The various stages of sleep

Did you know ?

What is sleep for ?

Our brain is active throughout our lives. Depending on our state, the type and intensity of activity varies. When we are awake, our brain is constantly being stimulated by external inputs (from our senses), receiving information from our own body, and on top of all this, it generates thoughts, emotions, performs billions of precise calculations before we even make a single movement. Such an intense brain activity requires tremendous amounts of energy and also generates metabolic “waste”.

The brain cannot remain active indefinitely: it must “rest”, slow down and even “clean up”.

This change in our brain’s activity is what we call “sleep” and can be monitored using electrodes on a subject’s head – the kind of assessment we conduct in our sleep laboratory during a polysomnographic examination.

An awake and active subject will show very rapid brain electrical activity and body movements, meaning we can easily detect the signals of wakefulness.
When the person calms down and closes their eyes, we observe typical sinusoidal patterns (called “alpha” patterns), indicating the person is now relaxed, and potentially on the verse of falling asleep.

The stages of sleep

Light sleep N1 stage

The transition from a relaxed state to the first stage of sleep – called light sleep or N1 – can happen easily when falling asleep at once.

But it can also be much more difficult if the subject is disturbed (by noise for instance).

In this case, one can go back and forth several times between wakefulness and the lightest stage of sleep, called N1.

This N1 stage is characterised by even slower brain activity than during the relaxation state, with very slow eye movements.
This stage usually lasts a few minutes. If the subject is left undisturbed, the sleep will “enter a deeper stage”.

Slow sleep - N2 stage

This transition is reflected in changes in the brain’s electrical activity, which is called the N2 stage.
During this phase, the subject becomes less and less reactive to stimuli and their brain is about to enter the N3 stage.

Deep sleep - N3 stage

The N3 stage is also called “deep sleep” or “slow-wave sleep”, and is characterised by very slow brain activity.

The amount of deep sleep is key in determining whether sleep will be restorative. Indeed, this stage is very important not only for the brain recovery process, but it also plays an important role in memory consolidation.

Sleep cycles

The different stages of sleep – including the N1, N2 and N3 stages – are called sleep cycles.
The duration of a cycle is usually 90 to 120 minutes. During the night, the brain goes through 3 to 5 complete sleep cycles (depending on sleep duration and the length of each cycle).


It is important to know that the first 2 to 3 sleep cycles feature a large amount of slow-wave sleep (N3), whereas the remaining cycles consist mainly of light N2 and REM sleep.

A polysomnographic sleep examination  gives us a clear overview of a patient’s sleep stages, durations, percentages, as well as a complete picture of their “sleep architecture”.

This picture is called “hypnogram”, and its detailed description is part of the results of a polysomnographic examination.

cenas hypnogramme