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?

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.