Why do we sleep?
The short answer
We sleep in cycles of about 90 minutes, each running light, deep, light and REM sleep. Deep sleep comes mostly early in the night and REM grows toward morning; five cycles last 7 hours 30 minutes. Sleep pressure builds while we are awake, and a body clock that repeats about every 24 hours shapes when we feel sleepy. Replay during sleep is associated with storing the day’s experiences as long-term memory.
Chapters and transcript
- 0:00
Sleep comes in cycles, not in one long block.
A night is a series of cycles, not one unbroken block. The night in the film is illustrative, not a measured one.
- 0:12
Each cycle is about 90 minutes, from 70 to 110 across people.
The standard figure for the length of an adult's sleep cycle is 90 minutes; across people the range is about 70 to 110 minutes.
- 0:17
Five cycles of 90 minutes last 7 hours 30 minutes: 23:00 to 06:30.
Five cycles of 90 minutes make 450 minutes, which is 7 hours 30 minutes. From 23:00, that ends at 06:30.
- 0:30
Light, deep and REM sleep take turns in every cycle.
Each cycle moves through light sleep, deep sleep, light sleep and REM sleep.
- 0:37
Deep sleep comes mostly early in the night.
Deep sleep is greater earlier in the night, while the proportion of REM sleep increases in the cycles just before waking.
- 0:46
REM periods grow longer in the cycles before you wake.
In the film's illustrative night, the last two cycles hold 86 minutes of REM, against 38 minutes in the first two.
- 0:57
Sleep pressure builds the longer you are awake.
The sleep-pressure curve is schematic: it builds while awake and clears during sleep. It is a model, not a measurement.
- 1:15
A body clock that repeats about every 24 hours shapes when you feel sleepy.
The body clock repeats roughly every 24 hours, so sleepiness rises and falls on a daily rhythm.
- 1:29
During sleep, the day’s experiences are replayed and stored.
Replay during sleep, in deep sleep in particular, is associated with stabilising the day's experiences as long-term memory.
Sources
Every figure in the film comes from these. The animation is illustrative.
- Wikipedia, Sleep cycle: the standard figure of 90 minutes for an adult, with a range of 70 to 110 minutes
- Wikipedia, Sleep: deep sleep is greater earlier in the night, REM increases in the two cycles before waking, and slow-wave sleep is associated with hippocampal replay that supports long-term memory
- Wikipedia, Circadian rhythm: a rhythm that repeats roughly every 24 hours