REM Sleep, or rapid eye movement sleep, is a major sleep state characterised by rapid eye movements, a relatively activated pattern of brain activity, vivid dreaming in many cases and profound inhibition of most skeletal-muscle activity. REM sleep alternates with non-REM sleep throughout the night and becomes increasingly prominent during later portions of a typical sleep period. The term refers specifically to the rapid movements of the eyes observed during this state, although the physiological characteristics extend far beyond eye movement. REM sleep is sometimes called paradoxical sleep because brain activity can resemble wakefulness in some respects while the body is in a state of marked motor inhibition.
REM sleep is characterised by rapid, low-amplitude and relatively desynchronised electroencephalographic activity compared with deep non-REM sleep, although modern sleep staging uses multiple physiological signals rather than EEG alone. Muscle tone is strongly reduced through active neural inhibition of spinal motor systems. The diaphragm continues to function, allowing breathing, but other skeletal muscles are largely inhibited. Heart rate and breathing can become more variable than during stable non-REM sleep. Rapid eye movements occur in bursts rather than continuously, and autonomic activity can fluctuate substantially during REM periods.
Dreaming is particularly common and often especially vivid during REM sleep, but dreaming is not exclusive to REM. Dreams can occur during non-REM sleep as well, although their typical phenomenology may differ. REM dreams can include complex imagery, narrative structure, emotional content and unusual combinations of memories or perceptions. The relationship between REM sleep and dreaming is therefore strong but not absolute. Dream reports depend partly on whether a person is awakened and asked to recall an experience, meaning that the absence of a remembered dream does not prove that no dreaming occurred.
REM sleep has been studied extensively in relation to memory, emotional processing, brain development and neural plasticity, although its precise functions remain an active area of scientific investigation. Evidence indicates that REM sleep contributes to normal brain function, but it is difficult to assign one exclusive function to the state. Different forms of learning and memory involve interactions among REM sleep, non-REM sleep and wakefulness. REM sleep is also particularly prominent in early life, suggesting an important relationship with developing nervous systems, although the amount and interpretation of infant REM-like sleep differ from adult sleep staging.
REM sleep typically first appears after a period of non-REM sleep rather than immediately after falling asleep. As the night progresses, individual REM episodes generally become longer, while deep slow-wave sleep is more concentrated earlier in the sleep period. This changing distribution contributes to the characteristic architecture of normal overnight sleep. Circadian timing and homeostatic sleep pressure both influence REM expression, and substances such as alcohol and some medications can alter REM quantity, timing or continuity.
REM sleep has distinctive pathological associations. Narcolepsy involves abnormal regulation of REM sleep and can produce unusually rapid entry into REM-related states. REM sleep behaviour disorder occurs when normal REM-related muscle atonia is impaired, permitting dream-enactment behaviours. Sleep paralysis can occur when REM-associated motor inhibition persists into wakefulness. These conditions illustrate that the components of REM physiology can become expressed at inappropriate times or fail to operate normally, producing recognisable clinical phenomena.
Sleep staging in a laboratory commonly identifies REM sleep using an electroencephalogram, electrooculogram and electromyogram. The EEG helps characterise brain activity, the eye channels detect rapid movements and the muscle channel identifies the marked reduction in skeletal-muscle tone. This multimodal approach is important because eye movements alone are insufficient to diagnose REM sleep. Sleep laboratories divide recorded sleep into standard stages using established scoring criteria, allowing the timing and amount of REM sleep to be quantified across the night.
REM sleep is often inaccurately described as the only stage in which people dream or as the stage that represents the deepest sleep. It is neither. Deep sleep occurs within non-REM sleep, particularly slow-wave sleep, while dreaming can occur in several sleep states. REM sleep is instead defined by a characteristic physiological configuration involving brain activity, eye movements and muscle atonia. Its importance arises from its integration into normal sleep architecture and its interaction with memory, emotion, development and neurological regulation rather than from a single simple function.
