The 5 phases of sleep: from slow waves to REM

Formerly it was believed that sleep 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.

Sleep analysis is carried out from its division into phases, each one with its distinctive characteristics. In this article we will describe the five phases of sleep, which in turn can be divided into the 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 cycles of sleep

The dream was little understood until the middle of the 20th century, when it began to be scientifically studied through the records of EEG activity.

In 1957, physiologists and researchers William C. Dement and Nathaniel Kleitman described five phases of sleep. Its model is still in force today, although it has been modernized thanks to the development of new analytical instruments.

The phases of sleep that Dement and Kleitman proposed and that we will detail in this article are given continuously while we sleep. Sleep is structured in cycles, that is, successions of phases, lasting between 90 and 110 minutes. approximately: our body goes through between four and six sleep cycles each night we rest from appropriate form.

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

• You may be interested: "10 curiosities about dreams revealed by science"

Slow wave or non-REM sleep

Slow sleep makes up about 80% of total sleep. During the four phases that compose it, the cerebral blood flow decreases compared to wakefulness and REM sleep.

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

Phase 1: numbness

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

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

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

Thus, the brain activity of phase 1 is similar to that which occurs while we are awake, and therefore in these periods it is usual for relatively low intensity noises to wake us up, for example.

Phase 2: light sleep

Light sleep follows periods of drowsiness. During phase 2 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 waves, and the appearance of sleep spindles and K complexes; These terms describe oscillations in brain activity that promote deep sleep, inhibiting the ability to wake up.

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

Phases 3 and 4: delta or deep sleep

In Dement and Kleitman's model, deep sleep is composed of phases 3 and 4, although Theoretical differentiation between the two has lost popularity and today it is often talked about both jointly.

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 sleep. That is why these periods are commonly known as "slow wave sleep".

During slow sleep, physiological activity is greatly diminished, although muscle 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; Specifically, during these phases most of the episodes of night terrors, sleepwalking, somniloquia and nocturnal enuresis occur.

• You may be interested: "Sleep paralysis: definition, symptoms and causes"

Fast wave or REM sleep (phase 5)

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

REM phases are also known as paradoxical sleep because during this phase it is difficult for us to wake up even though the predominant brain waves are beta and theta, similar to those of wakefulness.

This phase constitutes 20% of 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. Similarly, in the REM phase nightmares occur.

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