Rapid eye movement, or REM, sleep is a state of diffuse bodily activation. Its EEG patterns (tracings of faster frequency and lower amplitude than in NREM stages 2 and 3) are superficially similar to those of drowsiness (stage 1 of NREM sleep). Whereas NREM is divided into three stages, REM is usually referred to as a single phase, despite the fact that a complex set of physiological fluctuations takes place in REM sleep.

REM sleep is named for the rapid movements of the eyes that occur in this stage. These movements, however, are not constant in REM sleep and are thus described as phasic. The other hallmark finding in REM sleep physiology is a reduced or nearly absent muscle tone (except for the diaphragm, one of the key muscles for maintaining breathing). Muscle activity in REM sleep may be nearly absent (tonic REM sleep) or may be characterized by brief bursts of activity (phasic REM sleep).

Most autonomic variables exhibit relatively high rates of activity and variability during REM sleep. For example, there are higher heart and respiration rates and more short-term variability in these rates than in NREM sleep, increased blood pressure, and, in males, full or partial penile erection. In addition, REM sleep is accompanied by a relatively low rate of gross body motility but includes some periodic twitching of the muscles of the face and extremities, relatively high levels of oxygen consumption by the brain, increased cerebral blood flow, and higher brain temperature. This brain activation during REM sleep has been shown to be localized in several areas of the brainstem and thalamus as well as in neural structures usually involved in the regulation of emotion (the limbic structures).

An even more-impressive demonstration of the activation of REM sleep is to be found in the firing rates of individual cerebral neurons, or nerve cells, in experimental animals: during REM sleep such rates exceed those of NREM sleep and often equal or surpass those of wakefulness. However, REM sleep also displays some localized areas of neural deactivation, particularly in the frontal (anterior) and parietal (posterior and lateral) regions of the brain cortex. The reasons for these distributed patterns of activations and deactivations remain hypothetical; some researchers have suggested that these responses may represent neural processes involved in REM sleep generation and in the production of dreams, which are known to be prominent during REM sleep.

For mammals, REM sleep is defined by the concurrence of three events: low-voltage mixed-frequency EEG, intermittent REMs, and suppressed muscle tone. The decrease in muscle tone and a similarly observed suppression of spinal reflexes are indicative of heightened motor inhibition during REM sleep. Animal studies have identified the locus ceruleus (or locus coeruleus), a region in the brainstem, as the probable source of that inhibition. When that structure is surgically destroyed in experimental animals, the animals periodically engage in active, apparently goal-directed behaviour during REM sleep, although they still show the unresponsivity to external stimulation characteristic of the stage. It has been suggested that such behaviour may be the acting out of the hallucinations of a dream.

As mentioned above, an important theoretical distinction is that between REM sleep phenomena that are continuous and those that are intermittent. Tonic (continuous) characteristics of REM sleep include the low-voltage EEG and the suppressed muscle tone. Intermittent events in REM sleep include the rapid eye movements themselves and spikelike electrical activity in those parts of the brain concerned with vision and in other parts of the cerebral cortex. The latter activations, which are known as ponto-geniculo-occipital waves, also occur in humans. Functional brain imaging studies have revealed that in humans those waves are closely associated with rapid eye movements.

REM sleep is the stage of sleep during which dreams prevail. Knowledge of human dreams is based largely on subjective reports recorded upon awakening from sleep. Although dreaming can also occur during NREM sleep, dreaming reports from people waking from REM sleep are more frequent, and the content of their dreams is florid, vivid, and hallucinatory.

Although the functions of dreaming remain largely elusive, the patterns of brain activity during REM sleep provide some clues about the characteristic properties of dreams. For instance, the activation of limbic structures during REM sleep may be linked to the high emotional content of dreams, and the deactivation of frontal areas may account for the “bizarreness” of dreams (distortion of time and space, lack of insight and control, etc.).

Sequences of NREM and REM sleep

As an individual matures into adulthood, an adult sleep pattern is established, characterized by the development of sleep-onset NREM sleep, the emergence of NREM sleep substages, the reduction or elimination of napping, and the decline of slow-wave activity. The usual temporal progression of the two kinds of sleep in adolescent and adult humans is for a period of approximately 70–90 minutes of NREM sleep (the stages being ordered 1–2–3–2) to precede the first period of REM sleep, which may last from approximately 5 to 15 minutes. NREM-REM cycles (ultradian cycles) of roughly equivalent total duration then recur through the night (about four to six times during a normal night’s sleep), with the REM portion lengthening somewhat and the stage 3 contribution to NREM portion shrinking correspondingly as sleep continues. Overnight sleep often is divided into three time periods: the first third of the night, which consists of the highest percentage of deep NREM sleep; the middle third of the night; and the last third of the night, the majority of which is made up of REM sleep. In the typical adult and adolescent, approximately 25 percent of total accumulated sleep is spent in REM sleep and 75 percent in NREM sleep. Most of the latter is EEG stage 2. The high proportion of stage 2 NREM sleep is attributable to the loss of stage 3 in the NREM portion of the NREM-REM cycles after the first two or three.