Sleep comes in cycles, not in one long block.

Each cycle is about 90 minutes, from 70 to 110 across people.

Five cycles of 90 minutes last 7 hours 30 minutes: 23:00 to 06:30.

Light, deep and REM sleep take turns in every cycle.

Deep sleep comes mostly early in the night.

REM periods grow longer in the cycles before you wake.

Sleep pressure builds the longer you are awake.

A body clock that repeats about every 24 hours shapes when you feel sleepy.

During sleep, the day’s experiences are replayed and stored.