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EEG During Sleep

For medical students2 min readUpdated 2026-10-10

Electroencephalography (EEG) allows for the objective assessment of sleep stage transitions via changes in the electrical activity of the brain. Depending on sleep depth, high-frequency waking rhythms are sequentially replaced by slow waves, while during the paradoxical phase, the pattern once again resembles the active state.

WakefulnessLow-amplitude high-frequency activity (β-rhythm, 15–30 Hz)
Non-REM SleepFrequency decreases from the α-rhythm (8–13 Hz) to the ultra-slow δ-rhythm (0.5–3 Hz)
Autonomic ActivityHeart rate, blood pressure, and body temperature reach minimums during delta sleep
First CycleThe duration of the initial slow-wave sleep phase is 1–1.5 hours
REM SleepEEG desynchronization and a return to the fast β-rhythm typical of wakefulness

General Dynamics of EEG Rhythms

The rhythmic activity of the brain changes continuously depending on a person's functional state. The electroencephalogram (EEG) clearly delineates periods of wakefulness and various phases of sleep.

In active wakefulness, high-frequency and low-amplitude "fast" activity is recorded—the β-rhythm with a frequency of 15–30 Hz. As soon as a person lies down, closes their eyes, and prepares for sleep, the fast beta waves are replaced by the slower α-rhythm (8–13 Hz). This transitional moment is accompanied by the onset of skeletal muscle relaxation.

As sleep deepens, the frequency of electrical oscillations continues to decrease. The θ-rhythm (corresponding to the theta range of 4–6 Hz) appears, and at the peak of relaxation, the slowest waves dominate—the δ-rhythm (0.5–3 Hz). Notably, the overall sequence of rhythm transitions and the duration of each individual stage are strictly individual.

Stages of Non-REM Sleep

The process of falling asleep and descending into deep sleep is accompanied by predictable EEG changes and pronounced autonomic shifts. Non-REM sleep upon falling asleep lasts on average 1–1.5 hours and proceeds in several stages:

  1. Drowsiness stage. Occurs when closing the eyes. Alpha waves are recorded on the EEG, and the state is characterized by ease of awakening.
  2. Sleep spindle stage. Develops within the first 30 minutes after falling asleep. Alpha waves on the encephalogram begin to aggregate into specific patterns resembling spindles.
  3. Theta wave stage. Replaces sleep spindles after about half an hour. High-amplitude slow theta waves appear on the EEG. Arousing the sleeper at this stage is significantly more difficult. Autonomic changes begin: heart rate (HR) and blood pressure (BP) decrease, body temperature drops, and the respiratory and cardiac rhythms become regular.
  4. Delta wave stage (Delta sleep). The period of deepest sleep. High-amplitude ultra-slow delta waves increase on the EEG, which may slightly accelerate as sleep deepens further. The arousal threshold reaches a maximum: a person can be awakened only by very loud noise or physical stimuli (shaking intensely). Autonomic parameters (pulse, pressure, temperature) drop to their minimum values.

Paradoxical Sleep (REM Sleep) and Nocturnal Dynamics

A special place in sleep architecture belongs to the paradoxical phase (REM sleep). Despite the fact that the person is asleep, the electrical activity of the brain becomes fast again and is practically indistinguishable from the state of wakefulness.

Rhythm desynchronization occurs: the β-rhythm (14–30 Hz) is recorded on the EEG. A "paradox" arises between high brain activity and the state of sleep.

Throughout the night (typically represented on charts from 23:00 to 06:00), the phases of non-REM and paradoxical sleep continuously alternate. The key pattern of this process is that with each new cycle closer to morning, the duration of the paradoxical sleep phase steadily increases.

Mnemonic

To remember the sequence of EEG rhythms during sleep onset, you can use the phrase: "Awake Animals Think Deeply" (Alpha — Beta — Theta — Delta, noting the general transitional trend).

Frequently asked questions

Which brain structures act as pacemakers for non-REM sleep?

While the specific term "non-REM pacemakers" varies in literature, the following structures are confirmed to participate in the mechanisms of non-REM sleep:

  • Posterior hypothalamus sleep center: its neurons, upon removal of frontal cortical inhibitory influences, begin to inhibit the brainstem reticular formation, leading to a blockade of ascending activating influences on the cortex and the onset of non-REM sleep.
  • Thalamic nuclei: participate in the mechanisms of the slow-wave sleep phase.
In which sleep phase is complete skeletal muscle relaxation with loss of muscle tone (atonia) observed?

Complete skeletal muscle relaxation with loss of muscle tone (atonia) is observed during the rapid (fast-wave, paradoxical, or REM) sleep phase.

During this period, motor output is completely blocked, causing a sharp drop in muscle tone. A specific group of cells—neurons in the posterior part of the brainstem reticular formation—is responsible for this process. They actively inhibit muscle tone via axons projecting directly to the spinal cord.

Which neurotransmitters are involved in triggering the paradoxical sleep phase?

The neurotransmitter acetylcholine is involved in triggering the paradoxical phase (REM sleep).

The transition to REM sleep occurs due to the sharp activation of giant neurons in the brainstem reticular formation. Excitation of these cells is directly driven by the release of acetylcholine. This process ensures predominantly cholinergic activation of the cortex originating from the brainstem, pons, and limbic system, forming an electroencephalographic picture of an "awake brain" in a sleeping organism.

In which sleep stage is it most difficult to awaken a person?

In the delta wave stage (delta sleep). The arousal threshold at this moment is maximal, requiring very strong auditory stimuli or mechanical force.

How do autonomic parameters change during non-REM sleep?

As non-REM sleep deepens (especially during the theta and delta wave stages), heart rate, blood pressure, and body temperature decrease, reaching their minimum values during delta sleep.

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