Each sleep begins with non-REM sleep (also called: sleep NREM). NREM stands for " N on R apid E ye M ovement (NREM)" or in French "without quick eye movement".
Non-REM sleep consists of three phases of sleep that follow each other:
- Sleeping phase (light sleep) : phase 1
- Light sleep phase: phase 2
- Deep sleep phase: phase 3
Each of these phases is characterized by a specific brainwave model: as these phases progress, brain activity slows down and the muscles gradually relax.
A sleep cycle with its different phases of sleep can be represented as follows:
Sleep → Light sleep → Deep sleep → Light sleep → Deep sleep → REM sleep (paradoxical)
A complete sleep cycle, from sleep (phase 1) to REM sleep phase, lasts between 90 and 110 minutes on average.
Depending on the duration of sleep (and criteria specific to each person), we travel about 4 to 6 cycles of sleep per night. The first 3 cycles are essential for our sleep: they are called core sleep. The following cycles - which our body does not really need - are called optional sleep.
Here are the different phases of non-REM sleep:
Non-REM sleep - falling asleep
(sleep phase 1)
This is the transition phase between waking state and sleep.
During this phase 1, the frequency of brain activity decreases, we could say that the brain gradually regains its calm: eye movements become slow and muscles begin to relax.
The brain flows change to pass alpha waves (8 to 13 Hz on standby) to theta waves (4 to 7 Hz).
During this phase of sleep, spontaneous thrills may be observed.
It lasts about 10 minutes.
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Non-REM sleep - light sleep
(sleep phase 2)
The light sleep phase marks the beginning of sleep literally. In this phase of light sleep, the muscular tension continues to decrease and the conscious perception of the external environment decreases little by little. The sleeper does not wake up at the slightest sound, but does not sleep deeply yet.
During this light sleep, the electroencephalogram records mainly theta waves (4 to 7 Hz), but also what is called " complex K " (K means "knock" = "knock" in English): Specific waves occur either spontaneously or when the brain responds to external stimuli (eg a sound or light stimulus).
In parallel with these phenomena, spindles are observed regularly: a typical sequence of several fast brain waves whose amplitude increases and decreases between 11 and 16 Hz. Generated by the thalamus (deep zone of the brain), the spindles are designed to prevent external disturbances from disturbing sleep: they isolate the brain, thus stabilizing sleep.
This sleep phase lasts about 15 minutes.
Non-REM sleep - deep sleep
(sleep phase 3)
Phase 3 (Non-REM) is deep sleep, also known as Slow Wave Sleep (SWS) or Delta Sleep. It is characterized by slow and wide delta waves (0.1 to 4 Hz).
During this phase, the sleeper is awake with difficulty (unlike previous phases). It is also in this phase that we can witness phenomena such as sleepwalking or grinding of teeth.
This phase 3 lasts about 20 to 30 minutes.
Previously, non-REM phases were divided into 4 stages. Stage 4 then resembled stage 3, but in a more extreme version, with even deeper sleep, even lower brain frequencies. Today, we tend to integrate this phase in phase 3.
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