"To determine whether someone suffers from a sleep-arousal disorder, or parasomnia, researchers often videotape the person in an attempt to record a mid-slumber episode. They also use a polysomnograph, which monitors several bodily functions, including breathing, eye movements, and brain-wave patterns. It's thought that homicidal somnambulism—killing someone while you're asleep—is related to night terrors. Both usually occur in the first two hours after someone has nodded off, during deep, slow-wave sleep. Dreams and nightmares, which are far more common than night terrors, occur during the lighter, REM stage of sleep—when people tend to be silent and immobile. <snipped>