One of 5 parts or stages of the sleep cycle based on the type of brain activity that occurs during the stage. During stages 1 to 4, a person will feel drowsy, fall asleep, and move into a deep, dreamless sleep. Stage 5 is called rapid eye movement (REM) sleep, and it is during this stage that dreams occur. During several hours of normal sleep, a person will go through several sleep cycles that include REM sleep and the 4 stages of non-REM sleep (light to deep sleep).

Periods of sleep manifested by changes in EEG activity and certain behavioral correlates; they formerly included Stage 1: sleep onset, drowsy sleep; Stage 2: light sleep; Stages 3 and 4: delta sleep, light sleep, deep sleep, telencephalic sleep. In 2007, sleep stages were redefined by The American Academy of Sleep Medicine (AASM) as: N1-N2 (sleep onset - light sleep), N3 (SLOW-WAVE SLEEP), and REM SLEEP.