You brush your teeth, slip under the covers and turn off the lights, but if you think that's always the end of the story, you're wrong.
Sleepwalking is far more common than previously thought, affecting nearly one-third of adults at some point in their lives, according to the first major study to assess the disorder's prevalence in the U.S. in more than three decades.
"This is a big number," said Dr. Maurice Ohayon, director of the Sleep Epidemiology Research Center at Stanford University and an author on the study. "The question that follows is 'do we find some associations that are meaningful for sleepwalking?' And the answer is yes."
Sleepwalking is a type of parasomnia -- a category of common sleep disorders including things like sleep-related eating, night terrors and sleep paralysis -- that affects about 10 percent of Americans, according to the National Sleep Foundation. Sleepwalking typically occurs during non-REM sleep and can last from a few seconds to 30 minutes or more.
For the study, researchers relied on the broader term "nocturnal wandering," as they were unable to observe patients in a lab and give them a diagnosis. The possibility therefore remained that among the more than 15,000 adults who researchers interviewed about their sleep habits, some experienced wandering that occurred when they were actually awake, just very confused.
According to the findings, nearly 30 percent of the participants reported at least one experience with nocturnal wandering at some point in their lives, and more than 3.5 percent said they'd experienced it in the past year. A small group indicated they dealt with it regularly; 1 percent reported having two or more episodes per month. The results were published in the journal Neurology on Monday.
In addition to tracking prevalence, the researchers also looked at other conditions linked with sleepwalking in order to unpack possible causes or triggers of the disorder.
They found certain mental disorders were clearly linked. People with depression were three-and-a-half-times more likely to sleepwalk than those without, and people with obsessive compulsive disorder were four times more likely. Alcohol dependence also was tied to nocturnal wandering.
Other factors, such as a lack of sleep, appeared to make a difference. People who got less than seven hours per night did have a higher risk of having at least one nocturnal wandering episode in the past year. One-third of individuals with a family history of sleepwalking experienced it themselves, although Ohayon said it was too soon to tell what the genetic connection might be.
When it comes to understanding underlying mechanisms and causality, the researcher stressed that there's a long way to go. "This is a cross-sectional study," he said. "So what I can see is the association. I cannot tell you the direction of the causality. It will be necessary to do a longitudinal study to do that."
But while such questions remain, treatment options exist. "A lot of people who experience parasomnias, experience them when they're under a lot of stress, so stress management can help," said Dr. Philip Gehrman, clinical director of the behavioral sleep medicine program the University of Pennsylvania, who was not associated with the new study.
He explained that medication can be a next step for some patients, although for many, simply knowing that sleepwalking is fairly common and relatively normal can be enough to put them and their bedmates at ease.
More serious consequences, while very rare, can occur. Last year, a New Jersey woman's drowning reportedly may have occurred when she fell into a lake after she walked out of her home while unconscious.
"If this is happening in a hotel, or not in your usual location, it could [result in] a big accident," Ohayon said. "These extreme cases must make us very cautious about sleepwalking. Happily, it's an exception that something like this happens."