No one knows exactly why we sleep, but researchers continue to uncover important discoveries about the mysterious phenomenon, including some that seem counterintuitive. Here are five surprising findings about human sleep:
Interrupted sleep is worse than staying awake
Not getting enough sleep may make you grouchy, but you could feel even worse if your sleep is frequently interrupted, compared to if you simply go to bed late, a recent study suggests. Researchers found that people who experience frequent interruptions during their sleep were less happy and less energetic the next day than people who went to bed late but were able to sleep continuously for a few hours.
This may be because people whose sleep is frequently interpreted spend less time in deep sleep — a restorative phase of sleep — than people who go to bed late, the researchers said.
Modern life isn't actually stealing our sleep
The technologies of the past century, from light at night to electronic gadgets, are often blamed for keeping people awake long past the time they should have hit the sack. But a 2015 study suggests that modern humans don't get less sleep than their prehistoric ancestors — in fact, we may get more.
In the study, researchers looked at the sleep habits of people living in contemporary hunter-gatherer societies in parts of Africa and South America, as a proxy for our prehistoric ancestors. They found that people in these societies sleep less than 6.5 hours a night — less than the average American, who gets between 7 and 8 hours of sleep, according to the National Sleep Foundation.
"We find that contrary to much conventional wisdom, it is very likely that we do not sleep less than our distant ancestors," study author Jerome Siegel, a researchers at the University of California, Los Angeles, told Live Science in a 2015 interview.
We sometimes sleep with half our brain more alert
It's known that marine animals such as dolphins sleep by shutting down just one half of their brain at a time. But a recent study found evidence of a similar, albeit less drastic, phenomenon in people. People participating in a study on sleep, on the first night that they slept in the lab, showed more activity in the left hemisphere of their brains during deep sleep than in the right hemisphere.
Having one hemisphere remain more "vigilant" during sleep than the other may be a survival strategy when humans are in a new environment — the left hemisphere may serve as a "night watch" that wakes the sleeper up if there's danger, the researchers said.
Snowy days keep us in bed a little longer
Humans don't hibernate, but on a snowy day, it might just be a little harder to get out of bed. An analysis of data from a popular sleep tracking app found that people in snowier states sleep for a little longer during the winter months, compared to people in states with generally warmer weather.
For example, during the month of January 2015, users of the app who lived in snowy Northwestern states, including Montana, Wyoming, North Dakota and South Dakota, spent the longest time in bed — 7 hours and 20 minutes a night, on average. That's about 13 minutes longer than the average of people in the study in the Southeastern states (Tennessee, Mississippi, Alabama, Georgia, South Carolina and Florida), who spent on average 7 hours and 7 minutes in bed.
Sleep habits may change in the wintertime because of the reduction in daylight hours, which affects people's internal circadian clocks, experts say.
More people sleep too long
We often hear about Americans not getting enough sleep, but the percentage of U.S. adults who sleep for more than nine hours a night is also on the rise, according to a recent study.
The 2013 study found that, over two decades, the percentage of survey participants who reported sleeping for more than nine hours over a 24-hour period increased from 28 percent in 1985 to 37 percent in 2007.
"One does hear again and again…that people are sleeping less than they used to. [But] there's never been any good evidence for that," Diane S. Lauderdale, a professor of epidemiology at the University of Chicago's Department of Health Studies, told Live Science in a 2013 interview.
However, it's important to note that the way people define "sleep" can be ambiguous, and it's possible that in the study, participants recorded how long they spent in bed, rather than how long they actually spent sleeping, the researchers said.
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Rachael is a Live Science contributor, and was a former channel editor and senior writer for Live Science between 2010 and 2022. She has a master's degree in journalism from New York University's Science, Health and Environmental Reporting Program. She also holds a B.S. in molecular biology and an M.S. in biology from the University of California, San Diego. Her work has appeared in Scienceline, The Washington Post and Scientific American.