Sleep is not a continuous, uniform state but a complex process divided into distinct phases. These phases cycle throughout the night, each contributing uniquely to physical and mental restoration. Among these, rapid eye movement (REM) sleep stands out as an active and distinct period. It differs from the other non-REM (NREM) stages, characterized by unique physiological and neurological activity.
The Sleep Cycle
A night’s sleep involves a progression through several stages, forming a repeating cycle. Sleep begins with non-REM (NREM) sleep, divided into three stages. Stage N1 is a brief transition from wakefulness to sleep, followed by N2, a deeper sleep stage where heart rate and body temperature begin to decrease. The deepest stage is N3, often referred to as slow-wave or deep sleep, important for physical recovery.
After progressing through these NREM stages, the first REM period occurs about 90 minutes after falling asleep. A full sleep cycle, encompassing NREM stages and one REM period, lasts about 90 to 110 minutes. This cycle repeats multiple times throughout the night, with each subsequent REM period becoming longer. As the night progresses, REM sleep accounts for a larger proportion of the total sleep time.
What Happens During REM Sleep
During REM sleep, the brain exhibits a high level of activity, with brainwave patterns resembling those of an awake and alert person. This heightened neural activity is distinct from the slower, more synchronized waves seen in deep NREM sleep. One noticeable feature of this stage is the rapid, darting movements of the eyes beneath closed eyelids. These eye movements are how the stage earned its name.
A temporary paralysis of the body’s voluntary muscles, known as muscle atonia, also characterizes REM sleep. This protective mechanism prevents individuals from physically acting out their vivid dreams, minimizing the risk of self-injury. Despite this muscle inactivity, various physiological changes occur, including fluctuations in breathing patterns, heart rate, and body temperature regulation. The most vivid, narrative-driven, and often bizarre dreams are experienced during REM sleep, reflecting the brain’s active state.
The Function of REM Sleep
REM sleep plays a role in memory consolidation, particularly for certain types of learning. It processes and stores procedural memories, which relate to learning new skills and habits, such as riding a bicycle or playing an instrument. It also contributes to the consolidation of spatial memories, helping the brain map and remember physical environments. The emotional components of memories are also processed during this stage, which can help soften the impact of distressing experiences.
This sleep stage also contributes to emotional regulation, allowing the brain to process and integrate emotional experiences. This processing may help individuals manage stress and adapt to emotionally challenging situations by re-evaluating events in a less reactive state. In infants, REM sleep is abundant, comprising about 50% of their total sleep time, compared to 20-25% in adults. This higher proportion in early life suggests an important role in brain development and maturation, supporting the growth and organization of neural circuits.
Factors That Disrupt REM Sleep
Several lifestyle factors and substances can interfere with REM sleep. Alcohol consumption, especially when consumed close to bedtime, suppresses REM sleep during the early part of the night. This suppression often leads to a “REM rebound” later in the sleep cycle, where the body attempts to compensate by increasing the duration or intensity of REM sleep.
Certain medications can also alter REM sleep architecture. Some drugs, such as tricyclic antidepressants or stimulants, reduce the amount of time spent in REM sleep. The proportion of REM sleep naturally decreases with age, with older adults experiencing less REM sleep compared to younger individuals. Also, chronic sleep deprivation or inconsistent sleep schedules can disrupt the sleep cycle, leading to a reduction in REM sleep.
REM Sleep Disorders
Specific medical conditions can arise when the mechanisms governing REM sleep malfunction.
REM Sleep Behavior Disorder (RBD)
This condition occurs when the normal muscle atonia that prevents movement during dreams is absent. Individuals with RBD may physically act out their vivid dreams, leading to potential injuries to themselves or their bed partners. These actions can range from talking and punching to kicking and jumping out of bed.
Narcolepsy
Narcolepsy is characterized by the intrusion of REM sleep characteristics into waking hours. People with narcolepsy may experience sudden, irresistible sleep attacks, often accompanied by cataplexy—a sudden loss of muscle tone triggered by strong emotions. Other symptoms include sleep paralysis, the temporary inability to move upon waking or falling asleep, and vivid hypnagogic or hypnopompic hallucinations, which occur at sleep onset or offset.
Nightmare Disorder
Nightmare Disorder involves frequent and distressing nightmares that primarily occur during REM sleep. These nightmares are vivid, often elaborate, and result in awakening with a strong sense of fear or anxiety, causing distress or impairment in daily functioning.