Sleep Cycle: Understanding The Different Phases [+ Diagram].

Almost a third of our life is dedicated to sleep, yet it remains a mysterious function, but crucial for our health.

Sleep, although very variable from one subject to another, is made up of several cycles. The sleep phases follow each other but are not the same: the first one is light sleep, then comes the deep sleep and at the end of the night, it is a paradoxical sleep that we find.

Sleep is crucial for many biological functions such as memory, learning, immune system…

However, sleep disorders have not stopped increasing in recent years in our population. Real public health problems, they question the future evolution of our health.

Monsieurdream sheds light on your nocturnal activity, to help you better understand the functioning of your biological clock, the organization of the different phases of your sleep, as well as the role that sleep plays in your health.

Sleep: Definition.

Sleep is characterized by a decrease in consciousness, muscle tone, and perceptual processes. Two distinct physiological states characterize human sleep: slow-wave sleep and REM sleep.

What Are The Different Sleep Cycles?

Schematically, sleep corresponds to a sequence of 3 to 6 cycles, each lasting 90 to 120 minutes.

The number of sleep cycles that will follow one another during a night depends on the length of your night. For example, a 9-hour night’s sleep consists of an average of 6 sleep cycles.

A cycle is itself made up of alternating slow-wave and REM sleep. 

More specifically, falling asleep is followed by light slow-wave sleep, which leads to deep slow-wave sleep. After about 60 to 75 minutes, REM sleep appears, followed by an intermediate phase, also called micro-awakenings.

These different stages constitute the first sleep cycle.

How Are Sleep Cycles Measured?

Polysomnography is the reference examination to measure nocturnal sleep and it determines the different sleep phases.

Several physiological parameters are recorded:

  • Brain activity (by electroencephalography).
  • Muscle activity (electromyogram).
  • Eye movements (electrooculogram).

This recording allows us to obtain a hypnogram, as shown below, representative of the course of your night of sleep:

Each sleep cycle plays a specific role. This is what we propose to discover below.

Let’s start by studying the falling asleep phase.

Falling Asleep.

Falling asleep is a transitional phase of a few minutes, separating wakefulness and sleep. It is also observed during the first stage of light slow-wave sleep when you gradually begin to close your eyes, relax, and doze off.

But do you really know how we get to sleep at about the same time every night?

Our sleep rhythm is actually regulated by our brain which contains a real clock located in the hypothalamus, allowing us to fall asleep and wake up.

Biologically, falling asleep requires the convergence of several factors :

  • Homeostatic processes: Increases the need for sleep as the period of wakefulness is prolonged.
  • Circadian processes: Aims to synchronize the body and sleep with the day-night alternation. This is our biological clock!

Homeostatic Processes.

This process is comparable to an hourglass or a battery that tends to run out after a certain time. This process is called “homeostatic debt”.

Asleep debt increases as a function of the time spent awake and decreases during the sleep episode. In other words, the longer you stay awake during the day, the greater the sleep pressure at bedtime.

Circadian Processes: Your Biological Clock.

The second process that greatly influences the timing, duration, and quality of our sleep is the phase of our circadian rhythm. This is determined by our biological clock whose rhythm is endogenous but driven by daylight.

This clock, therefore, produces a cycle of about 24 hours during which optimal times for falling asleep, waking up, and performing during the day occur.

Our sleepiness increases significantly between 1 and 4 pm, and even more so between 2 and 5 am.

Several factors intervene to regulate our biological clock more finely, among which is a well-known hormone, melatonin.


Melatonin allows the induction of sleep. Commonly called the sleep hormone, melatonin is produced in darkness, at the beginning of the night, by the pineal gland (or epiphysis), located behind the hypothalamus.

When it is released at the beginning of the night, it favors the triggering of sleep. Conversely, when the retinal cells perceive light, its synthesis is inhibited.

Exposure to light, therefore, plays a fundamental role in its secretion, which can delay the sleep phase.

Slow-Wave Sleep.

Slow-wave sleep has this name because it is characterized by slow waves. It is associated with minimal mental activity, not very elaborate.

Slow-wave sleep is divided into three stages of gradual depth, each corresponding to a different cerebral activity revealed by electroencephalography.

We distinguish:

  • Stage 1 (N1) of light sleep, is mainly observed during sleep onset.
  • Stage 2 (N2) of light sleep.
  • Stage 3 (N3) deep slow-wave sleep.

Light Slow-Wave Sleep.

A cycle generally begins with light slow-wave sleep (stages N1 and N2). This is the most common stage of sleep during the night, lasting about 20 minutes. Sleep is confirmed at this time. But the sleeper is still easy to wake up at this stage.

In young adults, slow-wave sleep represents 70 to 85% of the total nocturnal sleep time. The N1 stage represents less than 5%. The N2 stage, from 45 to 60%.

Deep Slow-Wave Sleep.

Sleep then evolves towards a phase of progressively deeper sleep (N3) which lasts several tens of minutes.

During this stage of deep sleep (N3), the slowing down of brain activity increases. The electroencephalogram shows the presence of high amplitude and low-frequency waves.

Oxygen consumption is reduced and therefore cerebral metabolism is slowed down. Muscle tone is also reduced. In other words, the person is sleeping deeply and is difficult to wake up.

Deep slow-wave sleep is between 15 and 25% of our total sleep time.

Role Of Slow-Wave Sleep. 

  • Recovery from physical fatigue (especially during deep sleep).
  • Secretion of growth hormone.
  • Secretion of prolactin.
  • Brain synthesis of glycogen and proteins.

At the end of deep slow-wave sleep, our brain goes into light slow-wave sleep, which leads either to an awakening or to REM sleep in order to continue our night.

REM Sleep.

The sleep cycle ends with a phase of REM sleep. Accompanied by frequent rapid eye movements (under the closed eyelids), REM sleep is also called the REM period (Rapid Eye Movement).

Unlike slow-wave sleep, REM sleep is a period of intense brain activity, similar to that observed during wakefulness.

The term paradoxical comes from the fact that muscle tone is totally abolished during this phase, the muscles are as if paralyzed, which contrasts with the intensity of brain activity. It represents between 20 and 25% of our total sleep time.

Role Of REM Sleep. 

  • Resolution of tensions accumulated during the day and recovery from nervous fatigue, thanks to dreams.
  • Organization of the information recorded during the day.
  • Fixation in memory of what has been learned during the day and suppression of what is not useful to retain.

Sleep will be of good quality if the cycles follow each other harmoniously. On the contrary, when the cycles are disrupted, they lead to sleep disorders.

REM sleep is also conducive to the most elaborate dreams. It should be noted, however, that light slow-wave sleep can also include dreams, but they are less intense.

The Awakening.

We are finally at the moment of awakening. Unlike falling asleep, it is characterized by a return to a complete state of consciousness. There is a sudden acceleration of brain activity and an increase in muscle tone.

The transition from sleep to wakefulness does not take place at the same time for all the cerebral structures, which may explain the phenomenon of sleep inertia, characterized by an impression of being tired, or completely elsewhere, and unable to feel operational for a few minutes after waking.

Abnormalities in this transition from sleep to wakefulness can lead to certain dissociated states, such as somnambulism (a state dissociated between deep slow-wave sleep and wakefulness) or sleep paralysis (a state dissociated between REM sleep and wakefulness). 

The Rhythm Of Sleep Cycles.

The rhythm of sleep varies during the same night: at the beginning of the cycle, deep slow-wave sleep is the most predominant, while the proportion of REM sleep is more important at the end of the night.

The duration and the number of sleep cycles are variable throughout life.

Slow-wave sleep is deepest during childhood and adolescence, until about age 20. With advancing age, slow-wave sleep becomes lighter, explaining the increase in sleep disorders in the elderly.

In children, sleep cycles are much more numerous and shorter.

Finally, the quantity and quality of sleep required vary greatly from one person to another. There are early risers and late sleepers, but also heavy and light sleepers, and this is all dictated by our genetics. This is our chronotype.

The short sleepers would have very short light sleep phases.

But regardless of how much sleep each person needs, deep sleep would be relatively constant, while light and REM sleep durations would vary.

See how the duration of stage 2 (N2) and REM sleep is reduced in short sleepers compared to long sleepers and control (normal) subjects. The duration of deep slow-wave sleep (N3) is identical in all three types of subjects.

Excerpt from the book “Sleep disorders” by Yves Dauvilliers.

Sleep and Health.

What if everything happened while we were sleeping? Still largely underestimated, it is nevertheless recognized that sleep has effects on both physical and mental health.

In terms of memory, slow-wave sleep and REM sleep seems to be involved. The latter constitutes the major part of the sleep time of newborns. It has been demonstrated that disrupting or suppressing it would lead to disorders of the brain architecture.

In adults, sleep stimulates the secretion of several hormones. Lack of sleep can therefore create numerous hormonal disorders such as abnormalities in cortisol or insulin. These play an essential role in weight management.

Therefore, lack of sleep favors weight gain. Several studies show, for example, a greater tendency to snack after a night of insomnia.

Good quality sleep helps prevent certain diseases. It has an immune action against infections and offers metabolic protection against hypertension, cardiovascular diseases, and diabetes.

Epidemiological observations show an increased risk of arterial hypertension, hypercholesterolemia, or cardiovascular events (stroke, coronary heart disease) in people with poor sleep quality.

At the same time, studies have pointed to a link between sleep disorders and impaired cognitive abilities. Mainly, attentional and executive functions are affected, with vigilance disorders in the foreground.

Sleep deprivation, on the other hand, affects work productivity and increases traffic fatalities.

Epidemiological data also suggest a bidirectional relationship between sleep and mood disorders, including the presence of depressive and anxiety symptoms.

Sleep disorders do make people more vulnerable to the risk of developing a depressive and/or anxiety disorder, but in turn, having a mood disorder would influence sleep quality.


During the last decades, major advances have been made in the recognition of the different stages of sleep. Sleep is relatively stable in the same individual and respects a very specific organization.

However, important inter-individual differences exist, probably due to the genetic heritage of each individual. The duration and organization of sleep also vary according to age, lifestyle, professional constraints, and the environment of individuals.

Sleep is not just for recuperation, we have seen that it has essential benefits for good health, so don’t neglect it any longer: sleeping well means better health!


What Is The Ideal Amount Of Deep Sleep?

The amount of deep slow-wave sleep is independent of the total duration of sleep. However, it is related to the duration of the wakefulness that leads up to sleep, and to the quality of this wakefulness: significant physical activity increases the amount of deep sleep.

After an afternoon nap, there is relatively little deep slow-wave sleep, in favor of lighter slow-wave sleep. On the other hand, in the case of sleep deprivation, the deficit in deep slow-wave sleep is compensated in priority.

Deep slow-wave sleep decreases with age, in favor of much lighter sleep. Many insomnia in the elderly is, in fact, only “bad sleep impressions”, too light sleep, while the total sleep duration is very good, even increased.

What Is Restful Sleep?

There is really only one definition of restorative sleep: it is when, in the morning, we wake up not only with the impression of having slept well but also with the impression of being rested and in great shape.

These impressions will be obtained after different times of sleep according to the subjects.

We are very unequal when it comes to sleep:

Most of us need 7.5 to 8 hours of sleep, spread over 4 cycles of 2 hours or 5 cycles of 1.5 hours.

Some people, known as “short sleepers”, need less than 6 hours per night (probably 4 cycles of 1 hour and 30 minutes). Others, much rarer, will need only 4 hours of sleep to be in good shape. These light sleepers represent about 5% of the population.

On the other hand, “heavy sleepers” will need an average of more than 9 hours of sleep per day. They represent about 10 to 15% of the population.

These sleep needs are probably natural, largely determined hereditarily. They evolve during childhood and then generally remain remarkably constant after the end of adolescence.

Therefore, it is not the quantity of sleep that defines restful sleep but rather it’s quality.

How Many Hours Of Sleep Per Age?

The number of hours of sleep varies from person to person, but on average:

  • An infant needs 18 to 20 hours of sleep.
  • A teenager needs 9 to 10 hours of sleep.
  • An adult needs 7 to 8 hours of sleep.

An adult should therefore sleep at least 7 hours a night to feel rested.

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