It depends on the person, and the sleep, but the first stage averages at about 90 minutes, with stages 2-4 taking on average 100-120 minutes. The cycle generally proceeds from NREM (non-REM) stages 1-3 to REM (stage 4), back down to NREM, and back to REM, several times a night, generally ending in an REM stage before awakening.
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