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The 5 phases of sleep: from slow waves to REM

The 5 phases of sleep: from slow waves to REM

November 27, 2021

Formerly it was believed that the dream was simply the decrease in brain activity that occurs during wakefulness. However, we now know that sleep is an active and highly structured process during which the brain recovers energy and reorganizes memories.

The analysis of the dream is carried out from its division into phases, each one of them with its distinctive characteristics. In this article we will describe the five phases of sleep , which in turn can be divided into periods of slow waves and those of fast waves, better known as "REM sleep".

  • Related article: Types of brain waves: Delta, Theta, Alpha, Beta and Gamma

Phases and sleep cycles

The dream was little understood until the mid-twentieth century, when it began to study scientifically through the registers of electroencephalographic activity .


In 1957, physiologists and researchers William C. Dement and Nathaniel Kleitman described five phases of the dream. Its model is still valid today, although it has been modernized thanks to the development of new analysis tools.

The phases of the dream that Dement and Kleitman proposed and that we will detail in this article they occur continuously while we sleep . The dream is structured in cycles, that is, successions of phases, between 90 and 110 minutes approximately: our body goes through between four and six sleep cycles each night we rest properly.

During the first half of the night the slow phases of sleep predominate, while fast sleep or REM is more frequent as the night progresses . Let's see what each one of these types of dreams consists of.


  • Maybe you're interested: "10 curiosities about the dreams revealed by science"

Sleep of slow waves or no REM

Slow sleep constitutes approximately 80% of total sleep. During the four phases that comprise it, cerebral blood flow decreases compared to wakefulness and REM sleep.

Non-REM sleep is characterized by the predominance of slow brain waves , which indicate a decreased electrical activity in the central nervous system.

Phase 1: numbness

Phase 1 of sleep, which accounts for less than 5% of total sleep, is constituted by the transitional periods between wakefulness and sleep. It not only appears when we are falling asleep but also between the different dream cycles.

In this phase we progressively lose our awareness of the environment. Frequently appear prodromes of dream activity known as hypnagogic hallucinations, especially in children and in people with narcolepsy.


During numbness mainly alpha waves are recorded , which also occur when we are relaxed during the vigil, especially with our eyes closed. In addition, theta waves begin to appear, indicating an even greater relaxation.

Thus, the brain activity characteristic of phase 1 is similar to that which occurs while we are awake, and therefore in these periods it is usual to be awakened by relatively light noises, for example.

Phase 2: light sleep

Light sleep follows periods of numbness. During phase 2 the physiological and muscular activity decrease significantly and the disconnection with the environment intensifies, so that the dream becomes deeper and deeper.

This is related to the greater presence of theta waves, slower than alpha, and the appearance of sleep spindles and K complexes; These terms describe oscillations in brain activity that promote deep sleep, inhibiting the possibility of waking up.

Phase 2 of sleep it is the most frequent of the 5 , reaching approximately 50% of the total night's sleep.

Phases 3 and 4: delta or deep sleep

In the Dement and Kleitman model, deep sleep is composed of phases 3 and 4, although the theoretical differentiation between the two has lost popularity and nowadays it is common to talk about both together.

The slow sleep occupies between 15 and 25% of the total; approximately 3-8% corresponds to phase 3, while the remaining 10-15% is included in phase 4.

In these phases, delta waves predominate , which correspond to the deepest dream. That is why these periods are commonly known as "slow wave sleep".

During the slow sleep the physiological activity is very diminished, although the muscular tone increases. It is considered that our body rests and recovers more markedly in these phases than in the rest.

Many parasomnias are characteristic of slow wave sleep; in particular, during these phases most of the episodes of night terrors, sleepwalking, somnilochia and nocturnal enuresis occur.

  • Maybe you're interested: "Sleep paralysis: definition, symptoms and causes"

Fast wave dream or REM (phase 5)

The rapid eye movements that occur during this phase give it its best known name: MOR, or REM in English ("rapid eye movements"). Other physical signs of REM sleep are the strong decrease in muscle tone and the increase in physiological activity , as opposed to deep sleep.

The REM phases are also known as a paradoxical dream because during this phase it is difficult for us to wake up despite the fact that the predominant brain waves are beta and theta, similar to those of wakefulness.

This phase constitutes 20% of the total sleep. The proportion and duration of REM sleep progressively increases as the night progresses; this is related to the greater presence of vivid and narrative dreams during the hours preceding awakening. In the same way, nightmares occur in the REM phase.

It is believed that REM sleep is essential for brain development and the consolidation of new memories , as well as its integration with those that already existed. One argument in favor of these hypotheses is the fact that the REM phase is proportionally larger in children.


Sleep stages and circadian rhythms | Processing the Environment | MCAT | Khan Academy (November 2021).


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