Here is what can, and does, happen: The child gets out of bed and climbs out a window. Or gets out of bed, walks down a hallway, perhaps goes down a flight of stairs, navigates through a room or two, opens a door, walks out on the patio and, maybe, steps into the backyard swimming pool. The child is sleepwalking. According to some estimates, up to 17 percent of children have a sleepwalking experience between ages 4 and 12, peaking between ages 8 and 12. It is less prevalent among adults, affecting perhaps 4 percent of that population, although it can be more hazardous in that group. Sleepwalking has been part of the human experience probably since there has been a human experience -- it was mentioned in literature before Hippocrates -- but not much is known about it even now. How it happens, yes; why, not so much. It usually occurs during the first third of the night's sleep, and the characteristics are similar: Sleepwalkers often have open eyes, although they have a confused or glassy look. They might talk, although not clearly. Waking them will result in serious confusion, so experts say it is best not to. 'Monitor it, but let it run its course, unless they are headed out the front door,' said Jodi Mindell, a clinical psychologist and professor of psychiatry at St. Joseph's University in Philadelphia. It can help to guide them back to bed, where they will probably fall back asleep and have no memory of what transpired after they wake up. What you have with a sleepwalker is a 'sleeping brain and an awake body,' said Donna Arand, clinical director of the sleep disorder center at Kettering Medical Center, near Dayton, Ohio. 'Sleepwalkers tend to do things typical of daytime activities. They have some, but generally a very diminished, awareness of their surroundings. They can negotiate routine patterns.' There is also a genetic impulse: The chances of a child's sleepwalking increase tenfold if a parent or another sibling were sleepwalkers. Odd behaviors occasionally occur. Some -- boys in particular -- have a tendency to mistake the closet for the bathroom. There have also been instances of sleepwalkers making a sandwich and eating it, and, rarely, of adult sleepwalkers driving. The vast majority of kids will outgrow sleepwalking, which is why doctors don't consider it a serious medical problem. 'When parents come in, what I do is explain and reassure them,' said Judith Owens, a pediatrician at Brown University Medical School who has been studying sleep for 15 years. Safety can be a concern, however: tripping over something, falling down stairs, climbing out a window, wandering out the door in the middle of winter in your pajamas. So what parents of sleepwalkers need to think about, Mindell said, is putting gates at the top of stairs, locking outside doors and windows, hanging bells on bedroom doorknobs, putting away sharp objects and generally picking up. 'A Lego village in the middle of the bedroom floor won't do well,' she said. Baby monitors in rooms can alert parents that a child is on the move, she said, 'but they're not so good if someone is just walking quietly rather than crying.' What experts call sleep architecture plays a significant role in sleepwalking, also called somnambulism and formally known as partial arousal parasomnia. Everyone has a definitive sleep cycle, which lasts between 90 and 120 minutes and repeats itself through the night. The cycle is divided into REM (rapid eye movement) sleep and non-REM sleep. The non-REM part occurs first and is itself split into three stages. Sleepwalking nearly always occurs in the third stage of non-REM sleep, generally the deepest sleep of the night. The stages are marked by different brain wave activity, and in stage three, the brain waves become reallly sloooww. Then suddenly, in some people, bam! Out of bed and sleepwalking. How come? Don't know. 'We have no idea of what's going on in the brain,' said Mindell, who has been studying sleep for more than 20 years. Arand, who has been working in the field for 30 years, theorizes that the brain's chemistry may not make the complete switch from one sleep stage to another, and an 'ambiguous state' of sleepwalking can result. So why do kids grow out of it? There are some answers, particularly in the dynamics of stage three non-REM sleep. That stage closes dramatically as one ages. It can be as much as 50 percent of non-REM sleep in the very young and usually starts decreasing by about age 8 or 9. It's generally down to 25 percent by age 18, says Arand, and most people lose stage three altogether by age 40. Usually, as stage three disappears, so does sleepwalking. Adult sleepwalking can be a different animal. It can occur during what's left of stage three non-REM sleep, but it can also occur later in the sleep cycle and be more dangerous. To understand why, let's go back to sleep architecture and the REM/non-REM divide. Different things happen in each: Non-REM sleep restores one physically, while REM sleep, which lengthens during the night, is about settling things in the brain. In REM, said Helene Emsellem, a physician and medical director of the Center for Sleep and Wake Disorders in Chevy Chase, Md., the brain 'is rehashing the events of the day, throwing all the data points up in the air, then sorting through and interconnecting all this information, deciding what becomes memories, what fits in the learning process, and making you become the interesting, creative, well-balanced, fun person you can be.' Most dreaming occurs during the REM cycle, which is where trouble can lurk. The brain essentially paralyzes the skeletal muscles of the body during the REM cycle's sorting activity, said Emsellem. 'Maybe that's to prevent dangerous things from happening' -- such as responding to perceived danger in dreams in which you are being attacked, or chased by somebody like, say, the repo guy. But the paralysis doesn't always work. In the late 1980s, 'REM sleep behavioral disorder' was reported in humans, periods 'when one isn't fully paralyzed and can respond motorically (by overcoming the paralyzed skeletal muscles), without the ego saying, `Don't do that,' ' Emsellem said. Violent episodes can result when the lack of paralysis combines with a response to suggested danger in a dream. Which is why, said Emsellem, one should be very careful about waking an adult sleepwalker. 'If they are fighting somebody off in their dream, they don't know who you are and you could get hurt,' she said. 'It's best to leave the room, turn the lights on, and call to the person from outside the room.' Or, said Arand, 'prepare to duck, and wear a helmet.' Cures? Reducing stress and sleep deprivation, which have been identified as causes in adults. For the young, the re-introduction of naps, some experts say, can help. If sleepwalking episodes are predictable, Owens suggests 'scheduled awakenings': Wake the child up about a half-hour before the anticipated episode, make sure he's fully awake, and then let him go back to sleep. Keep it up for a month; 'it seems to work in about 50 percent of the cases,' she said. Different sleep environments can also be a cause -- sleepovers, for instance. But for the young, the most reliable cure is growing up. 