Rapid Eye Movement Sleep
Overview
Rapid Eye Movement (REM) sleep is a unique phase of sleep in humans and other mammals, characterized by random and rapid movement of the eyes, accompanied by low muscle tone throughout the body, and the propensity of the sleeper to dream vividly. The REM phase of sleep is the fifth and last stage of a normal sleep cycle, which is preceded by four stages of non-REM sleep.
Physiology of REM Sleep
During REM sleep, the electrical activity of the brain, as recorded by an electroencephalogram (EEG), appears very similar to the brain activity recorded during wakefulness. This stage is often referred to as 'paradoxical sleep' because while the brain and other body systems become more active, muscles become more relaxed. Dreaming occurs due to increased brain activity, but voluntary muscles become paralyzed.
In the brain, REM sleep is generated by the interaction of various neurotransmitters, such as acetylcholine, dopamine, histamine, norepinephrine, serotonin, and orexin. The exact mechanisms of these interactions are not fully understood, but the activation of the monoaminergic cell groups in the brain stem, pons, and midbrain are known to suppress REM sleep.
Stages of Sleep
A typical sleep cycle takes about 90 to 110 minutes, with the first REM period occurring about 70 to 90 minutes after falling asleep. The first period of REM typically lasts about 10 minutes, with each recurring REM stage lasting longer, and the final one may last up to an hour. The proportion of REM sleep is highest during infancy and early childhood, and it decreases with age.
A complete sleep cycle progresses through the stages of non-REM sleep from stage 1 to stage 4, then reverses back from stage 4 to stage 1 before REM sleep is entered. This cycle is repeated 3 to 5 times during a good night's sleep.
Functions of REM Sleep
The exact functions of REM sleep are not well understood. However, it is known that it plays a crucial role in the formation of memories. A number of theories have been proposed, including that REM sleep aids in brain development in infants and young children, provides a form of overnight therapy for dealing with traumatic experiences, and plays a role in learning and memory.
Disorders Associated with REM Sleep
There are several sleep disorders associated with REM sleep. These include REM sleep behavior disorder (RBD), narcolepsy, and sleep paralysis. RBD is a condition in which the paralysis that normally occurs during REM sleep is incomplete or absent, causing the person to "act out" their dreams. Narcolepsy is a neurological disorder characterized by excessive daytime sleepiness, sleep paralysis, and sometimes hallucinations.