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Neurobiology of Dreams: Electrical Activity

Neurophysiological evidence indicates that sleep is not composed of only one type of brain rest, but also of distinct stages of neural activity.

About 90 minutes after the onset of sleep, several abrupt physiological changes occur. The EEG becomes desynchronized, showing a low-voltage, fast activity pattern similar, but not identical, to that of the waking state. As a result, this sleep state has been variously called paradoxal sleep, active sleep, and desynchronized sleep.

The basic stages of sleep, by convention, are divided into two main types: REM (Rapid Eye Movement, or dreaming) and Non-REM (NREM).

NREM generally is broken into four stages: 1, 2, 3, and 4 . In each stage, brain waves become progressively larger and slower, and sleep becomes deeper. After reaching stage 4, the deepest period, the pattern reverses, and sleep becomes progressively lighter until REM sleep, the most active period, occurs.

This cycle typically occurs about once every 90 minutes in humans. About 75% of total sleep is spent in non-REM and 25% in REM. A good night's sleep depends on the proper balance of these components.

When a person becomes sleepy, the alpha and beta waves increasingly give place to other waves of low amplitude known as the theta waves (from four to seven per second). In light sleep theta waves predominate and there appears the so called sleep cycles (bunches of synchronized electrical activity from 12 to 17 HZ).

While sleep phases follow, it becomes increasingly deeper, and the person becomes progressively less reactive to sensorial stimuli.

Electrophysiological Activity in Sleep

What are brain "waves"?

In the brain, the total sum of the electrical activity of millions of neurons, located principally in the cortex, can be observed with the electroencephalograph (EEG), a device that registers the brain's cells [activity] through a person's several states, from the awaken state to deep sleep.

Nervous cells present electrical potential differences with relation to the liquid in which it is immersed. The action potential refers to a brief fluctuation of the electrical charge in the membrane of the neuron, caused by the fast opening and closing of ionic channels, which depends on the voltage (ion flux).

The action potential flows as waves through the neurons' axons, transferring information from one place to the other in the nervous system. A wave can be of high or low amplitude (voltage) and high or low frequency (regularity).

Functions of the Different Brain Waves

One hypothesis for sleep-related rhythms is that they are the brain's way of disconnecting the cortex from sensory input. When we are asleep, thalamic neurons prevent penetration of sensory information upward to cortex.

Stages of sleep and Brain Waves

Beta waves
(very low amplitude, high frequency; 13 to 30 waves/sec)
A person is awaken and active (in a state of alertness ). They are the fastest [EEG] waves and signal an active cortex and an intense state of attention. Irregular register (unsynchronized).
Alpha waves
(low amplitude, 8 to 13 waves/sec)
A person is awaken and relaxed, with closed eyes. Neurons are firing at different times. Regular register (synchronized).
Theta waves (low-medium amplitude, spike-like waves; 3-7 waves/sec) A person is sleepy, already sleeping, or in a sleep transition. It can be observed in from the hippocampus. Theta rhythm is also observed in REM sleep. Because the hippocampus is involved in memory processing, the presence of theta rhythm during REM sleep in that region of the brain might be related to that [memory] activity.
Delta waves
(high amplitude, low frequency; 3 waves/sec)
A person is deep asleep*. Neurons, which are not engaged in the processing of information, are firing all at the same time. Therefore, the activity is synchronized. Waves are large and slow.
60 to 70
Maximal retraction of the pupil and nictating membrane follow the volleys of ocular movements

*Ponto geniculo-occipital waves (PGO) are also observed in deep sleep. Spontaneus, intermitent, high voltage peaks that appear simultaneously in the pontine structures, the lateral geniculate and occipital cortex. Both electrophysiological and neurochemical studies have exploited this phenomenon in an effort to identify the pontine structures involved in this phasic response and also to map their central pathways.

Stages of sleep through one night divided in cycles

A typical night of sleep consists of the repetition of a 90 to 110-minute cycle of REM and NREM (non-REM) sleep. The time spent in REM sleep is represented by a light-blue bar. The first REM period is usually short (5-10 min), but tends to lenghten in sucessive cycles. Conversely, stages 3 and 4, which together are often referred to as "delta sleep", dominate the slow-wave sleep periods in the first third of the night, but are often completely absent during the later, early morning cycles. The amount of stage 2 slow-wave sleep increases progressively until it completely occupies the slow waves period toward the end of the night. (Based on Kelly, 10).

The electroencephalograph (EEG) shows patterns of electrical activity during different stages of sleep. Brain waves of an awaken person and of those of a person in rapid eye movement (REM) sleep (when dreaming occurs) are similar in frequency and amplitude. In non-REM (NREM) sleep (stages 1, 2, 3, 4) brain waves have a higher amplitude and a lower frequency, indicating that neurons in the brain are firing more slowly and in a synchronized fashion.

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Author: Silvia Helena Cardoso, PhD. Psychobiologist, master and doctor in Sciences by the University of São Paulo and post doctoral fellowship by the University of California, Los Angeles. Invited Professor and Associate Researcher of the Center for Biomedical Inofrmatics, State University of Campinas (Unicamp), Brazil. Email:

Translation: Silvia Helena Cardoso

Center for Biomedical Informatics
State University of Campinas, Brazil

Silvia Helena Cardoso, PhD

Copyright 1997 State University of Campinas