It was formerly believed that sleep consisted simply of 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.
Dream analysis 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 in turn can be divided into periods of slow waves and periods of fast waves, better known as “REM sleep.”
Sleep was little understood until the mid-20th century, when it began to be studied scientifically. through electroencephalographic activity records
In 1957, physiologists and researchers William C. Dement and Nathaniel Kleitman described five phases of sleep. Its model is still valid today, although it has been modernized thanks to the development of new analysis instruments.
The phases of sleep that Dement and Kleitman proposed and that we will detail in this article They occur continuously while we sleep Sleep is structured in cycles, that is, successions of phases, lasting between approximately 90 and 110 minutes: our body goes through between four and six sleep cycles each night that we rest adequately.
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.
Slow sleep makes up 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 decreased electrical activity in the central nervous system.
Phase 1: numbness
Phase 1 of sleep, which accounts for less than 5% of total sleep, It 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 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, which indicate even greater relaxation.
Thus, the brain activity typical of phase 1 is similar to that which occurs while we are awake, and therefore in these periods it is common for us to be woken up by relatively mild noises, 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 sleep becomes increasingly 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 possibility of waking up.
Phase 2 of sleep It is the most frequent of the 5 constituting 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 made up 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 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. This is why these periods are commonly known as “slow wave sleep.”
During slow sleep, physiological activity is greatly reduced, 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 episodes of night terrors, sleepwalking, somniloquy and nocturnal enuresis occur.
Fast wave sleep or REM (phase 5)
The rapid eye movements that occur during this phase give it its better-known name: REM, or REM in English (“rapid eye movements”). Other physical signs of REM sleep are 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 waking up. Similarly, nightmares occur in the REM phase.
It is believed that REM sleep It 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 greater in children.
