On average, a person spends a third of his life in a dream. Sleep (or, at least, the 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 to maintain life activity at the optimal level. Surprisingly, our ideas about such an important issue as the purpose of sleep are primitive and amorphous. To develop fundamental concepts in this area, more research is needed. Nevertheless, below are basic information about the physiology of sleep, including the basic mechanisms of its regulation and hypotheses explaining its functions.
Patients often ask the question - how much they need to sleep. Although the answer is usually about 8 hours, some individuals need only sleep 4.1 / 2 hours, while others need 10 hours of sleep. Thus, 8 hours is only an average value, and on the whole this indicator is subject to significant individual variations. Nevertheless, since people who have significantly different sleep times from the average are an absolute minority, they need an appropriate survey to identify possible sleep disorders.
In different biological species, the time of onset, duration and structure of sleep are different. It is peculiar for a person to fall asleep at night and wake up after sunrise. With the advent of artificial lighting and the need to work at night, sleep and wakefulness in many people deviated significantly from the usual rhythm, which is characterized by rest at night and active activities during the day.
Laboratory studies show that the degree of wakefulness or drowsiness depends on at least two factors:
- duration of previous wakefulness and
- 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 at daytime, which coincides with the traditional hour of the siesta - an after-dinner rest taken in many countries. Because of post-dinner fatigue and circadian physiological processes, it is difficult for many people to maintain active wakefulness at this time.
Most of the information on the structure of sleep, its stages and time characteristics accumulated to date has been obtained thanks to a special method that registers biopotentials during the whole sleep, polysomnography - PSG. Appearing in the 1940s, polysomnography is widely used nowadays for both scientific research and diagnosis of primary sleep disorders. For polysomnography, patients usually come to the somnological laboratory in the evening. The standard procedure of polysomnography involves placing at least two electrodes on the scalp (most often on the vertex and occiput) - for recording electroencephalography). Two electrodes are designed to record eye movements, and one electrode is placed on the chin muscle to assess the state of the muscle tone during the transition from sleep to wakefulness and during various stages of sleep. Additionally, sensors are used to measure airflow, respiratory effort, oxygen saturation, ECG recording and limb movements. To solve these or other 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 him to record his movements and diagnose disorders such as somnambulism or behavioral disorders in sleep with fast eye movements (BDG). 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 it is important to get information about the condition of the penis during sleep during the diagnosis of impotence.
The subject goes to bed at the usual hour (for example, at 23 h). The interval between turning off the light and the moment of falling asleep is designated as a latent period of sleep. Although some people fall asleep in a few minutes, in most cases this period takes 15-30 minutes. If the subject does not manage to fall asleep more than 45 minutes, he becomes uneasy. Difficulties of falling asleep are often due to the well-known phenomenon of the first laboratory night. As for the patient with insomnia, and for a healthy volunteer, the first night in the somnological laboratory causes stress, which leads to a significant lengthening of the latent period of falling asleep. A similar phenomenon is observed in many people who spend the night in unfamiliar surroundings, for example, in a hotel room. The reason for lengthening the latent period of falling asleep may be various factors: stress, a feeling of discomfort from an unusual bed or furniture, physical activity or a tight dinner just before sleep.
I stage of sleep - transitional between wakefulness and sleep. At this stage, a person feels only a slight nap and can respond to his name, even if he is pronounced quietly enough. This stage, apparently, does not contribute to rest or recovery of forces and normally takes only 5-8% of the total duration of sleep. An increase in the representation of the first stage is characteristic of restless intermittent sleep, which can be caused by sleep apnea, restless legs syndrome, or depression.
Stage II usually takes from one-half to two-thirds of the total sleep time. In a way this is the "core" of sleep. It represents a single, well-delineated phase, which on the electroencephalogram is characterized by the presence of two phenomena: carotid spindles and K-complexes.
Usually the transition from stage II to stages III and IV (stages of deep sleep) occurs rather quickly.
Steps III and IV are usually combined under the names "slow (slow-wave) sleep" or "delta-sleep". On the EEG, a slow sleep is characterized by pronounced high-amplitude slow delta waves. During a slow sleep, the muscle tone decreases, and the vegetative indices (pulse, respiratory rate) slow down. To wake a person to this phase of sleep is very difficult, if this happens, then at first it is disoriented and confused. Slow sleep is considered the period that is most "responsible" for rest and recovery of energy during sleep. Usually the first episode of slow sleep begins 30-40 minutes after falling asleep, that is, as a rule, at night. Slow sleep is usually more represented in the first third of the total sleep period.
The last stage of sleep is a dream with rapid eye movements, or a quick sleep. It is widely known that dreams are mainly associated with this stage of sleep. Only 10% of dreams fall on other stages of sleep. The stage of sleep leaves its imprint on the nature of dreams. Dreams during slow sleep are usually more vague, not structured - both in content and in those feelings that a person experiences. While dreams in a dream with fast eye movements, on the contrary, leave vivid sensations and have a clear plot. From the neurophysiological positions, fast sleep is characterized by three main features:
- low-amplitude high-frequency activity, reminiscent of the picture of the EEG in a state of intense wakefulness;
- rapid eye movements;
- deep muscular atony.
The combination of the "active" brain (low-amplitude high-frequency EEG activity) and the "paralyzed" body (muscle atony) was the impetus for the appearance of yet another name for this stage - the "paradoxical dream". Muscular atony, which develops during sleep with rapid eye movements, appears to be an evolutionary device that prevents a physical reaction to dreams. Usually, the first episode of sleep with rapid eye movements begins 70-90 minutes after falling asleep. The interval between the onset of sleep and the onset of the first episode of sleep with rapid eye movements is commonly referred to as a latent period of sleep with rapid eye movements. Normally, sleep with rapid eye movements takes about 25% of the total sleep time.
The first cycle of sleep involves sequential passage through all the stages described. The second and subsequent cycles in the rest of the night begin with stage II, followed by a slow sleep and sleep with rapid eye movements. As already mentioned, episodes of slow sleep are more prolonged in the first third of the night, whereas sleep with rapid eye movements is more represented in the last third of the night.
When evaluating the results of a laboratory sleep study, several indicators are analyzed: the latent period of sleep, the total duration of sleep, the effectiveness of sleep (the ratio of time a person slept to the total time of recording), the degree of fragmentation of sleep (the number of complete or incomplete awakenings, during which a person was awake after the onset of sleep), the architectonics of sleep (representation and duration of the main stages of sleep). Additionally, other physiological indices are analyzed, for example, associated with respiration (apnea, hypopnea), oxygen saturation, periodic limb movements, and heart rhythm. This makes it possible to reveal the influence of certain physiological processes on sleep. An example is the episodes of sleep apnea, which lead to the fragmentation of sleep.
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