REM Sleep

REM sleep, or fully known as Rapid Eye Movement Sleep, is one of the two main stages of sleep. It is also called paradoxical sleep because brain activity is as high as during wakefulness, while most skeletal muscles in the body are temporarily paralyzed (atonia). This stage derives its name from its characteristic rapid and conjugate eye movements.

Historical Development

The scientific study of sleep gained a new dimension with Hans Berger’s development of electroencephalography (EEG) in 1929. In 1937, Loomis and colleagues classified sleep into five distinct stages based on EEG recordings, but they were unable to fully define the unique structure of one of these stages. The identification of REM sleep as a distinct stage and the demonstration that sleep consists of a periodic cycle of REM and NREM (Non-Rapid Eye Movement) phases were achieved through the work of Eugene Aserinsky, William C. Dement, and Nathaniel Kleitman in 1957.

Position and Architecture Within the Sleep Cycle

A normal night’s sleep consists of cycles alternating between NREM and REM stages. Each cycle lasts approximately 90 to 120 minutes and repeats four to six times throughout the night.

• Onset and Duration: The first REM period typically emerges about 90 minutes after sleep onset. This interval, known as REM latency, represents the time between sleep initiation and the first REM period. The initial REM phase is usually short, lasting 5 to 15 minutes.

• Nighttime Distribution: As the night progresses, the duration of REM periods increases while deep slow-wave sleep (NREM stage 3) decreases. Consequently, NREM sleep predominates in the first half of the night, whereas REM sleep becomes more dominant in the second half.

• Proportion: In healthy adults, REM sleep constitutes approximately 20 to 25 percent of total nighttime sleep.

Neurobiological Basis

The generation and regulation of REM sleep are controlled by complex neuronal networks located in the brainstem.

Brain Regions

The key neurons responsible for REM sleep are localized in the pons. Specifically, cholinergic neuron groups known as the laterodorsal tegmental (LDT) and pedunculopontine tegmental (PPT) nuclei play a critical role in initiating and maintaining REM sleep.

Neurotransmitters

The balance of neurotransmitters in the brain changes markedly during REM sleep.

• Acetylcholine: One of the most important neurotransmitters in REM sleep, its levels are elevated during this stage. Cholinergic agonists enhance REM sleep.

• Serotonin and Norepinephrine: The release of these neurotransmitters reaches its lowest levels during REM sleep. The cessation of activity in noradrenergic neurons of the locus coeruleus and serotonergic neurons of the raphe nuclei is necessary for the onset of REM sleep.

• GABA: The primary inhibitory neurotransmitter involved in the overall regulation of sleep.

Ponto-Geniculo-Occipital (PGO) Waves

These phasic electrical potentials are unique to REM sleep and originate in the pons, propagating to the lateral geniculate nucleus and then to the occipital cortex. PGO waves are thought to contribute to dream imagery by inducing cortical desynchronization and linking to perception and arousal mechanisms.

Physiological Characteristics

REM sleep is characterized by a series of physiological changes, both tonic (sustained throughout the stage) and phasic (occurring intermittently).

• Electroencephalogram (EEG): Low-voltage, mixed-frequency activity is observed. This pattern resembles wakefulness or NREM stage 1. Rhythmic theta/delta bursts known as “sawtooth waves” are distinctive to this stage.

• Muscle Tone (EMG): There is a marked loss of muscle tone or complete atonia in skeletal muscles, except for vital respiratory muscles such as the diaphragm.

• Eye Movements (EOG): Rapid, jerky, and conjugate eye movements are the most prominent feature of this stage.

• Autonomic Nervous System: Heart rate, respiratory rate, and blood pressure increase and become irregular. Brain temperature also rises.

• Other Features: Vivid and narrative dreams occur predominantly during this stage. Additionally, sleep-related penile erections may also occur during this period.

Functions

While the precise functions of REM sleep have not been fully elucidated, several hypotheses have been proposed:

Mental and Psychological Restoration

REM sleep is believed to support psychological restoration and is associated with long-term emotional well-being.

Memory and Learning

It is suggested that REM sleep contributes to neural plasticity, supports memory consolidation, and plays a role in programming genetic memory for species-specific behaviors.

Rebound Phenomenon

When individuals are deprived of REM sleep, they subsequently experience more intense and prolonged REM sleep in the next sleep period, compensating for the deficit. This phenomenon demonstrates that REM sleep is a physiological necessity for the organism.

Age-Related Changes

The proportion of REM sleep within total sleep varies significantly with age.

• Infancy: In newborns, approximately 50 percent of total sleep consists of REM sleep (also called active sleep).

• Childhood and Adolescence: This proportion declines to about one-third during the first year of life and reaches approximately 25 percent by age 2 to 3.

• Adulthood and Old Age: The proportion of REM sleep remains stable between 15 and 25 percent throughout adulthood. Although slow-wave sleep (NREM stage 3) declines markedly in advanced age, the proportion of REM sleep within total sleep is largely preserved.

Clinical Significance

The characteristics of REM sleep are used in the diagnosis and understanding of various neurological and psychiatric conditions.

REM-Related Parasomnias

These disorders involve abnormal behaviors occurring during REM sleep. The most well-known include:

• REM Sleep Behavior Disorder (RBD): Characterized by the loss of REM-associated muscle atonia, leading individuals to physically act out their dreams, potentially causing harm to themselves or their bed partners.

• Nightmare Disorder: A condition marked by recurrent, disturbing dreams.

• Recurrent Isolated Sleep Paralysis: A condition in which muscle atonia persists during awakening from or falling into REM sleep.

REM Latency

A shortened REM latency (less than 90 minutes) is a biological marker observed in conditions such as depression. In narcolepsy, the direct onset of sleep into REM sleep (SOREM) is a characteristic finding.