Sleep is essential to normal cognitive and emotional processing. Nearly every mental health disorder is associated with impairment in sleep. It is [Page 2464]therefore important for psychologists and psychiatrists to have a good understanding of sleep and sleep disorders. Parasomnias are a group of disorders characterized by a behavior that is abnormal during sleep. Historically, parasomnias have been classified by behaviors; however, with the evolving understanding of sleep physiology and pathophysiology, a much more convenient classification is based on sleep stage.

Sleep is generally divided into rapid eye movement (REM) and non–rapid eye movement (NREM) sleep, which have different brain wave patterns on an electroencephalogram (EEG). REM sleep occupies about 20% of total sleep time. REM sleep is characterized by complex dreaming, muscle tone inhibition (atonia), ...