrophages, which gobble up diseased and damaged cells, were present in the cervical ganglia of mice with heart disease, and the rodents' nerves showed signs of inflammation and scarring. The mice also had fewer axons in their pineal glands and less melatonin in their blood than healthy mice did. The rodents' circadian rhythms — the internal processes that regulate how the body responds to day and night — were also disrupted, as evidenced by changes in their metabolic rates and activity levels, for example.

Giving mice melatonin completely reversed this disruption, the team found. Additionally, when drugs were used to destroy the macrophages in the rodent's SCGs, their melatonin levels were restored.

Because these analyses were conducted in mice and only 16 humans, the findings "call for further studies" to reveal the mechanisms that drive immune cells to the SCG, the researchers noted in the paper. This may involve studying the nerve cells that link the heart and spinal cord, as well as messenger proteins called cytokines that summon macrophages.

In time, the team believes the study may pave the way for the development of new drugs to treat sleep disturbances caused by heart disease. 

"It will be now pivotal to obtain evidence in a randomized clinical trial to determine whether therapeutic melatonin is indeed effective in treating sleep disorders in patients with chronic heart disease," Engelhardt told Live Science in an email. If it proves effective, "then this could spare many patients the unnecessary side effects that come with standard sleeping pills."

If you've ever passed by a field of horses at night, you've probably noticed that they rarely lie down to sleep. So why do these mammals sleep standing up? 

The answer is one of survival: Horses slumber while standing to balance their need for sleep against the ever-looming threat of predators.

Horses come from a long line of prey animals in the family Equidae, and sleeping on their hooves is an adaptation against predation. "Standing gives them a literal jump on predators and a better chance of getting away than if they were [lying] down," Karen Waite, an equine specialist at Michigan State University, told Live Science in an email. 

Simply put, horses are big animals, and it takes time and energy for them to get off the ground. A standing horse is therefore much better able to run away when roused from sleep. According to BBC Science Focus, the same goes for other large prey herbivores, such as zebras, bison, elephants and giraffes, all of which are capable of sleeping on their feet.

Related: Why do horses wear shoes?

Horses have specialized anatomical features that enable them to stay on their hooves while catching some z's. These features, known as a "stay apparatus" include a series of tendons and ligaments — soft tissues that connect muscle to bone, and bone to bone, respectively — that run throughout both the forelegs and hind legs, according to Waite. When a horse relaxes its leg muscles, the stay apparatus ligaments and tendons act as tension bands that stabilize the shoulder, knee and ankle joints in the legs. This enables the horse to remain standing without having to maintain much tension in their muscles.

But while horses do most of their sleeping while standing, they do need to lie down for more restorative sleep. "Horses will spend most of their time sleeping standing up, but they don't actually reach full REM sleep when they're standing," Sarah Matlock, a senior instructor of equine behavior at Colorado State University, told Live Science. During rapid eye movement (REM) sleep in humans — when a sleeping individual's eyes move rapidly under closed eyelids — dreaming can occur, and so muscles become temporarily paralyzed so we don't act out our dreams. During REM sleep,the brain also consolidates and processes new information for long-term memories. People who don't get enough REM sleep may experience problems with mental concentration and mood regulation, a weakened immune system and less cell growth.

Adult horses are often able to get by on as little as 5 hours of sleep per day, and they can get most of it while standing up. And while standing horses can achieve "slow-wave," or deep, dreamless sleep, but an adequately rested horse requires at least 25 minutes of REM sleep per day, which can only happen while lying down, Matlock said.

Because of this, horses must lie down every day. If they don't, they may experience sleep deprivation, which can be a serious health risk for a horse. For instance, putting it at risk of injury from falling, according to the American Association of Equine Practitioners. "Horses that don't get enough REM sleep can be misdiagnosed with narcolepsy," Matlock said. "Like, they might fall over while you're riding them."

Because standing while sleeping is an adaptation to avoid predators, horses need to feel comfortable and safe before they are willing to lie down and sleep, Matlock said. In feral horse populations, if multiple horses lie down to get REM sleep at the same time, there will always be at least one horse that remains standing, likely to watch for potential predators. 

"If they don't feel safe in their environment, or they don't have other horses with them, or if they're isolated, then they're less likely to feel safe enough to lay down to sleep," Matlock said.

Scientists have filmed an octopus exhibiting strange behaviors in a laboratory in New York that could be explained by it having nightmares. Over the course of a month, researchers watched as the octopus appeared to jolt out of a restful sleep and thrash around, in a behavior that seemed almost like the animal was suffering from some kind of sleep disorder.

But was this octopus really having nightmares? There are some other potential explanations for why the animal might have acted this way, and experts expressed caution in interpreting the animal's behavior too quickly – but nonetheless, this behavior is certainly unusual. 

"For all the studies that have been done" on octopuses and other cephalopods, "there’s still so much we don't know," said Eric Angel Ramos, a postdoctoral researcher at the University of Vermont who helped film the octopus.

Video footage from a laboratory at The Rockefeller University in New York captured four episodes in which an Octopus insularis named Costello appeared to sleep calmly in a tank before suddenly flailing its tentacles around in a frenzy. In two of these instances, Costello also shot a jet of black ink into the water, a common predator-defense mechanism. 

Related: Octopuses torture and eat themselves after mating. Science finally knows why.

"It was really bizarre, because it looked like he was in pain; it looked like he might have been suffering, for a moment," Ramos told Live Science. "And then he just got up like nothing had happened, and he resumed his day as normal."

Some of these behaviors are similar to what an octopus might do when encountering a predator in the wild, according to the research team, who described these behaviors in a preprint (which has not been peer-reviewed) posted to the server bioRxiv this month. 

That led the authors to speculate that “the animal may have been responding to a negative episodic memory or exhibiting a form of parasomnia,” meaning a sleep disorder. But they also cautioned that nothing can be definitively concluded from these observations.

Recently, researchers have learned more about octopus sleep. In 2021, scientists published a study documenting evidence of a two-stage sleep pattern in the animals, consisting of “active” and “quiet” sleep — similar to how humans fluctuate between rapid eye movement (REM) and non-REM sleep each night. In humans, most dreaming occurs during REM sleep, so some scientists have wondered if octopuses may also dream during their “active” sleep stage.

However, one expert who wasn’t involved in the observations expressed caution in interpreting the octopus's actions as dreams.  

We don't know enough about the neuroscience of sleep in cephalopods to know if they dream at all, let alone have nightmares, Robyn Crook, a comparative neurobiologist at San Francisco State University, told Live Science. And even if octopuses do dream, they might dream in a completely different way than humans do, she said.

“It's not something that we could easily answer,” Crook said. “It's a very philosophical question.”

So although the behaviors in this video are "very interesting," they could very likely have been spurred by something other than dreams, she said. 

For example, the octopus might have just been startled by something, Crook said. This octopus also might have been exhibiting signs of senescence, she said. This is the stage of an octopus’s life that occurs right before death, when their bodies start to break down. 

In another octopus species, the giant Pacific octopus (Enteroctopus dofleini), Crook and her colleagues recently found an association between senescence and nervous system degradation. To her, the arm movements in the video seemed more like evidence of a lack of motor control,  which she says is associated with senescence, rather than anti-predator behavior.

Indeed, the species Costello belongs to lives for about 12 to 18 months, Ramos said, and Costello died shortly after these incidents. "I don't exclude that senescence could be one of the drivers of this," he told Live Science. 

It's possible that this behavior seemed unusual because many laboratory octopuses are euthanized before they start to senesce, Ramos said. Plus, most labs aren't filming their octopuses 24/7, he added, so other laboratories might have missed chances to spot similar behaviors.

It's well known that the sleep-promoting hormone melatonin plays a key role in sleep. But other hormones, including the reproductive hormones estrogen and progesterone, can also affect our sleep — and potentially even our dreams. 

In particular, people may notice differences in the quality of their sleep and the way they dream at different stages of their menstrual cycle, experts told Live Science. Here's how the menstrual cycle might affect sleep and dreaming.  

Related: Does melatonin work? 

How the menstrual cycle affects sleep and dreams 

Sleep is broken down into stages, broadly known as non-rapid eye movement (NREM) and rapid eye movement (REM) sleep. People dream during REM sleep, which usually begins about 90 minutes after a person falls asleep, said Dr. Tania Adib, a consultant gynecologist at The Lister Hospital in the U.K. 

NREM sleep is broken up into four stages: easily disturbed light sleep, a deeper intermediate sleep, and two stages of deep sleep, which occurs just before REM sleep, according to the Mayo Clinic. Deep sleep is the most restorative and where the body and brain have time to repair.

Estrogen and progesterone can affect how much time people spend in REM and NREM sleep and whether they wake up often while dreaming.

"There are lots of estrogen and progesterone receptors in the parts of the brain that regulate sleep, and because the levels of these hormones change during the menstrual cycle, it means our sleep also varies across the month," said Dr. Zoe Schaedel, co-founder of Myla Health, a private clinic for women experiencing menopause, perimenopause and reproductive health issues in England. 

The menstrual cycle has four stages: menses (when the period occurs), the follicular phase, ovulation, and the luteal phase, according to the Cleveland Clinic. The menstrual cycle "begins" on the first day of a period. Prior to ovulation, which occurs roughly around day 14, estrogen levels rise, Schaedel told Live Science. "Sleep tends to be better-quality in this phase, known as the follicular or proliferative phase."  

After ovulation, progesterone levels then begin to rise and eventually overtake estrogen; if a pregnancy doesn't occur, the levels of both hormones drop in the lead-up to menstration. "This is known as the 'luteal phase' and is where we see changes in the structure of sleep," Schaedel said.

Shifting progesterone levels during the luteal phase may partly cause disturbed sleep in the lead-up to a person's period, Schaedel said. 

Compared to estrogen, progesterone is thought to be more strongly tied to premenstrual symptoms (PMS), which occur during the luteal phase and commonly include insomnia or oversleeping, according to Mount Sinai. These sleep changes aren't fully understood, but rising progesterone in the luteal phase is linked to increased body temperatures and sweating, which may interfere with sleep because normally, body temperatures decline before and during sleep. The overall ratio of REM to NREM sleep may also decrease during this phase, according to a 2021 review in the journal Frontiers in Neuroscience. 

Progesterone and estrogen help modulate chemical messengers in the brain — namely, serotonin and gamma-aminobutyric acid (GABA) — which regulate mood and stress levels, according to Mount Sinai. Anxiousness and depressed mood due to PMS may undermine people's attempts to fall asleep, and physical symptoms, like body aches and nausea, may also help explain why people experiencing PMS are prone to sleep disruptions, per the Frontiers in Neuroscience review. 

Melatonin production and sensitivity may also change during the luteal phase, some studies hint, but the research is "not sufficient" to draw strong conclusions.

Sleep disturbances in the luteal phase may increase the likelihood that individuals remember their vivid dreams, Schaedel said. 

"If sleep fragments REM, or you wake up shortly after the REM stage, you are far more likely to remember your dreams," Adib told Live Science.

One 2019 study hinted that hormones may also affect the types of dreams a person has. The study asked hundreds of women to recall the previous night's dreams. Of the women who could remember, those in the luteal phase more often reported their dreams as being "pleasant," while those in other parts of the menstrual cycle were more likely to report their dreams being "unpleasant." 

However, it's unclear whether this result would carry over to other, larger groups of people.

People who have irregular sleep schedules may have an increased risk of dying from cancer and heart disease, compared with people who have regular sleep schedules, according to a seven-year study of middle-age and older adults.

Scientists have previously found that a lack of sleep may increase the risk of dying from any cause, but it was unclear whether irregular sleep schedules similarly affected mortality rates. Now, new research, which was posted April 17 on the preprint database medRxiv and has yet to be peer-reviewed, suggests they do.

"To my knowledge, our study is the largest to look at how sleep irregularity affects all-cause mortality," or death from any cause, Matthew Pase, an associate professor of psychology at Monash University in Australia and a co-author of the study, told Live Science. "We show that sleep regularity may impact mortality independently of sleep duration."

Pase and his colleagues recruited nearly 90,000 people living in the U.K. to wear movement-tracking devices on their wrists for around a week; the participants ranged from about 40 to 70 years old. By using a statistical model to estimate when people were asleep or awake from the recorded movement data, the researchers calculated a sleep regularity score for each participant, defined as the probability that a person was awake or asleep at the same time between any two consecutive days. 

Related: What is sleep hygiene? 

One group of participants was scored as having "low sleep regularity," as they had a 41% chance of being asleep or awake at the same time for two days in a row; this put the group in the fifth percentile of sleep scores, overall. Those with "average" sleep regularity, who were in the 50th percentile, had a 61% chance of being asleep at the same time on consecutive days.

Within about seven years of the team collecting this data, 3,010 of the participants died. Those with low sleep regularity scores were 46% more likely to die of any cause within the follow-up period, compared with people with average scores. 

Of the people who died, 1,701 died of cancer and 616 died of cardiovascular disease. People with low sleep regularity scores were 33% more likely to die of cancer, and 73% more likely to die of cardiovascular disease, than people with average scores. 

(A third group of people with "high" sleep regularity scores had a slightly lower-than-average chance of death, overall, and from cancer and heart disease, specifically, but their risk wasn't significantly different than the "average" group.)

The analysis took into account differences in participants' age, ethnicity, sex, smoking status, diabetes, existing heart conditions and cancer, education level, income, and how long they slept each night, on average.

"Shifts in sleep and wake times can disrupt bodily processes, such as repair of tissues and metabolism, so this may then increase chronic disease risk," Pase said. However, it's also possible that bodily changes that contribute to the development of cancer and cardiovascular disease may have led to the observed differences in sleep regularity, he said. 

"Of course, we can't yet confirm whether there's a causal association," he said, meaning based on their data, the team can't determine whether irregular sleep schedules directly drive up rates of deadly disease, or vice versa.

Another limitation of the study is that the researchers relied on only movement data to estimate sleep and wake times, so the team couldn't perfectly differentiate between when people were truly asleep and when they were lying relatively still. "If we could increase the accuracy of our measurements further we could be seeing an even more powerful effect," Pase said.

How can people with inconsistent sleep schedules get back on track? Dr. Johan Meurling, who treats people with sleep disorders, recommends several tips for sleeping regularly, such as setting an alarm clock to wake at the same time on weekdays and weekends; avoiding napping during the day; and not using devices to browse social media, in bed. Exercising during the day but not within the few hours before bedtime and avoiding caffeine after about 3 p.m. can also help, he said.

"The one consistent advice I have for my patients with irregular sleep-wake schedules from weekdays to weekends is to examine where their inherent biological clocks lie," Kin Yuen, a doctor in sleep medicine, wrote in an email to Live Science. Trying to align your schedule with your internal rhythm of being a morning or evening person will help you sleep regularly, she wrote. 

For an astonishing 11 days and 25 minutes, 17-year-old Randy Gardner didn't sleep for a high school science fair project in California in 1963, setting the world record for the longest someone has stayed awake. Other people have reportedly broken this record — Robert McDonald went 18 days and nearly 22 hours without sleeping in 1986 — but none were monitored as closely or by a doctor as Gardner was.

Guinness World Records doesn't cover this feat anymore; in 1997 they stopped accepting new submissions due to "inherent dangers associated with sleep deprivation."

But what are these dangers? What happens to people who experience prolonged sleep deprivation?

Sleep is necessary for executive, emotional and bodily functions, and insufficient sleep may raise the risk of several health conditions, including diabetes, heart disease, obesity and depression, according to the Centers of Disease Control and Prevention. Experts say humans require a consistent six to eight hours of sleep at the same interval every 24 hours. But it's not uncommon for people, especially students, to pull an all-nighter and stay awake for 24 hours.

At this stage of sleep deprivation, it can be challenging to distinguish between sleep and wakefulness, said Dr. Oren Cohen, a sleep medicine fellow at Mount Sinai Hospital in New York City.  As someone starts to push 24 hours without sleep, their brain activity already shows signals that they are on the sleep-wake border, even though they appear to be awake, Cohen said. 

Related: Does drinking chamomile tea really help people fall asleep?

This is called sleep intrusion or micro-sleep. People who forgo hours of sleep appear awake, but their brain involuntarily goes into a sort of abnormal sleep, which could include intervals of inattention or hallucination. "But sleep takes over, inevitably the brain goes into sleep. When a person tells me 'I haven't slept in weeks,' that's nearly impossible," said Dr. Alon Avidan who leads the Sleep Disorders Center at the University of California, Los Angeles. 

"I would be hard-pressed to believe that someone can stay awake for more than 24 hours without these episodes," Cohen added.

But exactly how long people can go without sleep and the timeline of side effects that unfold can be hard to pinpoint. Chronic sleep deprivation, when an individual doesn't sleep for an extended period, is so devastating that it's unethical to research it in humans, Avidon said. It's even been used as a form of psychological torture. 

Though prolonged sleep deprivation can’t be studied, we do have data on people with the rare inherited disease called fatal familial insomnia (FFI). These patients have a genetic mutation that causes an abnormal protein to accumulate in the brain and progressively worsens sleep. Their bodies begin to deteriorate and they eventually die because the abnormal protein builds up and damages their brain cells. The disorder kills most patients within an average of 18 months. 

In animals, a study from 1989 in rats showed the animals could only go without sleep for between 11 and 32 days before it killed them.

A 2019 human study published in Nature and Science of Sleep found participant alertness and vigilance were relatively normal up to 16 hours into sleep deprivation. But after 16 hours, their lapses in attention rose significantly and were even worse for the participants with chronic insomnia. A study from 2000 found that staying awake for 24 hours reduced hand-eye coordination on par with a blood alcohol content of 0.1%. The effects of sleep deprivation at 24 hours included reduced reaction time, slurred speech, impaired decision-making, diminished memory and attention, irritability, impaired vision, hearing and hand-eye coordination, and tremors, according to the Cleveland Clinic.

At 36 hours, sleepless people may have increased inflammatory markers in their blood and even develop hormone imbalances and slowed metabolism, Cleveland Clinic reported. There's little research on what happens at 72 hours, but people can become anxious, depressed, hallucinate and have trouble with executive function.

Research in U.S. physicians showed that impaired sleep increased burnout and self-reported medical error. Surgical residents in Israel showed "greater impulsivity, slower cognitive processing, and impaired executive function" than before their 26-hour shift, according to a 2021 study in the journal Medical Education. Shift workers are also at high risk for the consequences of impaired sleep because they tend to get insufficient sleep, aren't always able to sleep at the same time, and must often sleep while it's light out, which clashes with humans' natural sleep-wake cycle.

And it's critical to know that you can't just make up for sleep deprivation tomorrow or over the weekend. Sleep deprivation is cumulative, so those who go without sleep incur a sort of sleep debt. And for every hour of sleep lost, it requires a full eight hours of sleep to recover, Avidon said.

Sleep restriction is risky for another reason, too: even though the impact on attention can be severe, you might not notice it yourself. Just like someone who is under the influence of alcohol thinks that they are fine to drive, someone getting limited sleep might feel fine or even get a second wind. They don't know they are having these lapses in attention, Cohen said. 

Going just one night without sleep may make the brain look older, as if it had suddenly aged one to two years overnight, a new study suggests. 

However, these changes seem to disappear after a good night's rest.

In the study, researchers used machine learning to generate "brain age" estimates from magnetic resonance imaging (MRI) scans of sleep-deprived people's brains, which they compared to MRIs of those same people’s brains after a full night's sleep. The results, published Feb. 20 in the Journal of Neuroscience, suggest that one night of complete sleep deprivation produces changes in the brain similar to those seen after one or two years of aging.

Brain age is "a very interesting measure in terms of looking at how that changes from the sleep loss," said Judith Carroll, an associate professor of psychiatry and biobehavioral sciences at the University of California, Los Angeles who was not involved in the study.

Related: What happens in your brain while you sleep?

The researchers pulled from five existing data sets, which included data from 134 participants in four groups: total sleep deprivation (no sleep for one night), partial sleep deprivation (three hours time in bed for one night), chronic sleep deprivation (five hours in bed each night for five nights) and a control group (eight hours in bed each night). Each group had at least one night of baseline sleep, where they spent eight hours in bed, before sleep deprivation; most groups also had a full night of recovery sleep afterwards. 

Everyone had an MRI taken after each night, allowing researchers to compare how their brains looked before and after sleep deprivation, and after a full night's rest.

The researchers determined the apparent ages of the participants' brains using a machine-learning algorithm called brainageR, which was trained on data from more than 3,000 people. The publicly available algorithm predicts a person's chronological age from their brain MRIs based on how healthy brains typically look at given ages, in terms of their tissue and fluid volume. In past tests, researchers found that brainageR could accurately predict age within about four years.

In their new study, the researchers found that, for the group that got no sleep for one night, brainageR estimated that they were one to two years older, on average, than they were predicted to be at baseline. These differences vanished after a night of recovery sleep.

The partial and chronic sleep deprivation groups didn't have significant differences in their age predictions, compared to control.  

These results jibe with earlier research on the effect of sleep deprivation on the brain. There's evidence that several types of changes take place in the brains of sleep-deprived people, including changes in fluid distribution and gray matter volume.

This "widespread change in brain morphology … would be captured with this method of brain age as well," study senior author Dr. David Elmenhorst, a professor in the Institute of Neuroscience and Medicine at the research institution Forschungszentrum Jülich in Germany, told Live Science. Crucially, he framed the results not as actual aging but rather as changes that the machine-learning algorithm interpreted as aging.

Because the study found this effect only in the total sleep deprivation group, it's hard to say what the results might mean for the effects of sleep deprivation in real life, Carroll said. "I'm not sure that we can say anything about long-term effects of chronic sleep loss, because even the chronic condition's only five days," she said.

The study was also relatively small. Elmenhorst said a larger sample size might highlight smaller effects in the other groups, like a brain age increase of a few months. Future research could also incorporate people who experience chronic sleep deprivation, such as people who do shift work, Carroll said.

"A lot of individuals really struggle to sleep [during the day] when they're awake all night," she said. "Something that looks more closely at this in those groups, I think could be really valuable and more informative."