Definition

One of four stages of the sleep cycle based on the type of brain activity that occurs during the stage. Stages one, two, and three occur during non-rapid eye movement (non-REM) sleep, which is when a person first falls asleep and then moves from a light sleep into a deep sleep. Stage four occurs during REM sleep, which is when the eyes move rapidly while closed and when most dreams occur. During normal sleep, a person goes through four to five sleep cycles that last about 90 minutes each and include both non-REM sleep and REM sleep.