Sleep Cycles

Sleep Cycles

By Arkemmus | Arkemmus' Blog | 12 Jun 2021

Sleep organization (phases)

Sleep occupies about a third of our lives. Understanding how it is made of allows us to better clarify our needs and especially the importance of each phase, for truly restorative sleep.
First of all, you should know that, contrary to what has been thought for a long time, sleep is an active phenomenon. Also, only at the beginning of the twentieth century it was highlighted that sleep divides itself into 2 main states. Thus, there is no longer just wakefulness and sleep but good wakefulness, slow sleep, and REM sleep. Let's recap all of this:
- The awakening time
- Slow sleep, itself divided into 3 phases: falling asleep, light slow sleep, deep slow sleep
- REM sleep or REM sleep (REM is the acronym of the Rapid Eye Movements that occur during this phase).
During the sleep period, 3 to 5 cycles of about 90 minutes follow one another. But this is an average for adults and it differs by age but also by the individual. And within a cycle, the duration of each stage varies depending on the time of sleep. The first cycles contain more time for deep slow sleep and the last more time for REM sleep.
Please note that we are voluntarily not talking about the night but the sleep period. Indeed, even apart from any external synchronizer, we will sleep anyway.

Slow sleep

Falling asleep, stage 1
Breathing becomes slower, muscles relax, consciousness decreases. During this half-sleep stage, the muscles may show small contractions, often with the impression of falling into a vacuum.
Slow light sleep, stage 2
Slow light sleep, as the name suggests, is not very deep and accounts for about 50% of total sleep time. It is still easy to wake up at this stage - a noise or a light is enough - but you will remember sleeping. During this phase, the eye and muscle activities are reduced.
Deep slow sleep, stage 3
The sleeper is isolated from the outside world by sleep, his brain activity is reduced to a minimum. It is difficult to wake him up during this phase. This is the time of the cycle when we recover the most from the accumulated physical fatigue. The whole organism is at rest and recovering. The brain emits slow, wide waves. It accounts for approximately 20-25% of total sleep time and occurs at the end of this phase.  
REM sleep
This phase is called "paradoxical" because the individual simultaneously shows signs of very deep sleep and signs of wakefulness. The brain activity is very intense and our eyes show incessant rapid movements while we are paralyzed with completely sluggish muscles. This is the phase during which we have the longest and most elaborate dreams.
This phase represents from 20 to 25% of the total sleep time with greater amplitude at the end of the night. This is probably why we sometimes wake up saying "Ah, I was dreaming!".
Note also that we often think that it is during this phase only that parasomnias take place, but this is false since each of the phases can bring its share of parasomnias, sometimes putting us in wacky situations but which for some are really not to be taken lightly, in case of nightmares.  
Sleep regulation
By simplifying as much as possible, note that there are main 2 types of regulation:
- Homeostatic regulation (internal balance): the propensity to fall asleep increases according to sleep deprivation and decreases during the occurrence of a sleep episode.
- Circadian regulation under the control of our central clock: the sleep pressure is maximum between 1 a.m. and 5 a.m. and is also quite intense between 2 p.m. and 4 p.m.
Remember that light is the main synchronizer of our internal clock because it is its absence that triggers the secretion of melatonin, the hormone that makes us fall asleep.

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