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Changes in Higher Nervous Activity and Behavior During Sleep

For medical students2 min readUpdated 2026-10-10

During sleep, higher nervous activity and human behavior undergo regular changes that reflect inhibitory processes in the cerebral cortex. The primary criterion for the transition from wakefulness to sleep is the breakdown of the law of strength for stimuli, which is accompanied by the loss of goal-directed activity and characteristic autonomic shifts objectively recorded by instrumental methods.

Law of StrengthDisrupted during sleep onset: the nervous system's reactions to stimuli become distorted.

PolymetrySynchronous recording of EEG and autonomic parameters using a multichannel polygraph.

NREM SleepHeart rate, respiration rate, and blood pressure decrease.

REM SleepMetabolism increases, along with elevated oxygen consumption and brain temperature.

Phase Changes in Higher Nervous Activity

The transition from active wakefulness to sleep (sleep onset) is accompanied by specific shifts in higher nervous activity. These changes were studied in detail within classical neurophysiology laboratories using the method of classical conditioned reflexes.

The main sign of sleep onset is the disruption of the fundamental physical law of the strength of stimuli. Normally, a strong signal elicits a strong reaction, and a weak signal elicits a weak one. However, when drifting into sleep, this rule ceases to apply.

Three main phases of higher nervous activity changes during sleep onset are distinguished:

Equaling phase (Equoral phase). At this stage, subjects begin to produce completely identical conditioned reflex responses to signals of vastly different physical strength.
Paradoxical phase. Characterized by a distorted reaction: the body's responses to weak conditioned stimuli turn out to be greater in magnitude and intensity than reactions to objectively strong signals.
Narcotic (inhibitory) phase. The final stage of falling asleep, during which there is a complete cessation of any responses to all conditioned stimuli, regardless of their intensity.

Behavioral Changes and Research Methods

With the onset of sleep, behavior changes dramatically. First and foremost, there is a complete loss of active and, especially, goal-directed activity. Both humans and animals during this period adopt a strictly defined posture that promotes maximum relaxation and rest.

To objectively characterize the state of a sleeping organism in modern physiology, polymetry is used. This is a complex method involving the simultaneous recording of the electroencephalogram (EEG) and a range of autonomic parameters using a multichannel polygraph.

Conditions for performing polymetry:

The study is conducted in a special sound-attenuated chamber to eliminate the influence of external noise.
The subject is comfortably positioned on a bed.
A special helmet with sensors for EEG recording is placed on the head, while autonomic parameter sensors are attached in parallel.
Parameter recording is carried out continuously: both during the actual process of falling asleep and throughout the entire night rest period.

The polygraph recording must include: EEG in multiple leads, electrocardiogram (ECG), respiration parameters, blood pressure (BP) levels, body temperature, and gas exchange indicators.

Somatovegetative and Motor Manifestations

Changes in the electrical activity of the brain (EEG) are always matched by completely specific somatovegetative parameters.

Motor reactions: A sleeping person does not remain completely motionless all night. Changes in posture and body position occur predominantly during periods of phase transitions (for example, during the transition from slow-wave sleep to REM sleep and vice versa). Directly during REM sleep, subjects generally lie quietly. It is important to note that phenomena such as somnambulism (sleepwalking) belong exclusively to the slow-wave phase.

Autonomic reactions: The dynamics of internal organ activity are tightly linked to the current phase.

In the slow-wave phase, autonomic inhibition processes predominate: blood pressure drops predictably, and the pulse and respiratory rate decrease.

In the REM sleep phase, the picture changes sharply:

Increased: blood pressure, pulse rate, and local brain temperature.
Decreased: muscle tone (reaches a minimum).
Specific manifestations are observed: minor twitching of facial muscles and fingertips, respiration becomes irregular, snoring decreases (if present before this phase), and penile erection may occur in men.
Oxygen consumption increases sharply, which reliably indicates an elevation in the overall metabolic level of the body.

Mnemonic

To remember the phases of the disruption of the law of strength during sleep onset, use the acronym EPN: Equaling, Paradoxical, Narcotic.

Frequently asked questions

What EEG rhythms are recorded during the various stages of slow-wave sleep?

During the various stages of slow-wave sleep, alpha, theta, and delta rhythms are sequentially recorded.

Drowsiness stage — alpha waves are recorded on the EEG.
Sleep spindles stage — alpha waves group into characteristic spindles.
Theta wave stage — high-amplitude slow theta waves appear.
Delta wave stage — an increase in high-amplitude ultra-slow delta waves occurs.

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

The generation of the REM sleep phase is associated with the concerted activity of neuronal groups in the brainstem reticular formation and the locus coeruleus, as well as cholinergic activation.

Locus coeruleus — during wakefulness, its neurons are active and exert an inhibitory influence on the giant neurons of the reticular formation; their activity decreases during the transition to sleep.
Giant neurons of the brainstem reticular formation — sharply activated during the REM phase.
Acetylcholine — excitation of giant neurons is driven by the release of acetylcholine.
Central region of the reticular formation — the activity of its neurons determines rapid eye movements.

In REM sleep, predominantly cholinergic activation of the cortex from the pons/brainstem and limbic system is noted.

What mechanisms ensure the sharp drop in muscle tone during REM sleep?

The decrease in muscle tone during REM sleep is associated with the inhibitory influence of neurons in the caudal part of the brainstem reticular formation.

Neurons of the caudal reticular formation — inhibit muscle tone.
Projections to the spinal cord — axons of these neurons project to the spinal cord.

In the REM phase, despite cortical activation, the motor output is blocked, manifesting as muscular atonia.

In which sleep phase does somnambulism (sleepwalking) most commonly occur?

Sleepwalking is a motor reaction characteristic exclusively of the slow-wave (NREM) phase of sleep.

When does a sleeping person most frequently change posture?

Changes in body position (posture shifts) occur predominantly during phase transitions — upon transitioning from slow-wave sleep to REM sleep and vice versa.

What happens to muscle tone during rapid eye movement (REM) sleep?

During the REM sleep phase, muscle tone is significantly reduced, despite blood pressure, heart rate, and brain temperature increasing at that moment.

What is the essence of the paradoxical phase of higher nervous activity during sleep onset?

In this phase, the law of strength is broken: the organism's conditioned reflex responses to weak stimuli become larger in magnitude than responses to strong signals.

Go deeper

Methodological features of classical conditioned reflexes in classical neurophysiology
Correlation of EEG changes with specific somatovegetative shifts
Physiological mechanisms of muscle tone reduction in the REM sleep phase
Dynamics of gas exchange and metabolic indicators across different sleep stages