It was once believed that sleep consisted simply of the decreased brain activity that occurs during wakefulness. Today, however, we know that sleep is an active and highly structured process during which the brain recovers energy and reorganizes memories.

The analysis of sleep is carried out by dividing it into phases, each with its distinctive characteristics. In this article we will describe the five phases of sleep , which can be divided into periods of slow wave and periods of fast wave, better known as “REM sleep”.

Sleep phases and cycles

Sleep was poorly understood until the middle of the 20th century, when it began to be scientifically studied through the records of electroencephalographic activity .

In 1957, physiologists and researchers William C. Dement and Nathaniel Kleitman described five phases of sleep. Their model is still valid today, although it has been modernized by the development of new analytical tools.

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

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

Slow wave or non-REM sleep

Slow sleep constitutes approximately 80% of total sleep. During the four phases that compose it, the cerebral blood supply decreases in comparison with wakefulness and REM sleep.

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

Phase 1: Numbing

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

In this phase we progressively lose awareness of our surroundings. Frequently, prodromes of dream activity known as hypnagogic hallucinations appear, especially in children and 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 typical of phase 1 is similar to that which occurs while we are awake, and therefore it is common during these periods to be woken up by relatively low intensity noises, for example.

Phase 2: light sleep

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

This is related to the increased presence of theta waves, which are 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 possibility of waking up.

Phase 2 of sleep is the most frequent of the 5 phases , constituting approximately 50% of total nightly 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 the theoretical differentiation between the two has lost popularity and today they are often spoken of together.

Slow sleep occupies 15-25% 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. This is why these periods are usually 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; in particular, most episodes of night terrors, sleepwalking, somnolence and nocturnal enuresis occur during these phases.

Rapid wave or REM sleep (phase 5)

The rapid eye movements that occur during this phase give it its most famous name: REM. Other physical signs of REM sleep are a strong decrease in muscle tone and an increase in physiological activity , as opposed to deep sleep.

REM sleep is 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 increases progressively as the night progresses; this is related to the greater presence of vivid and narrative dreams during the hours preceding awakening. Similarly, nightmares occur in REM sleep.

It is believed that REM sleep is fundamental for brain development and the consolidation of new memories , as well as their integration with those that already existed. An argument in favour of these hypotheses is the fact that the REM phase is proportionally greater in children.