Sleep physiology

On average, a person spends a third of their life sleeping. Sleep (or at least alternation of periods of activity and rest) is an integral mechanism of physiological adaptation in all living beings. This confirms the theory that sleep performs important functions in maintaining vital activity at an optimal level. Surprisingly, our understanding of such an important issue as the purpose of sleep is primitive and amorphous. Further research is needed to develop fundamental concepts in this area. However, below is a basic overview of the physiology of sleep, including the main mechanisms of its regulation and hypotheses explaining its functions.

Patients often ask how much sleep they need. Although the most common answer is 8 hours, some individuals need 4.5 hours of sleep, while others need 10 hours. Thus, 8 hours is only an average, and in general, this figure is subject to significant individual variations. However, since people whose sleep duration deviates significantly from the average constitute an absolute minority, they need appropriate examination to detect possible sleep disorders.

The time of occurrence, duration and structure of sleep vary among different biological species. Humans tend to fall asleep at night and wake up after sunrise. With the advent of artificial lighting and the need to work at night, the sleep and wakefulness patterns of many people have deviated significantly from the usual rhythm, which is characterized by rest at night and active activity during the day.

Laboratory studies show that the degree of wakefulness or sleepiness depends on at least two factors:

1. duration of previous wakefulness and
2. circadian rhythm.

Therefore, the main peak of sleepiness occurs in the late evening hours, which coincides with the usual time of going to bed. An additional peak of sleepiness occurs during the daytime, which coincides with the traditional hour of siesta - the afternoon rest accepted in many countries. Due to afternoon fatigue and circadian physiological processes, many people have difficulty maintaining active wakefulness at this time.

Most of the information accumulated to date about the structure of sleep, its stages and temporal characteristics was obtained using a special method that records biopotentials throughout sleep - polysomnography - PSG. Having appeared in the 1940s, polysomnography is now widely used both for scientific research and for diagnosing primary sleep disorders. For polysomnography, patients usually come to a somnology lab in the evening. The standard polysomnography procedure involves placing at least two electrodes on the scalp (most often on the crown and back of the head) - to record electroencephalography). Two electrodes are designed to record eye movements, and one electrode is placed on the mental muscle to assess the state of muscle tone during the transition from sleep to wakefulness and during various stages of sleep. Additionally, sensors are used to measure air flow, respiratory effort, blood oxygen saturation, record ECG and limb movements. To solve certain problems, various modifications of polysomnography are used. For example, additional EEG leads are used to diagnose nocturnal epileptic seizures. In some cases, the patient's behavior during sleep is recorded on videotape, which allows recording his movements and diagnosing disorders such as somnambulism or rapid eye movement (REM) sleep behavior disorder. In addition, this technique can be further modified to solve special diagnostic problems. For example, in some cases it is necessary to study the secretion of gastric juice during sleep, and to diagnose impotence it is important to obtain information about the state of the penis during sleep.

The subject goes to bed at a normal hour (e.g. 11 p.m.). The interval between turning off the lights and falling asleep is called the sleep latency period. Although some people fall asleep within a few minutes, most people fall asleep within 15-30 minutes. If the subject fails to fall asleep within 45 minutes, he or she becomes restless. Difficulty falling asleep is often due to the well-known phenomenon of the first laboratory night. For both the patient with insomnia and the healthy volunteer, the first night in the sleep laboratory causes stress, which leads to a significant extension of the latency period of falling asleep. A similar phenomenon is observed in many people who spend the night in an unfamiliar environment, such as a hotel room. The extension of the latency period of falling asleep can be caused by various factors: stress, a feeling of discomfort from an unfamiliar bed or environment, physical exertion, or a heavy dinner shortly before bedtime.

Stage I sleep is a transitional state between wakefulness and sleep. At this stage, a person feels only lightly drowsy and can respond to their name even if it is spoken quietly. This stage does not seem to promote rest or recovery and normally accounts for only 5-8% of the total duration of sleep. An increase in the presence of stage I is characteristic of restless, intermittent sleep, which may be caused by sleep apnea, restless legs syndrome, or depression.

Stage II typically takes up between one-half and two-thirds of the total sleep time. In some ways, it is the "core" of sleep. It is a single, well-defined phase that is characterized on the electroencephalogram by the presence of two phenomena: sleep spindles and K-complexes.

Typically, the transition from stage II to stages III and IV (deep sleep stages) occurs quite quickly.

Stages III and IV are usually combined under the names "slow (slow-wave) sleep" or "delta sleep". On the EEG, slow sleep is characterized by pronounced high-amplitude slow delta waves. During slow sleep, muscle tone decreases, and vegetative indicators (pulse, breathing rate) slow down. It is very difficult to wake a person in this phase of sleep, and if this happens, he is initially disoriented and confused. Slow sleep is considered the period that is most "responsible" for rest and restoration of strength during sleep. Usually, the first episode of slow sleep begins 30-40 minutes after falling asleep, that is, as a rule, late at night. Slow sleep is usually represented to a greater extent in the first third of the total sleep period.

The last stage of sleep is rapid eye movement sleep, or REM sleep. It is widely known that dreams are mainly associated with this stage of sleep. Only 10% of dreams occur in other stages of sleep. The stage of sleep leaves its mark on the nature of dreams. Dreams during slow-wave sleep are usually more vague, unstructured - both in content and in the feelings experienced by a person. Whereas dreams in REM sleep, on the contrary, leave vivid sensations and have a clear plot. From a neurophysiological standpoint, REM sleep is characterized by three main features:

1. low-amplitude, high-frequency activity resembling the EEG pattern in a state of intense wakefulness;
2. rapid eye movements;
3. deep muscular atony.

The combination of an "active" brain (low-amplitude, high-frequency EEG activity) and a "paralyzed" body (muscle atonia) has given rise to another name for this stage: "paradoxical sleep." Muscle atonia that develops during REM sleep appears to be an evolutionary adaptation that prevents physical responses to dreams. Typically, the first episode of REM sleep begins 70 to 90 minutes after falling asleep. The interval between sleep onset and the onset of the first episode of REM sleep is called the REM sleep latency period. Normally, REM sleep accounts for about 25% of total sleep time.

The first sleep cycle involves a sequential progression through all of the stages described. The second and subsequent cycles for the remainder of the night begin with stage II, followed by slow wave sleep and rapid eye movement sleep. As mentioned, slow wave sleep episodes are longer in the first third of the night, while rapid eye movement sleep is more prevalent in the last third of the night.

When evaluating the results of a laboratory sleep recording study, several parameters are analyzed: the latency period of falling asleep, the total duration of sleep, sleep efficiency (the ratio of the time during which a person slept to the total recording time), the degree of sleep fragmentation (the number of complete or incomplete awakenings, the time during which a person was awake after the onset of sleep), and sleep architecture (the number and duration of the main stages of sleep). Other physiological parameters are also analyzed, such as those related to breathing (apnea, hypopnea), blood oxygen saturation, periodic limb movements, and heart rate. This makes it possible to identify the influence of certain physiological processes on sleep. An example is episodes of apnea, which lead to sleep fragmentation.

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