Description of sleep phases and their roles

A night's sleep is characterized by a succession of cycles (4 or 5 on average), lasting from 1h30 to 2h.

Slow wave sleep phase

Each cycle begins with a period of slow wave sleep which lasts on average 1h10 to 1h40 and can be broken down into 4 phases of increasing depth

• Phase 1: falling asleep or pre-awakening 
• Phase 2: light sleep
• Stages 3 and 4: deep sleep

REM sleep phase

This is followed by a period of very deep REM sleep, lasting between 10 and 15 minutes. The duration of REM sleep increases from one cycle to the next during the course of a night.

Intermediate sleep phase

This is followed by a very short pre-awakening period (less than 3 minutes). At this point, the sleeper will be very sensitive to external stimuli, but if no particular disturbance wakes him up, he will continue on a new cycle and will not remember this "micro-awakening" when he wakes up. 

Slow wave sleep

REM sleep

• Almost total immobility, but muscle tone preserved (fingers clenched, muscles firm)

• Poker face

• Slow, regular pulse and respiratory rate

• Eyes unmoving behind closed eyelids

• Brainwaves slow down and gradually increase in amplitude, leading to slow, full electrical activity

• Low mental activity

• Energy saving

• Complete relaxation (most muscles relaxed, fingers open...), with occasional brief body movements

• Mobile, expressive face (reflection of dreams)

• Pulse and respiration as rapid as during wakefulness, but more irregular

• Rapid eye movements behind closed eyelids

• Intense electrical activity in the brain, with fast, short waves similar to those of the waking phase

•  True "internal" mental awakening (without perception of the environment) that corresponds to dreaming

•  Glucose and oxygen consumption as high as during wakefulness

Slow wave sleep

REM sleep

• Recovery from physical fatigue, during phases 3 and 4 (deep sleep)
• Growth hormone secretion
• Prolactin secretion
• Brain glycogen and protein synthesis

• Resolution of daytime tensions and recovery from nervous fatigue, thanks to dreams.

• Organization of information recorded during the day

• Memorize what has been learned during the day and delete what is not useful to remember