Why do we sleep?
The short answer
We sleep so the brain and body can clean, repair and consolidate memories. A night runs in roughly 90-minute cycles of light, deep and REM sleep: deep sleep clears waste and releases growth hormone, while sleep as a whole moves the day’s memories into long-term storage. When we sleep is set by sleep pressure, which builds while we are awake, and a 24-hour body clock.
Chapters and transcript
- 0:00
A night is a series of cycles
A night of sleep moves through light sleep, deep sleep and REM in cycles of about 90 minutes. Deep sleep comes mostly early in the night; REM periods grow longer towards morning. The night shown is illustrative.
- 0:09
Deep sleep cleans and repairs
In deep sleep the brain’s waves become slow and large. Studies, mostly in mice, show fluid clearing waste from the brain faster during sleep, and most growth hormone is released in deep sleep, helping tissues repair.
- 0:18
Sleep files the day’s memories
During sleep the day’s experiences are replayed, and important ones are moved from the hippocampus, which holds them briefly, into long-term storage in the cortex. Both deep sleep and REM take part, and losing sleep weakens next-day recall.
- 0:27
Two forces decide when you sleep
Sleep pressure builds the longer you are awake, as adenosine accumulates, and sleep clears it. A roughly 24-hour body clock keeps you alert by day. You fall asleep easily when pressure is high and the clock’s alerting signal dips. Caffeine blocks adenosine’s signal; most adults need seven to nine hours.