A period of sleep characterized by decreased metabolic activity, slowed breathing and heart rate, and the absence of dreaming. In humans and certain other animals, the sleep cycle occurs in five stages, the first four consisting of non-REM sleep and the last stage consisting of REM sleep. This cycle repeats itself about five times during a normal episode of sleep. In non-REM sleep, Stage I is characterized by drowsiness, Stage II by light sleep, and Stages III and IV by deep sleep. In adult humans, non-REM sleep accounts for about 75-80 percent of total sleep. Also called NREM sleep. Compare REM sleep. See more at sleep.