What actually happens in your brain during a night's sleep?
Your night is a loop of stages, and the deep part comes early.
▶ Start the storyYour night is not one long switch-off. Your brain loops through a repeating set of stages: light non-REM sleep, deep slow-wave sleep, and REM sleep, about every 90 minutes, four to six times a night. Deep sleep is heaviest early on and REM sleep takes over towards morning.
Sleep scientists call the stages N1, N2 and N3, then REM. N1 is the doorway: the alpha waves of the resting brain give way to slower theta waves, and you may get a hypnic jerk, a sudden twitch. N2 is lighter sleep with brief bursts of fast activity called sleep spindles. N3 is slow-wave sleep, named after the big, slow, synchronized waves that sweep the brain. A cycle normally runs N1, N2, N3, N2 and then REM, which is the main occasion for dreams, with fast brain waves and a body that has lost its muscle tone.
Step 1: N1
Doorway: theta waves, hypnic jerks
Step 2: N2
Sleep spindles and K-complexes
Step 3: N3
Slow-wave deep sleep
Step 4: N2
Back up to lighter sleep
Step 5: REM
Fast brain waves, dreams, no muscle tone
There is a human twist in stage 1. People woken from it often believe that they were fully awake; they can feel awake while already being in the first stage of sleep. And the whole thing runs on a lower power setting: the brain uses significantly less energy asleep than awake, even though it is an organ that normally burns about a fifth of the body's energy.
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Recap
A night is four to six loops of light, deep and REM sleep, with deep sleep early and REM late.
💡 A trick to remember it · A night is a playlist on repeat: N1, N2, N3, then REM as the encore, with the long slow tracks first.
Surprising fact · People woken from stage 1 often think they were fully awake.
Sources (3)
No source, no claim. Every fact in this lesson (16 claims) cites at least one of these.