What happens to brain waves during sleep?

In this brief guide we are going to talk about the brain waves during sleep, we will explain the sleep process, its stages, and the brain waves that take place.

What happens to brain waves during sleep?

During sleep, our brain waves cycle from beta to alpha, theta and finally delta. There are cycles that last about 90 minutes.

Neurons communicate with each other through small electrical impulses that can be measured. We call these brain waves. These waves have different types of frequency, some are faster and others slower. If they are separated through filters, we can observe them more clearly.

We will have to perform an Electroencephalogram (EEG) recording the brain’s electrical activity through sensors placed on the scalp that allows us to see these electrical potentials in the form of waves.

The frequency of brain waves is measured in cycles per second or Hertz (Hz). The electrical current in Europe has a frequency of 50 Hz, i.e. each second has 50 cycles.

Brain waves are a direct reflection of the function of the Central Nervous System.

What happens when there is an imbalance in the brain waves?

If the Central Nervous System has an imbalance, it can be observed that these waves are altered and this will be reflected in the brain map or QEEG. The imbalance leads to discomfort and even pathology that will affect the quality of life.

The brain map will give information on the proportion of these waves, the conduction speed and the intensity of communication. If the aforementioned is not well regulated, treatment with Neurofeedback will be of great help, since it will teach the brain to regulate itself without the need for medication.

Stages of sleep

When sleep occurs, the brain begins to emit particular waves, which (some) are not observed during wakefulness. We differentiate a period of sleep into five phases:

  • Phase 1: Alpha waves would be combined, with a new activity, called theta. These are broad, low-frequency waves that would indicate a state of extreme relaxation and drowsiness.
  • Phase 2: The person would already be asleep and the work done by the brain in this phase would give rise to the sigma rhythm (also called sleep spindles) and K-complexes (broad, negative wave followed by a smaller, positive wave). It would be a phase with light sleep, in which it is easy to wake up.
  • Phases 3 and 4: These are the deep sleep phases called slow-wave sleep (delta waves). It allows the brain to rest and recover from the hustle and bustle of the day. This period has an essentially restorative function and lasts about 30 minutes.

Approximately 90 minutes after leaving us in the arms of Morpheus, the electroencephalogram begins to desynchronize: the dream activity that leads to the REM phase appears.

Then the same brain waves occur as when we are alert, the beta waves. In the first studies on sleep, this fact surprised researchers so much that it has also been called the “paradoxical sleep” phase, in fact, the wave recorded by the encephalogram and the eye movement wave are the same as when a person wakes up and sees an image.

Throughout the night, periods of slow waves are followed by periods of paradoxical sleep, so in a night sleeping eight hours, we can experience 4 or 5 periods of REM.

What can be seen in a brain map?

The waves recorded in the EEG, or brain map, can be separated into groups for study. These will be divided according to their frequency into:

  • Delta waves: with a frequency of 0.2-4 Hz.
  • Theta waves: 4-8 Hz
  • Alpha waves: 8-12 Hz
  • Beta waves: 12-30 Hz
  • Gamma waves: 30-90 Hz

Types of brain waves

The types of brain waves are as follows:

Delta waves

These are the very slow waves (or frequencies) but also the ones with the greatest amplitude. They are characteristic of when the individual is asleep and predominate during sleep.

They are also observed in meditation states. The production of the Delta rhythm coincides with the regeneration and restoration of the Central Nervous System. Sleep is restorative

Theta waves

Theta waves predominate when the senses are processing internal information and the individual is disconnected from the outside world, self-absorbed. They are very important during learning and memory. They are produced during the transition between wakefulness and sleep.

We produce Theta in states of intuition or processing unconscious information, for example processing traumas, nightmares or fears.

Alpha Waves

Alpha predominates when the Central Nervous System is at rest, relaxed but awake and attentive.

If there is an alpha deficit, the individual has difficulty relaxing.

In Alpha we would say that the brain is idle, relaxed, at rest, but at the same time ready for action if necessary.

This frequency helps mental coordination, mind/body integration, calmness and alertness.It is also a frequency that the brain uses as a reward after a job well done.

Beta waves

Beta predominates during the waking period.

It appears in states in which attention is directed to external cognitive tasks, as opposed to Theta waves, which appear during internal cognitive states.

The frequency is fast, it is present when we are attentive and involved in the resolution of daily tasks or problems, also during decision making or when we are concentrated.

Beta can be subdivided into:

Beta 1 (from 12-15 Hz),

Beta 2 (from 15-22 Hz):

When the Central Nervous System is engaged in a task.

Beta 3 (from 22-30 Hz):

When the Central Nervous System is engaged in highly complex cognitions or integrating new experiences.

May signify a state of anxiety or excitement as would be the case with individuals suffering from Generalized Anxiety Disorder.

An excess of Beta consumes a lot of energy. The brain map of a patient with anxiety disorder may show an excess of Beta 2 or 3, which would imply a lack of regulation. With Neurofeedback, it is possible to regulate and decrease the proportion of Beta waves and thus achieve improvement of anxiety.

Gamma waves

These are the fastest waves. They are produced in short bursts.

They are related to the simultaneous processing of information in several areas of the Central Nervous System.

Gamma wave bursts are observed when the brain is in a state of high resolution (e.g. during the calculation of a mathematical formula).

They are also observed in states of spirituality, feeling of altruism, love or other states of virtuality.

Gamma modulates perceptions and consciousness, and disappears during anesthesia.

Sleep etiology and evolution

Snyder (1966), theorizes about the etiology of sleep, which is produced in many mammals in order to conserve energy and to keep themselves safe.

He proposes REM sleep as a periodic awakening mechanism whose purpose is to activate the animal in case of being stalked while sleeping or when receiving any other external stimulus.

On the other hand, Muzio and Dement (1966) were struck by a large amount of REM sleep in newborn mammals.

And, finally, they conclude that these require the stimulus of REM sleep to compensate for that obtained during wakefulness, without losing hours of sleep.

The computational sense of the brain

Closer in time, many other theories related REM sleep to memory and learning. Understanding the brain in a way analogous to a computer.

That is, this organ would have the function of discarding the unimportant content of the previous day and making room for the next day.

At the same time, it also consolidates some learning which, as Breger (1967) calls it, is selective learning. Studies such as those of Gaarder (1966) or Hennevin and Leconte (1971) give REM sleep the function of processing and ordering information and memories.

Dewan (1969) elaborates his programming hypothesis, according to which, during sleep, a reprogramming necessary for activities of high associative level and emotional content takes place.


Theories have evolved, giving sleep (and especially its REM phase), functions linked to memory, survival, learning, rest and association in the cortex. It is considered a fundamental function of the organism, being part of a biological process that today we know as circadian rhythm.

In this brief guide we talked about the brain waves during sleep, we explained the sleep process, its stages, and the brain waves that take place.


Graven, S. (2006). Sleep and brain development. Clinics in perinatology, 33(3), 693-706.

Agnew Jr, H. W., Webb, W. B., & Williams, R. L. (1964). The effects of stage four sleep deprivation. Electroencephalography & Clinical Neurophysiology.