What goes bump in the night? In many U.S. households: people. That's according to new Stanford University School of Medicine research, which found that about 3.6 percent of U.S. adults -- or upward of 8.4 million -- are prone to sleepwalking. The work also showed an association between nocturnal wanderings and certain psychiatric disorders, such as depression and anxiety.

The study, the researchers noted, "underscores the fact that sleepwalking is much more prevalent in adults than previously appreciated."

Maurice Ohayon, MD, DSc, PhD, professor of psychiatry and behavioral sciences, is the lead author of the paper, which appeared in the May 15 issue of Neurology, the medical journal of the American Academy of Neurology.

Sleepwalking is a disorder "of arousal from non-REM sleep." While wandering around at night can be harmless and is often played for laughs -- anyone remember the Simpsons episode where Homer began wandering around and doing silly things in his sleep? -- sleepwalking can have serious consequences. Episodes can result in injuries to the wanderer or others and lead to impaired psychosocial functioning.

It is thought that medication use and certain psychological and psychiatric conditions can trigger sleepwalking, but the exact causes are unknown. Also unclear to experts in the field is the prevalence.

"Apart from a study we did 10 years ago in the European general population, where we reported a prevalence of 2 percent of sleepwalking," the researchers wrote in their paper, "there are nearly no data regarding the prevalence of nocturnal wanderings in the adult general population. In the United States, the only prevalence rate was published 30 years ago."

For this study, the first to use a large, representative sample of the U.S. general population to demonstrate the number of sleepwalkers, the researchers also aimed to evaluate the importance of medication use and mental disorders associated with sleepwalking. Ohayon and his colleagues secured a sample of 19,136 individuals from 15 states and then used phone surveys to gather information on participants' mental health, medical history and medication use.

Participants were asked specific questions related to sleepwalking, including frequency of episodes during sleep, duration of the sleep disorder and any inappropriate or potentially dangerous behaviors during sleep. Those who didn't report any episodes in the last year were asked if they had sleepwalked during their childhood. Participants were also queried about whether there was a family history of sleepwalking and whether they had other parasomnia symptoms, such as sleep terrors and violent behaviors during sleep.

The researchers determined that as many as 3.6 percent of the sample reported at least one episode of sleepwalking in the previous year, with 1 percent saying they had two or more episodes in a month. Because of the number of respondents who reported having episodes during childhood or adolescence, lifetime prevalence of sleepwalking was found to be 29.2 percent.

The study also showed that people with depression were 3.5 times more likely to sleepwalk than those without, and people with alcohol abuse/dependence or obsessive-compulsive disorder were also significantly more likely to have sleepwalking episodes. In addition, individuals taking SSRI antidepressants were three times more likely to sleepwalk twice a month or more than those who didn't.

"There is no doubt an association between nocturnal wanderings and certain conditions, but we don't know the direction of the causality," said Ohayon. "Are the medical conditions provoking sleepwalking, or is it vice versa? Or perhaps it's the treatment that is responsible."

Although more research is needed, the work could help raise awareness of this association among primary care physicians. "We're not expecting them to diagnose sleepwalking, but they might detect symptoms that could be indices of sleepwalking," said Ohayon.

Among the researchers' other findings:

    The duration of sleepwalking was mostly chronic, with just over 80 percent of those who have sleepwalked reporting they've done so for more than five years.
    Sleepwalking was not associated with gender and seemed to decrease with age.
    Nearly one-third of individuals with nocturnal wandering had a family history of the disorder.
    People using over-the-counter sleeping pills had a higher likelihood of reporting sleepwalking episodes at least two times per month. (Indeed, a sleeping pill was the trigger for Homer Simpson's middle-of-the-night shenanigans.)