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Paradoxical (REM) Sleep

Q: Which neurotransmitter systems are responsible for triggering and maintaining paradoxical sleep?

The cited sources indicate predominantly cholinergic activation for rapid/paradoxical sleep. The transition into the rapid sleep phase is accompanied by the resumption of active impulses to the cortex. The source of activation differs from wakefulness: it is predominantly cholinergic activation from the pons/brainstem and limbic system. During the rapid sleep phase, a sharp activation of giant neurons in the brainstem reticular formation occurs, driven by the release of acetylcholine in this area.

Rapid Eye Movement (REM)

For medical students2 min readUpdated 2026-10-10

Paradoxical sleep is a deep sleep stage during which the electrical activity of the cerebral cortex is nearly identical to that of active wakefulness. It is during this period that rapid eye movements occur, and the nervous system processes emotional experiences.

Adult proportionAbout 20% of total sleep architecture (averaging 1.5–2 hours per night)

Key featureRapid eye movements (REM sleep)

Lifetime totalAn individual spends a total of about 5 years of life in this dreaming phase

EEG Paradox and Arousal Specifics

In a normal sleep cycle, the paradoxical phase occurs immediately after slow-wave sleep. The electroencephalogram (EEG) records low-amplitude high-frequency activity, which is normally characteristic of full wakefulness. However, the patient is in a state of deep sleep. It is because of this stark mismatch between the EEG pattern and the actual depth of sleep that the stage was named paradoxical.

The arousal process during this period is highly specific. It is extremely difficult to wake a person using strong but neutral stimuli (e.g., intense background noise). At the same time, the sleeper easily wakes up from weak but subjectively significant sounds, such as the slightest rustle. This state echoes the concept of "paradoxical sleep" described by Ivan Pavlov.

This paradox is due to specific cortico-subcortical relationships. Other similar phenomena are known in electrophysiology:

Atropine paradox: following atropine administration, an animal behaves as if awake, yet the EEG registers slow high-amplitude oscillations typical of sleep.
Paradox in urethane anesthesia (described by V.G. Agafonov): the animal is under anesthesia that blocks wakefulness, while the EEG records slow activity. If a nociceptive (pain) stimulus is applied, EEG activation occurs (transition to fast waves), but the animal exhibits no behavioral signs of waking up.

Age Dynamics and Functional Significance

The duration of the rapid wave stage depends directly on age. A fetus spends the majority of its intrauterine life in the paradoxical EEG phase. In newborns, it accounts for 60% of the total sleep structure. As a person matures, this indicator steadily declines: in adults, paradoxical sleep occupies only 20% (approximately 1/4 of total rest time), and in the elderly, the duration of this phase drops even further.

A key function of paradoxical sleep is the regulation of psychological state. It acts as a sort of "valve" for relieving the consequences of emotional stress. It has been proven that the percentage of this phase increases significantly after intense emotional experiences (e.g., after watching stimulating movies).

Since the paradoxical EEG stage coincides precisely with rapid eye movements, the English-language literature uses the term REM sleep (from Rapid Eye Movement). Given the uniqueness of these physiological processes, several authors propose designating this phase as an independent "third state" of vital activity, located between wakefulness and ordinary sleep.

Consequences of REM Sleep Deprivation

Artificial deprivation of the paradoxical phase leads to severe central nervous system disorders. The body begins to actively compensate for the deficit: if on the first nights attempts to enter REM sleep are recorded up to 22 times, in subsequent nights their number increases to up to 30.

Daytime symptoms of deprivation include:

Marked agitation and extreme absent-mindedness.
Motor impairments (unsteadiness of movement).
Short-term and long-term memory disorders.
Hyperphagia (constant feeling of hunger).

With prolonged absence of rapid-wave sleep (after 5 nights of deprivation), severe psychotic manifestations develop. Frightening visual hallucinations occur: subjects see ghosts, animated furniture, or attacking plants. In classic experiments, studies had to be urgently aborted due to the critical severity of the participants' condition.

Mnemonic

To avoid confusing EEG phenomena: "Paradoxical sleep — the brain has woken up (fast waves), but the body is asleep." "Atropine paradox — the body has woken up, but the brain is asleep (slow waves)."

Frequently asked questions

What changes in muscle tone and autonomic parameters occur during the paradoxical sleep stage?

During the paradoxical sleep stage, motor output is blocked and muscle atonia develops; the stage also coincides precisely with rapid eye movements.

Change in muscle tone — associated with neurons in the caudal part of the brainstem reticular formation that inhibit muscle tone; their axons project to the spinal cord.
Rapid eye movements — the paradoxical EEG stage precisely coincides with rapid eye movements, which is why it is designated as REM sleep.
Autonomic parameters — not specified for this stage in the cited sources.

This state differs from true wakefulness by sensory input restriction and muscle atonia.

Which brain structures are responsible for generating the REM sleep phase?

The cited sources describe the following structures for the rapid/paradoxical sleep phase:

Brainstem reticular formation — the central region contains neurons whose activity determines rapid eye movements; the caudal part contains neurons that inhibit muscle tone. During REM sleep, a sharp activation of giant neurons in the brainstem reticular formation occurs.
Locus coeruleus — during wakefulness, its neurons are active and inhibit the giant reticular formation neurons; during the transition to sleep, their activity decreases.
Pons/brainstem and limbic system — indicated as the source of predominantly cholinergic activation to the cortex during REM sleep.
Hypothalamic centers and limbic structures — ascending activating influences on the cortex during paradoxical sleep are carried out due to their close morphofunctional connections.

Which neurotransmitter systems are responsible for triggering and maintaining paradoxical sleep?

The cited sources indicate predominantly cholinergic activation for rapid/paradoxical sleep.

The transition into the rapid sleep phase is accompanied by the resumption of active impulses to the cortex. The source of activation differs from wakefulness: it is predominantly cholinergic activation from the pons/brainstem and limbic system. During the rapid sleep phase, a sharp activation of giant neurons in the brainstem reticular formation occurs, driven by the release of acetylcholine in this area.

Why is paradoxical sleep called REM sleep?

The abbreviation REM stands for Rapid Eye Movement. This is because rapid eye movements of the sleeper are observed specifically during this EEG stage.

How does emotional state affect sleep structure?

Emotional shocks and stress increase the proportion of paradoxical sleep, as this phase serves as a physiological mechanism for relieving nervous tension.

What sounds are easiest to wake up to during the REM sleep phase?

Strong neutral stimuli cannot wake a person up. Arousal occurs from weak, but subjectively significant sounds (e.g., a quiet rustle).

Go deeper

Mechanisms of muscle atonia formation during paradoxical sleep
Specifics of thalamocortical interactions during the REM phase
Dream theories and their neurophysiological justification
The role of brainstem structures in generating rapid-wave sleep