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Sleep scientists divide a typical night into cycles of roughly ninety minutes, each containing distinct stages: light sleep, deep slow-wave sleep, and rapid eye movement (REM) sleep. Deep sleep, concentrated in the earlier cycles, is when the body performs most tissue repair and immune system reinforcement. REM sleep, which becomes longer in later cycles, is closely associated with memory consolidation and vivid dreaming. Interestingly, brain activity during REM can resemble waking activity on an electroencephalogram, even though the body's major muscle groups are temporarily paralyzed, a state researchers call atonia. Sleep deprivation studies show that missing REM sleep specifically impairs procedural memory more than missing deep sleep does, suggesting the two stages serve genuinely different cognitive functions rather than being interchangeable phases of rest.