Physiology, Sleep Stages

The human body cycles through 2 phases of sleep, (1) rapid eye movement (REM) and (2) nonrapid eye movement (NREM) sleep, which is further divided into 3 stages—N1 to N3. Each phase and stage of sleep includes variations in muscle tone, brain wave patterns, and eye movements. The body cycles through all stages approximately 4 to 6 times each night, averaging 90 minutes for each cycle.