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  4. Deep sleep and REM sleep: what happens at night

What happens at night

Short answer

Every night you go through four to six sleep cycles of roughly ninety minutes. Each cycle contains light sleep, deep non-REM sleep and REM sleep. Deep sleep repairs the body and cleans the brain; REM sleep processes emotion and anchors memory. You need both, and in the right proportion.

4 – 6
Sleep cycles per night
~90 min
Length of one complete cycle
13 – 23%
Share of deep sleep in adults

Deep sleep: the repair

Deep sleep, also called slow-wave sleep, is the most restorative stage. Heart rate and breathing drop, the muscles relax and brain waves become slow and high. In this stage growth hormone is released, tissue repairs itself and the brain's cleaning system runs at full power.

Deep sleep is concentrated in the first half of the night. That is at once the strongest argument for an early and fixed bedtime: anyone going to bed late doesn't lose the tail end of their night but the most restorative part of it.

REM sleep: the processing

REM is the dreaming stage. Brain activity resembles that of being awake, while the muscles are temporarily paralysed. REM processes emotion, consolidates memory and supports learning and creativity.

REM, by contrast, increases as the night goes on; the longest periods fall in the early morning. Sleeping two hours too few therefore costs you disproportionately much REM — and you notice that in your mood sooner than in your muscles.

Why one night says more than ten minutes

A complete cycle lasts ninety minutes, and the stages in which a bed has to do its work are spread across the whole night. In ten minutes of lying down you touch none of them.

What you judge in a showroom is the first sensation on lying down. What actually matters is how often you change position at four in the morning and how you get up the next day.

Sources

  1. Walker, M. (2017). Why We Sleep. Scribner.
  2. Rasch & Born (2013). About Sleep's Role in Memory. Physiological Reviews.
  3. Léger et al. (2018). Slow-wave sleep: from the cell to the clinic. Sleep Medicine Reviews.

Frequently asked questions

How much deep sleep do I need?

In adults, deep sleep takes up roughly thirteen to twenty-three per cent of the night. The exact figure says less than consistency does: a fixed rhythm usually yields more deep sleep than a long but irregular night.

Why do I wake up so often in the early morning?

In the early morning you are largely in light sleep and REM, and that is the stage in which you wake most easily. Light, noise or an over-warm room then break through faster than earlier in the night.