Sleepwalking is surprisingly common among Americans, with about 3.6% of U.S. adults — 8.5 million — rising in the wee hours of the night to wander around.

A disorder caused by arousal from non-REM sleep, sleepwalking is very common during childhood and tends to decrease with age. ”It’s dangerous because it is quite a lot of people,” says lead researcher Dr. Maurice Ohayon, a professor of psychiatry and behavioral sciences at Stanford University School of Medicine. “Sleep walking is less dangerous if you are home and there is someone to take care of you. But if you’re alone or not in your home, you could hurt yourself. You could fall or do something inappropriate.”