General dynamics of EEG rhythms
The rhythmic activity of the brain continuously changes depending on a person's functional state. The electroencephalogram (EEG) allows for a clear distinction between periods of wakefulness and the various phases of sleep.
In the active wakefulness state, 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 υ-rhythm appears (corresponding to the theta range of 4–6 Hz), and at the peak of relaxation, the slowest waves dominate—the δ-rhythm (0.5–3 Hz). It is important to note that the overall sequence of transitions between rhythms and the duration of each individual stage are strictly individual.
Stages of slow-wave sleep (Non-REM)
The process of falling asleep and plunging into deep sleep is accompanied by regular changes on the EEG and pronounced autonomic shifts. Slow-wave sleep upon falling asleep lasts on average 1–1.5 hours and proceeds in several stages:
- Drowsiness stage. Occurs upon closing the eyes. Alpha waves are recorded on the EEG, and the state is characterized by the ease of awakening.
- Sleep spindles stage. Develops during the first 30 minutes after falling asleep. Alpha waves on the encephalogram begin to form specific patterns resembling spindles.
- Theta wave stage. Replaces sleep spindles after about half an hour. High-amplitude slow theta waves appear on the EEG. Arousing a sleeping person at this stage is already significantly more difficult. Autonomic changes begin: heart rate (HR) and blood pressure (BP) decrease, body temperature drops, and the breathing and heart rate rhythm becomes regular.
- Delta wave stage (Delta sleep). The period of maximally deep sleep. High-amplitude ultraslow delta waves increase on the EEG, which may accelerate slightly as sleep deepens further. The arousal threshold reaches its maximum: a person can only be awakened by a very loud sound or physical stimulus (intense shaking). Autonomic parameters (pulse, pressure, temperature) drop to their minimum values.
Paradoxical sleep (REM sleep) and nocturnal dynamics
A special place in the structure of sleep is occupied by 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 shown on charts from 23:00 to 06:00), the phases of slow-wave and paradoxical sleep continuously alternate. The key regularity of this process is that with each new cycle closer to morning, the duration of the paradoxical sleep phase steadily increases.
Briefly: answers to topical questions
Which brain structures act as pacemakers for slow-wave sleep?
The sources do not explicitly use the term "slow-wave sleep pacemakers."
The involvement of the following structures in the development and mechanisms of slow-wave sleep has been confirmed:
- Posterior hypothalamic sleep center: upon removal of the inhibitory influence of the frontal cortex, its neurons begin to inhibit the brainstem reticular formation, which leads to a blockade of ascending activating influences on the cortex and the onset of slow-wave sleep.
- Thalamic nuclei: participate in the mechanisms of the slow-wave sleep phase.
In which phase of sleep is complete skeletal muscle relaxation observed with the loss of muscle tone (atonia)?
Complete skeletal muscle relaxation with the loss of muscle tone (atonia) is observed during the rapid (fast-wave, paradoxical, or REM) sleep phase.
During this period, the motor output is completely blocked, causing a sharp drop in muscle tone. A specific group of cells is responsible for this process—neurons of the posterior part of the brainstem reticular formation. It is they that carry out the inhibition of muscle tone via their axons, which project directly into the spinal cord.
Which neurotransmitters are involved in triggering the paradoxical phase of sleep?
The neurotransmitter acetylcholine is involved in triggering the paradoxical phase (rapid sleep).
The transition to rapid sleep occurs due to the sharp activation of giant neurons in the brainstem reticular formation. The excitation of these cells is directly caused by the release of acetylcholine. This process provides predominantly cholinergic activation of the cortex originating from the brainstem, pons, and limbic system, which forms the electroencephalographic picture of an "awake brain" in a sleeping organism.
To memorize the sequence of EEG rhythms when falling asleep, you can use the phrase: "Awake Stork Dozes Quietly" (Beta — Alpha — Theta(υ) — Delta).
Class and exam questions
In which stage of sleep is it most difficult to wake a person up?
During the delta wave stage (delta sleep). The arousal threshold at this moment is maximal; very strong auditory stimuli or mechanical impact are required.
How do autonomic parameters change during slow sleep?
As slow 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.
What else to study on this topic
- Individual fluctuations in the duration of sleep stages
- Features of the electromyogram (EMG) and electrooculogram (EOG) during sleep
- Mechanisms of "sleep spindle" formation
- Autonomic shifts during the paradoxical phase of sleep
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