Sleep cycles

The sleep cycle is a concept that everyone knows. REM sleep, deep sleep... You probably know these terms, or have already heard them. However, it is necessary to look into the subject, because it allows you to improve your sleep! To know how to sleep, the ideal is indeed to understand how sleep works. Let's dive into the complex structure of sleep!

How is sleep organized?

Sleep is not a straight line: between going to bed and waking up, we go through several phases of sleep. These are called "sleep cycles".
Sleep is made up of a succession of these phases. In general, 4 to 6 cycles follow one another during the night. Each cycle has three main phases: light slow-wave sleep, deep slow-wave sleep and paradoxical sleep, also called “rapid eye movement sleep”.
During a cycle, there are phases of falling asleep, then light slow-wave sleep, which lead to deep slow-wave sleep. At the end of the cycle, after a return to light slow-wave sleep, a phase of paradoxical sleep appears.

How long is a cycle?

A cycle lasts about 90 minutes, on average, and there are generally 3 to 6 cycles per night. However, this remains an average; sleep is different for each individual.
The first half of sleep contains mostly deep slow-wave sleep phases, while the second half of sleep is rich in light slow-wave sleep phases and REM sleep phases.
Sleep tends to change with age: older people tend to have fewer phases of deep slow-wave sleep.

Falling asleep

The falling asleep phase corresponds to the first phase of sleep. It makes the transition between the waking state and the sleeping state. During this phase, breathing becomes slower and muscles relax. During this phase of sleep, it is common to have the impression of falling into the void, or to have jolts. There is nothing to worry about: these sensations are perfectly normal and common.
This phase generally lasts between 5 and 20 minutes. Beyond this time, especially from 30 minutes, it may be due to insomnia; it will then be advisable to speak to a doctor.

Slow wave sleep

During slow wave sleep, the electrical activity of the brain is significantly reduced: normally, in a calm state, the brain reaches an electrical frequency of 8 to 12 hertz. When asleep, the brain has very low electrical activity, which can drop to 1 hertz in deep slow wave sleep.

Light slow wave sleep

This phase follows that of falling asleep. Sleep is not very deep, and it is easy to wake you up with the slightest noise or sufficient light.
This phase represents 50% of total sleep.

Deep slow wave sleep

The deep slow wave sleep phase is a phase where we are deeply asleep. The heart and breathing have a regular rhythm, the body does not move. It is a phase in which it is difficult to wake up, where the brain becomes more and more insensitive to external stimuli (lights, noises).
This is the time of sleep when we recover the most from accumulated fatigue. The whole body is at rest and recovering. It represents about a quarter, 20 to 25% of sleep time, and occurs particularly at the beginning of the night.

REM sleep

The REM sleep phase is the best known phase, because it is the one during which we dream the most. Its name comes from the word "paradox", which refers to the conflict between the signs of deep sleep (still body, totally relaxed) and the signs of wakefulness (eye movements, intense brain activity, which are caused by dreams). The heart rate can also increase, as well as breathing. However, it depends on what we dream about.
This phase represents approximately 20% of total sleep time. 

Which phase is the most restorative?

The deep sleep phase is the most restorative phase for the body. During this phase, the body recovers much of the accumulated fatigue.

In which phase do we dream the most?

We dream throughout the night, but particularly during the REM sleep phase. It is during this phase that dreams are the most abundant and the most vivid.