What Are the Phases of Sleep?

There are two main phases of sleep: Slow (NREM) sleep and REM (fast) sleep. Discover the subphases and their characteristics.
What Are the Phases of Sleep?
Laura Ruiz Mitjana

Written and verified by la psicóloga Laura Ruiz Mitjana.

Last update: 26 March, 2023

When we sleep, the body and the brain undergo great changes. Although we think that nothing happens, the reality is that neurons are always active. But what are the phases of sleep that we go through?

When we sleep, we go through 5 well-differentiated phases: The first four are in the slow or NREM sleep stage and the last one is the REM sleep phase (fast or paradoxical). Discover the characteristics of each phase, what percentage of time they occupy, and what disorders can appear in each of them.

We must also know that these phases are repeated throughout the night. We go through four or five cycles, and each one includes a journey through all these stages.

What are the phases of sleep?

According to Dement and Kleitman’s (1957) classification, sleep can be divided into two major stages: Slow (NREM) and REM (rapid eye movement). Within slow sleep (NREM), we find four phases, and within REM sleep, only one.

Stage 1 NREM sleep (drowsiness)

When we fall asleep, we enter phase 1; that’s why we talk about drowsiness. It’s the transition between wakefulness and sleep and lasts approximately 7 minutes.

This phase will be repeated later, as it’s also the transition between stages. When we experience a fragmented nighttime dream, it repeats itself.

At this stage, we still perceive most of the stimuli that happen around us (auditory and tactile). Phase 1 of slow sleep (NREM) occupies 5% of the total. The brain waves that appear here are alpha and theta, which are related to calm.

There are three phenomena associated with phase 1 of NREM:

  • Electrooculogram (EOG): Slow movements.
  • Electromyography (EMG): Decreased muscle tone.
  • Hypnagogic hallucinations: These are those hallucinations that appear right at the time of falling asleep, between wakefulness and sleep. They’re physiological.
A child goes through the phases of sleep.
The sleep phases are staggered and repeated in cycles during rest.

NREM sleep stage 2

In phase 2, the threshold for awakening is raised. That is, it’s more difficult for some noise or sound in the environment to wake us up. A disconnection from the environment occurs, which facilitates sleeping behavior.

It accounts for 50% of the total sleep phases and, therefore, it’s the longest stage. The waves that predominate at the brain level are theta. Regarding the associated phenomena, the following are common:

  • Electroencephalogram (EEG): Sleep spindles and K complexes. Sleep spindles are a type of sinusoidal wave generated in the thalamus and in the cerebral cortex. In the case of the K complexes, they’re biphasic slow waves characterized by a slow, negative discharge of high amplitude and with a positive deflection (deviation).
  • EOG: Infrequent movements.
  • EMG: Decreased muscle tone.
  • Decreased sympathetic activity, heart rate, blood pressure, and cerebral blood flow.

Stages 3 and 4 of NREM sleep (slow-wave sleep)

Phases 3 and 4 include the so-called slow sleep or slow waves. Here, the depth is much greater. If we wake up in this phase, we find ourselves confused and disoriented.

When a person has a restful sleep, the quality of phase 4 is very good. Phases 3 and 4 predominate in the first half of the night and their presence or amplitude decreases throughout the course of it. This is when dreams appear.

These phases occupy between 15 and 25% of the total sleep phases. Specifically, the third occupies between 3 and 8% and the fourth between 10 and 15%. The predominant waves are delta.

What are the EOG and EMG like in these phases? What else appears?

  • EOG: Infrequent movements.
  • EMG: Increased muscle tone.
  • Decreased sympathetic activity, heart rate, blood pressure, and cerebral blood flow (the same as in phase 2).
  • Night terrors and sleepwalking appear, two types of sleep disorders called parasomnias.

REM sleep

Slow (NREM) sleep is followed by REM sleep, also called fast sleep. REM stands for rapid eye movement.

This is the phase in which we have typical dreams, those that are presented in the form of a narrative. The first few episodes of REM sleep are short-lived, but they get longer as the night progresses.

This phase favors brain development and learning. It occupies 20% of the total and the brain waves are theta and beta. Zigzag and PGO (low voltage mixed frequency) spikes appear on the electroencephalogram.

As for the EOG, the characteristic rapid eye movements occur (even if the eyes are closed). Muscular atony is produced, which is verified through the EMG, due to the segregation of cholinergic neurons.

Sympathetic activity increases and the heart rate is faster than in wakefulness. Blood pressure is greater or equal, and there’s also an increase in cerebral blood flow. At the same time, respiratory disorders and the phenomenon of nightmares appear.

Nightmares and night terrors in the phases of sleep

As we’ve seen, nightmares and dreams typically appear in REM sleep or fast sleep. On the other hand, night terrors appear in stages 3 and 4 of slow NREM sleep.

Mother comforts a child with nightmares.
Nightmares and night terrors are sleep disturbances that occur late in each cycle.


Nightmares are considered parasomnias and are also called distressing dreams. They consist of repeated awakenings during the sleep period caused by terrifying and prolonged dreams that leave vivid memories, the content of which usually focuses on threats to one’s survival, safety, or self-esteem.

According to a 2018 protocol prepared by the Spanish Association of Child and Adolescent Psychiatry (AEPNYA), it’s estimated that around 20-30% of children with a sleep disorder will require some type of intervention. When these types of disorders appear, from 25 to 84% tend to persist in early childhood for a period that can reach three years.

Night terrors

Night terrors appear in stages 3 and 4 of slow NREM sleep, typically in the first third of the night. They consist of recurrent episodes of abrupt awakenings that begin with a cry of anguish. The person, when waking up, and unlike what happens in nightmares, doesn’t remember anything of what they dreamed.

They’re less elaborate than nightmares, and vegetative signs appear (tachycardia, sweating, and tachypnea). There’s intense fear in the subject. According to the Spanish Society of Extrahospital Pediatrics and Primary Care (SEPEAP), night terrors usually appear between 3 and 10 years of age.

The phases of sleep and a good night’s rest

Complying with all phases of sleep in various cycles is synonymous with getting a good rest. This is necessary for the integrity of the body and for health in general. Disorders of the phases of sleep have repercussions, not only in the immediate time after waking up, but also in daily life.

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  • SEPEAP – Sociedad Española de Pediatría Extrahospitalaria y Atención Primaria.

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