There are five stages of sleep that cycle over and over again during a single night's rest: stages 1, 2, 3, 4 and REM (rapid eye movement). Stages 1 through 4 are also known as non-rapid eye movement sleep (NREM). About 50% of sleep time is spent in stage 2 and about 20% is spent in REM (normally more than 2 hours a night in adults). A complete sleep cycle, from the beginning of stage 1 to the end of REM, usually takes about an hour and a half.
Stage 1 is light sleep during which the muscles begin to relax and a person can be easily awakened. During stage 2, brain activity slows down and eye movement stops. Stages 3 and 4 comprise deep sleep, during which all eye and muscle movement ceases. It can be difficult to wake a person during deep sleep. Stage 3 is characterized by very slow brain waves (delta waves), interspersed with small, quick waves. In stage 4, the brain waves are all delta waves.
It is during deep sleep that some people sleepwalk and children may experience bedwetting. It is during REM sleep that dreams occur. The muscles of the body stiffen, the eyes move, the heart rate increases, breathing becomes more rapid and irregular, and the blood pressure rises.