About 8.5 million Americans walk in their sleep, according to a new study, the largest ever to document the prevalence of these nighttime walkers.
Researchers from Stanford University interviewed nearly 16,000 adults in 15 states about their nocturnal habits. They found that 3.6 percent of them reported sleepwalking more than once during the previous year. About 1 percent said they had two or more sleepwalking episodes in a month.
Previous studies found that sleepwalking was pretty common, especially in children. But Dr. Maurice Ohayon, the study's lead author and director of Stanford's Sleep Epidemiology Research Center, said he was surprised to learn just how many sleepwalkers there were.
"There are very few sleep disorders with so high a prevalence," he said.
The study, published today in the journal Neurology, is the first in 30 years to look at how many Americans sleepwalk, and the only study to do so on so wide a scale.
Scientists still don't know exactly what makes people walk in their sleep. But it is clear that the behavior can be risky if they get into dangerous situations without being conscious of what they're doing.
"Sleepwalking is best thought of as the brain getting halfway caught between wakefulness and sleep. Your brain is literally part awake, and you may do some behaviors you wouldn't normally do," said Dr. David Schulman, medical director of Emory University's Sleep Laboratory, who was not involved in the study.
Usually, people are quick to blame certain medications, such as Ambien, or other sleeping aids for bouts of sleepwalking. But the study's findings suggest a more complicated picture of these nightly wanderings.
Ohayon and his colleagues found a number of factors showed up more often in people who reported sleepwalking. People who got less than seven hours of sleep each night were more likely to report sleepwalking, and those with sleep apnea (meaning they stop breathing in their sleep) were 3.9 times more likely to do so. People with major depression were 3.5 times more likely to sleepwalk, as were people who abused alcohol. Other psychiatric conditions such as obsessive-compulsive disorder and social phobias also showed up more often in sleepwalkers.
"These are psychiatric disorders that we know are associated with disrupted sleep. So it seems particularly logical that we have seen these risk factors with sleepwalking," Ohayon said.
The researchers also found that sleepwalking may be a family affair. About one-third of the study's participants reported having a family member who was also a sleepwalker.
Medications were linked to sleepwalking, particularly over-the-counter sleeping aids and hypnotics, although the data linking sleepwalking to such hypnotic drugs as Ambien was not as strong as some reports have suggested. Researchers also noticed a link between sleepwalking and selective serotonin uptake inhibitors, or SSRIs, which are antidepressants.
Ohayon said it's not that these factors cause someone to begin sleepwalking. Instead, they may trigger the disorder in people who are predisposed by genetics, physical or psychiatric conditions.
Experts say the numbers in the study are likely an underestimate of sleepwalking, especially since researchers did not observe people during sleep, relying only on a person's memory of a sleepwalking episode.