REM sleep in adult humans typically occupies 20-25% of total sleep, about 90-120 minutes of a night's sleep. During a normal night of sleep, humans usually experience about 4 or 5 periods of REM sleep; they are quite short at the beginning of the night and longer toward the end. Most people tend to wake, or experience a period of very light sleep, for a short time immediately after a bout of REM. The relative amount of REM sleep varies considerably with age. A newborn baby spends more than 80% of total sleep time in REM. During REM, the summed activity of the brain's neurons is quite similar to that during waking hours; for this reason, the phenomenon is often called paradoxical sleep. This means that there are no dominating brain waves during REM sleep. REM sleep is physiologically different from the other phases of sleep, which are collectively referred to as non-REM sleep. Vividly recalled dreams mostly occur during REM sleep.
Physiologically, certain neurons in the brain stem, known as REM sleep-on cells, (located in the pontine tegmentum), are particularly active during REM sleep, and are probably responsible for its occurrence. The release of certain neurotransmitters, the monoamines (norepinephrine, serotonin and histamine), is completely shut down during REM. This causes REM atonia, a state in which the motor neurons are not stimulated and thus the body's muscles don't move. Lack of such REM atonia causes REM Behavior Disorder; sufferers act out the movements occurring in their dreams.
Heart rate and breathing rate are irregular during REM sleep, again similar to the waking hours. Body temperature is not well regulated during REM. Erections of the penis (Nocturnal Penile Tumescence or NPT) frequently accompany REM sleep and their presence in the case of someone suffering erectile dysfunction while awake may suggest its having a psychological rather than an organic cause. Clitoral enlargement, with accompanying vaginal blood flow and transudation (i.e. lubrication) is also present during REM.
During dreaming, the primary visual cortex is inactive, while secondary areas are active. This is similar to when subjects are asked to imagine or recall a visual scene, and different from what happens when they are actually seeing the scene.
REM sleep can occur within about 90 minutes of falling asleep, but in people with a sleep onset REM period, it may be as little as 15-25 minutes; this is considered a sign of narcolepsy.
Rapid eye movement sleep behaviour disorder (RBD) is a parasomnia that involves loss of atonia (paralysis) during otherwise intact REM sleep. The loss of motor inhibition leads to a wide spectrum of behavioural release during dreaming. This extends from simple limb twitches to more complex integrated movements where sufferers appear to be unconsciously acting out their dreams.
Intimately related to views on REM function in memory consolidation, Mitchison and Crick have proposed that by virtue of its inherent spontaneous activity, the function of REM sleep "is to remove certain undesirable modes of interaction in networks of cells in the cerebral cortex", which process they characterise as "unlearning". As a result, those memories which are relevant (whose underlying neuronal substrate is strong enough to withstand such spontatneous, chaotic activation), are further strengthened, whilst weaker, transient, "noise" memory traces disintegrate.
One important theoretical consequence of the Onthogenetic Hypothesis is that REM sleep may have no essentially vital function in the mature brain, i.e., once the development of CNS has completed. However, because processes of neuronal plasticity do not cease altogether in the adult brain, REM sleep may continue to be implicated in neurogenesis in adults as a source of sustained spontaneous stimulation.
Some researchers argue that the perpetuation of a complex brain process such as REM sleep indicates that it serves an important function for the survival of mammalian and avian species. It fulfills important physiological needs vital for survival to the extent that prolonged REM sleep deprivation leads to death in experimental animals. In both humans and experimental animals, REM sleep loss leads to several behavioral and physiological abnormalities. Loss of REM sleep has been noticed during various natural and experimental infections. Survivability of the experimental animals decreases when REM sleep is totally attenuated during infection; this leads to the possibility that the quality and quantity of REM sleep is generally essential for normal body physiology.
The sentinel hypothesis of REM sleep was put forward by Frederic Snyder in 1966. It is based upon the observation that REM sleep in several mammals (the rat, the hedgehog, the rabbit, and the rhesus monkey) is followed by a brief awakening; this does not occur for either cats or humans, although humans are more likely to wake from REM sleep than from non-REM sleep. Snyder hypothesized that REM sleep activates an animal periodically, to scan the environment for possible predators. This hypothesis does not explain the muscle paralysis of REM sleep.
Koulack, D. To Catch A Dream: Explorations of Dreaming. New York, SUNY, 1991.