FIGURE 4.3 The First 90 Minutes of Sleep From wakefulness to the deepest sleep of stage 4 NREM, the brain’s activity, measured by EEG recordings, progressively diminishes, as demonstrated by larger and slower brain waves. The four NREM stages occupy the first 50 to 70 minutes of sleep. Then, in a matter of minutes, the brain cycles back to smaller, faster brain waves, and the sleeper experiences the night’s first episode of dreaming REM sleep, which lasts 5 to 15 minutes. During the rest of the night, the sleeper continues to experience 90-minute cycles of alternating NREM and REM sleep.

Source: Data from Carskadon & Dement (2005).