FIGURE 4.4 The 90-Minute Cycles of Sleep During a typical night, you experience five 90-minute cycles of alternating NREM and REM sleep. The deepest stages of NREM sleep, stages 3 and 4, occur during the first two 90-minute cycles. Dreaming REM sleep episodes become progressively longer as the night goes on. Shifts in sleep position, indicated by the dots, usually occur immediately before and after REM episodes.

Source: Data from Hobson (2004).