Nearly everyone has a story about people talking in their sleep. Though it tends to be more common in children, it can happen at any age: A 2010 study in the journal Sleep Medicine suggested that around two-thirds of people have at least one episode of sleep talking in adulthood.
Sleep talking is not considered a sleep disorder but a normal variation of human sleep behavior. The International Classification of Sleep Disorders lists sleep talking under "isolated symptoms, apparently normal variants and unresolved issues," along with things like snoring and sleep starts — the sudden jerking motion some people have when falling asleep, also known as hypnagogic jerks.
However, although sleep talking isn't a disorder, it can have unwanted impacts on a person's sleep and on the sleep of someone sharing a room or bed with them. Here, we look at the science behind sleep talking.
What is sleep talking?
Sleep talking, or somniloquy, is when a person makes vocalizations while asleep. These vocalizations can be full words and phrases, or they can be mumblings, shouts or even laughter.
Children commonly talk in their sleep, with half of all kids talking in their sleep once a year or more, and around a quarter sleep talking at least once a week, according to a 1980 paper in the journal Brain & Development. Most children grow out of these episodes of nighttime babble, though sleep talking can recur later in life, brought on by stress or sleep deprivation, Dr. Jennifer Martin, a professor of medicine and the president of the American Academy of Sleep Medicine, told Live Science.
Around half of sleep talking is incomprehensible, audio recordings from a 2017 study published in the journal Sleep suggested. The same study found that, out of 3,349 understandable recordings, the word that most sleep talkers said was "no."
As to whether people tell the truth while sleep talking, that's mostly a myth, Martin said. "It doesn't seem to be the case that [people say their] deep dark inner secrets," she said.
Dr. Jennifer L. Martin serves as president of the board of directors for the American Academy of Sleep Medicine (AASM) and is board-certified in behavioral sleep medicine by the American Board of Sleep Medicine (ABSM). She is a professor of medicine at the David Geffen School of Medicine at UCLA. Martin received her PhD in clinical psychology from the University of California, San Diego, as part of the SDSU/UCSD Joint Doctoral Program.
Some people sleep talk in their dreams, saying phrases that align with what they later remember dreaming about, a 2009 study in the Sleep journal found. But most sleep talking is unrelated to dreaming, as it happens while people are in a stage of sleep with fewer dreams, Martin said.
"Sleep talking tends to occur in a stage of sleep that we call non-rapid eye movement, or non-REM sleep," she said. "During this stage our brain is relatively quiet, compared to what we see during rapid eye movement sleep [where we dream]."
During REM sleep, the body is effectively paralyzed to prevent the acting out of dreams, said Martin, and this paralysis should stop people from talking. If sleep talking does occur during REM sleep, then it could be a sign of something more serious.
"There is a sleep condition called REM behavior disorder where the system that paralyzes your muscles — really so you can't hurt yourself during your sleep — is not working properly," Martin said.
If this is the case, an early diagnosis is important, said Dr. Erik K. St. Louis, the head of the Division of Sleep Neurology at the Mayo Clinic in Minnesota. "REM behavior disorder often evolves into violent sleep behaviors like screaming, shouting, punching and arm flailing, which may lead to injury to the patient or their bed partner," he told Live Science. "It may also be the initial presentation of disease in older adults, usually Parkinson's disease or dementia with Lewy bodies."
What causes sleep talking?
Researchers still don't know what causes sleep talking, but studies that measure brain activity can offer some insight.
Recent analyses show similarities between sleep talking and normal awake speech, St Louis said. Linguistic studies, like the 2017 paper in the journal Sleep, have also shown that the properties of sleep talk — the language, patterns, syntax and semantics — follow the same rules as people's day-to-day conversations and are therefore comprehensible.
These discoveries further neurologists' understanding of the sleeping brain and the purpose of sleep itself, which remains understudied, St. Louis said.
Sleep talking could be linked to memory consolidation, when the sleeping brain revisits experiences to commit the important ones to long-term memory. A 2018 review published in the journal Sleep Medicine Reviews suggested sleep talking could sometimes be a verbal replay of the memories the brain is sifting through at the time.
The cause of sleep talking could be different in children and adults, Martin told Live Science. Sleep talking and other unusual sleep behaviors are much more common in kids, and this could simply be the child’s brain "learning what not to do while it's asleep," Martin said. It could also be related to the phases of brain development during childhood, she said.
In adults, however, certain conditions and circumstances make sleep talking more likely. For one, sleep talking can have a genetic component: it runs in families, according to a 2001 study published in the journal Psychiatric Genetics. It has also been linked with obstructive sleep apnea — a condition in which people experience pauses in breathing, or shallow breathing during sleep — according to the Cleveland Clinic.
Can you stop sleep talking?
Sleep talking is usually considered a harmless trait, but it can be unpleasant for anyone in earshot — nearly 10% of sleep talking in the 2017 Sleep study contained profanities and swearing.
"Sleep talking has also been associated with sleep disturbance and shallower sleep, so may not be quite as benign as we presume," St Louis said.
To stop a person talking in their sleep, Martin advises giving them a little nudge. This gentle interruption can stop the behavior, she said.
Sleep talking, along with other sleep behaviors like sleep walking and snoring, tend to get worse when people are sleep deprived, Martin told Live Science. A 2013 study in the Journal of Sleep Research showed sleep deprivation increased disturbance in slow-wave, non-REM sleep, which can lead to sleep talking and sleepwalking.
"So, making sure that you're getting good healthy sleep tends to decrease how often it happens," Martin said. This means that staying up late to let a partner fall asleep first might actually make sleep talking worse.
This article is for informational purposes only, and is not meant to offer medical advice.
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Amy Arthur is a U.K.-based journalist with a particular interest in health, medicine and wellbeing. Since graduating with a bachelor of arts degree in 2018, she's enjoyed reporting on all kinds of science and new technology; from space disasters to bumblebees, archaeological discoveries to cutting-edge cancer research. In 2020 she won a British Society of Magazine Editors' Talent Award for her role as editorial assistant with BBC Science Focus magazine. She is now a freelance journalist, with bylines in BBC Sky at Night, BBC Wildlife and Popular Science, and is also working on her first non-fiction book.