When we sleep, our bodies become relaxed and our brains "turn off" our muscles… but not everyone experiences sleep as a temporary, necessary immobility. A new study from Montreal investigates sleepwalking and finds a probable genetic component to the disorder. In fact, a child’s odds of sleepwalking rise based on their parents’ sleep history, the researchers say: Kids with one sleepwalking parent were three times as likely, and those with two sleepwalking parents were seven times as likely to walk in their sleep compared to kids whose parents slept normally.

Sleepwalking is known to be higher among children than adults, though some people suffer from this problem throughout their life. While this sleep disorder may serve as a comic plot device in books and movies, it is not a laughing matter when it happens in real life. In fact, one example of how un-funny sleepwalking can be comes from, surprisingly enough, a comedian. Mike Birbiglia told NPR about the night he had a dream involving a guided missile aimed at his hotel room. Naturally enough, his dream-self quickly jumped out the window, yet this longtime sleepwalker also made the leap in real life. Luckily, Birbiglia survived, though he earned 33 stitches following his second-story fall.

Slow-Wave Sleep

The International Classification of Sleep Disorders describes sleepwalkers as “disoriented in time and space, with slow speech, with severely diminished mentation, and blunted response to questions or requests." Sleepwalkers may also suffer from "anterograde and retrograde memory impairment” — simply put, amnesia.

"The question of amnesia associated with sleepwalking episodes is fascinating although a large percentage of sleep walkers often remember part of the mental content associated with their nocturnal episodes," Dr. Jacques Montplaisir, lead author of the new study, told Medical Daily in an email. He explained the most likely cause is brain level dissociation, whereby part of the brain is awake while some regions remain in the sleep state.

Sleepwalking is a parasomnia that usually occurs as a result of partial arousal from slow-wave sleep, a phase of deep sleep which precedes the REM stage (dreaming) of sleep. During slow-wave sleep, our bodies get their necessary rest, while our minds consolidate memories. Past research has shown another parasomnia that shares many of the same characteristics as sleepwalking. Sleep terrors, which are essentially terrifying nightmares characterized by screams, intense fear, and prolonged inconsolability, also result from slow-wave disturbances.

With his colleagues at the Hopital du Sacre-Coeur de Montreal, then, Montplaisir investigated the prevalence of both sleepwalking and sleep terrors during childhood, seeking to understand whether a relationship existed between early sleep terrors and later sleepwalking. The team of researchers also explored possible links between a child's experience and parents’ histories of sleepwalking. After analyzing data from the Quebec Longitudinal Study of Child Development, Montplaisir and his co-researchers discovered some interesting relationships between all the data.

Overall prevalence during childhood was 56.2 percent for sleep terrors, while overall prevalence for childhood sleepwalking was 29.1 percent. The two sleep disorders, though, seemed to trend in opposite directions. The peak prevalence for sleep terrors occurred at age 1 and a half (about one-third of all cases) and for sleepwalking at age 10 (about 13 percent). So while more children suffer sleep terrors in their earliest years, the numbers drop off as they grow older; meanwhile sleepwalking, which is relatively infrequent during the preschool years, increases steadily until age 10.

Importantly, late childhood sleepwalkers were more likely to have had sleep terrors in early childhood than not (34.4 percent versus 21.7 percent).

Another key finding from the study linked a child’s odds of sleepwalking to their parents. The prevalence of sleepwalking was 22.5 percent for children whose parents did not sleepwalk. By comparison, 47.4 percent of children with one sleepwalking parent and 61.5 percent of children with two sleepwalking parents followed in their parents footsteps.

“These findings point to a strong genetic influence on sleepwalking and, to a lesser degree, sleep terrors,” wrote the authors in their conclusion. Overall, nearly 80 percent of sleepwalkers have at least one family member affected by sleepwalking, sleep terrors, or both.

Source: Petit D, Pennestri MH, Paquet J, et al. Childhood Sleepwalking and Sleep Terrors: A Longitudinal Study of Prevalence and Familial Aggregation. JAMA. 2015.