Regulation of the wake-sleep cycle
The periodicity of alternating states of wakefulness and sleep is strictly subject to the circadian rhythm. This biological rhythm is directly related to daily changes in ambient light levels.
The internal pacemaker of the circadian rhythm is the suprachiasmatic nucleus of the hypothalamus. It functions as the brain's master biological clock. By sending signals, this nucleus exerts a direct regulatory influence on:
- other hypothalamic nuclei;
- the thalamus;
- brainstem structures.
Melatonin, a hormone produced by the pineal gland, plays a huge role in the circadian regulation of the cycle as well as in the synchronization of endocrine functions. Normally, its secretion actively occurs in the dark.
Phases and stages of sleep
Human sleep consists of two main phases. Their differentiation is based on two main electrographic features: the presence or absence of rapid eye movements, and the dominance of fast or slow rhythms on the electroencephalogram (EEG).
- Rapid eye movement (REM) sleep phase — also known as rapid or fast-wave sleep.
- Non-rapid eye movement (NREM) sleep phase — slow or slow-wave sleep.
NREM sleep is heterogeneous, with three consecutive stages reflecting the depth of immersion:
- Stage I (drowsiness and light sleep). The EEG registers the disappearance of the alpha rhythm (the main indicator of relaxed wakefulness). It is replaced by low-amplitude beta and theta rhythms.
- Stage II (medium-depth sleep). Its key EEG marker is the appearance of so-called sleep spindles (specific grouped high-frequency oscillations).
- Stage III (deepest sleep). In modern classification, it combines the previously separate stages III and IV. Slow, high-amplitude delta waves begin to dominate on the EEG during this period.
Comprehensive assessment of sleep quality
To evaluate patients with suspected sleep disorders, a detailed data analysis is performed. First, subjective satisfaction with the quality of sleep and wakefulness is assessed using special questionnaires.
When taking the anamnesis, it is essential to consider:
- the nature of professional activity and labor;
- the patient's living conditions;
- the presence of comorbid somatic, neurological, and psychiatric pathologies;
- the impact of ongoing pharmacotherapy.
However, the main objective method of investigation is polysomnography, although its routine use is not required in every clinical case.
Polysomnography and hypnogram construction
Polysomnography involves continuous instrumental recording of a number of vital physiological parameters during night or day sleep.
During the study, the following are recorded simultaneously:
- EEG (electroencephalogram) — to assess the bioelectrical activity of the brain;
- Electrooculogram — to record eye movements;
- Electromyogram — to monitor muscle tone (most commonly assessing oral cavity muscles);
- ECG (electrocardiogram) — to monitor cardiac activity;
- Electropneumogram — to assess chest respiratory movements and airflow through the upper airways.
The specialist analyzes sleep latency, total sleep duration, duration of phases and stages, number of awakenings, nature of motor activity, as well as ECG and respiratory parameters at different stages of sleep.
The final result of polysomnography is a hypnogram — a graphical representation of the patient's sleep cycles throughout the night. The ordinate axis displays: wakefulness (W), REM sleep (REM), and NREM sleep stages (1, 2, 3, and according to the legend of some classical charts, stage 4 as well).
Briefly: answers to topical questions
Which neurotransmitter systems provide the transition between NREM and REM sleep phases?
The transition between phases and the maintenance of sleep are regulated by a complex of neurotransmitters and peptides.
- Serotonin — a key sleep-inducing factor.
- Melatonin — ensures the synchronization of sleep phases.
- Inhibitory and opioid systems — GABA, enkephalins, endorphins.
- Specific factors — delta-sleep-inducing peptide.
- Arousal and modulation mediators — acetylcholine, dopamine, adrenaline, histamine.
The transition into the REM sleep phase is due to the release of acetylcholine, which causes a sharp activation of the giant neurons in the brainstem reticular formation.
What is the average duration of a single complete sleep cycle in an adult?
The average duration of one complete sleep cycle is 1.5–2 hours. One cycle is a sequential change of NREM sleep stages, which necessarily ends with an episode of REM sleep. Throughout the night, such cycles repeat 6–7 times.
How exactly does skeletal muscle tone change during the REM sleep phase?
During the REM sleep phase, there is a sharp drop in muscle tone. The electromyogram registers low muscle tone, and the amplitude of biopotentials of the floor-of-the-mouth muscles sharply decreases or they are not registered at all. This phenomenon reflects the active inhibition of the motor system. Neurons of the caudal brainstem reticular formation are responsible for this process, inhibiting muscle tone via axons projecting to the spinal cord.
For quick memorization of NREM sleep stages by EEG, use the immersion association: Stage I — Alpha rhythm disappears (light drowsiness), Stage II — Sleep spindles appear (medium depth), Stage III — powerful Delta waves dominate (deepest sleep).
Class and exam questions
Which brain structure serves as the internal pacemaker for the circadian rhythm?
Suprachiasmatic nucleus of the hypothalamus. It acts as the internal pacemaker, controlling other hypothalamic nuclei, the brainstem, and the thalamus.
By what criteria is sleep divided into phases?
The main criteria are the presence or absence of rapid eye movements, as well as the appearance of specific fast or slow rhythms on the EEG.
What parameters does the polysomnograph record besides the EEG?
For a comprehensive assessment, the device simultaneously records an electrooculogram (eye movements), electromyogram (oral muscle tone), ECG, and electropneumogram (respiratory movements and airflow).
What else to study on this topic
- The impact of somatic, neurological, and psychiatric disorders on sleep architecture
- Interpretation of respiratory and ECG parameters in various stages of sleep
- The nature of motor activity during REM and NREM sleep
- The use of specialized questionnaires for the subjective assessment of sleep quality
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Other topics in "Chapter 32. Sleep-Wake Disorders"
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