Rapid eye movement (REM) sleep is an active phase of the sleep cycle, first identified in the 1950s. During this stage, the brain becomes highly active, mirroring levels seen during wakefulness, yet the body remains almost entirely still. This paradoxical state challenges the notion that sleep is merely a passive period of rest, revealing a dynamic process integral to how we function when awake.
REM’s Role Within the Sleep Cycle
Sleep is not a single, uniform state but a structured journey through several distinct stages that repeat throughout the night in a sleep cycle. A complete cycle involves progressing through stages of non-REM (NREM) sleep before entering the REM stage. This entire sequence lasts approximately 90 to 110 minutes, and a person experiences four to five of these cycles per night.
The sleep cycle begins with N1, a brief transitional phase between wakefulness and sleep. Following this is the N2 stage, which accounts for a large portion of total sleep time, where heart rate and body temperature decrease. During N2, the brain also exhibits short bursts of electrical activity called sleep spindles. From there, sleep deepens into the N3 stage, or slow-wave sleep, which is the most restorative phase for the body.
After the NREM stages, the brain transitions into REM sleep. The first REM period is short, lasting around 10 minutes and occurring about 90 minutes after falling asleep. As the night progresses, NREM stages shorten while REM periods grow longer. Later cycles can include REM stages lasting up to an hour, meaning most REM sleep occurs in the hours before waking.
Physiological Hallmarks of REM Sleep
The REM stage is defined by a collection of distinct physiological events. Its name is derived from the rapid, darting movements of the eyes that occur in bursts behind closed eyelids, which may correlate with the visual experiences of dreaming. This ocular activity stands in sharp contrast to the rest of the body, which experiences a temporary paralysis.
This state of muscle paralysis is known as atonia. During REM sleep, signals from the brainstem inhibit motor neurons in the spinal cord, preventing the body’s major muscle groups from moving. This paralysis affects all skeletal muscles except for those that control breathing and eye movement. Atonia is a protective function that ensures an individual does not physically act out vivid dreams, thereby preventing potential injury.
While the body is still, the brain is exceptionally active, a phenomenon that has led to REM sleep also being called “paradoxical sleep.” EEG recordings show that brain wave patterns during REM are high-frequency, resembling the patterns seen during an awake state. This heightened brain activity is accompanied by physiological fluctuations; breathing becomes faster and more irregular, and heart rate and blood pressure increase to levels near wakefulness.
Cognitive Functions of REM Sleep
The brain activity during REM sleep is closely linked to several cognitive functions, most notably memory consolidation. During REM, the brain actively processes information acquired throughout the day, helping to solidify new learnings and skills. The brain strengthens certain neural connections, transferring short-term memories into more durable long-term storage.
Beyond memory, REM sleep is also involved in emotional regulation. The amygdala, a brain region central to processing emotions, is highly active during this stage. REM sleep helps to process the emotional content of recent experiences, moderating their intensity. This function may explain why a good night’s sleep can help provide perspective on stressful events, with dreaming being the subjective experience of this processing.
This stage also appears to support creativity and problem-solving. By creating a unique neurochemical environment, REM sleep allows the brain to form novel connections between seemingly unrelated ideas. This process can lead to innovative insights and creative solutions that were not apparent during waking hours. The dream state provides a space for the brain to experiment with associations without the constraints of waking thought.
Factors That Influence REM Sleep
The amount and quality of REM sleep a person gets can be influenced by several factors. Age is a primary variable; infants spend a much higher proportion of their sleep time in the REM stage to support their rapid brain development. As people age, the total time spent in REM sleep tends to decrease.
Lifestyle choices and substances also have a direct impact. Alcohol consumption before bed suppresses REM sleep early in the night, followed by a “REM rebound” as the alcohol is metabolized. This rebound increases REM duration and intensity later, which can cause vivid dreams and fragmented sleep. Certain medications, like some antidepressants, can also suppress REM sleep.
A person’s sleep schedule is also a factor. Individuals who shorten their sleep time are disproportionately depriving themselves of REM sleep. Chronic sleep deprivation can lead to a REM rebound effect when a person finally sleeps longer, as the body attempts to compensate. Maintaining a regular sleep-wake cycle helps regulate the progression through all sleep stages.
Disruptions and Disorders of REM Sleep
Some medical conditions involve the malfunction of REM sleep’s core mechanisms, such as REM Sleep Behavior Disorder (RBD). In individuals with RBD, the normal muscle paralysis, or atonia, that should occur during REM sleep is absent. This failure of atonia causes people to physically act out their dreams, which can be violent and lead to injury to themselves or their sleeping partners. The onset of RBD is often gradual and is associated with neurodegenerative conditions like Parkinson’s disease.
Narcolepsy is another condition linked to REM sleep, characterized by overwhelming daytime sleepiness and an unstable sleep-wake cycle. People with narcolepsy often enter the REM stage very quickly, sometimes within 15 minutes of falling asleep. Elements of REM sleep can also intrude into wakefulness, such as cataplexy—a sudden loss of muscle tone triggered by strong emotions, which is the muscle atonia of REM sleep occurring while a person is conscious.