Traditional understanding of the biological significance of sleep
For a long time, normal physiology was dominated by the classical viewpoint, according to which sleep was regarded solely as a period of passive rest. It was believed that the main and only task of this state is to ensure the restoration of energy resources in brain cells that were depleted after a prolonged and active period of wakefulness.
A number of past researchers even drew a direct parallel between human sleep and the phenomenon of anabiosis in protozoa. Anabiosis represents a specific state of sharply slowed metabolism, balancing on the verge between life and death, which allows microorganisms to survive under critical and extreme environmental conditions. By analogy, it was assumed that sleep is a temporary maximal reduction of all vital processes.
Modern concept: sleep as an active process
In recent years, scientific views on the fundamental functions of sleep have undergone radical changes. Today, physiologists confidently state that sleep can by no means be viewed as a banal decrease in the intensity of life processes. On the contrary, it is an exceptionally active physiological process.
- Central nervous system (CNS) activity. It has been established that brain activity during this period not only does not fade, but often even exceeds the usual levels of daytime wakefulness. In particular, researchers record a significant increase in the electrical activity of neurons across a number of brain structures.
- The role of the autonomic nervous system. Concurrently with processes in the CNS, the autonomic nervous system does not rest either: a pronounced activation of a whole range of vital autonomic functions is objectively recorded.
Based on these data, modern researchers define sleep as a full-fledged "second state of vital activity" of the body, existing alongside ordinary wakefulness.
Sleep as preparation for future activity
Another important concept of normal physiology views the biological significance of sleep through the prism of preparation for future loads. According to this theory, sleep is a state during which the body (and, first and foremost, the brain) actively prepares its systems for upcoming activity.
To better understand this mechanism, physiologists often cite a striking biological analogy: the state of a sleeping person is compared to the state of a developing fetus in the mother's womb. Just as the fetus covertly but continuously prepares for independent existence after birth, the sleeping person's brain forms the functional basis every night for successful wakefulness in the coming day.
Objective signs of sleep
The key and most striking objective sign of the onset of sleep is loss of consciousness. In the physiological context, this phenomenon is characterized by the complete disconnection of all active links of the subject with the objective surrounding world.
In a state of deep sleep, the central nervous system blocks the inflow of most sensory signals. As a result of this process, a person ceases to consciously perceive what is happening around them and completely loses the ability to respond to many environmental stimuli. It is precisely this deep depression of consciousness that allows the brain to seamlessly implement the active internal processes necessary for recovery.
Briefly: answers to topical questions
What phases and stages of sleep are distinguished on the electroencephalogram?
The electroencephalogram distinguishes the slow-wave sleep phase (NREM sleep) and the fast-wave (paradoxical) sleep phase (REM sleep). Slow-wave sleep includes the following stages:
- Drowsiness stage (Stage I) — disappearance of the $\alpha$-rhythm, appearance of $\beta$- and $\theta$-rhythms.
- «Sleep spindles» stage (Stage II) — appearance of sleep spindles.
- Theta wave stage — high-amplitude theta waves.
- Delta wave stage (Stages III-IV, delta sleep) — high-amplitude delta rhythm.
The rapid eye movement phase (REM sleep) is characterized by low-amplitude high-frequency activity, flattening of the curve, and the appearance of fast rhythms.
Which neurotransmitters are involved in the regulation and maintenance of sleep?
The alternation of phases and the maintenance of sleep are regulated by a complex of neurotransmitters and peptides. These include:
- Serotonin — a key sleep-inducing factor.
- Melatonin — ensures the synchronization of sleep phases.
- Inhibitory and opioid systems — GABA (gamma-aminobutyric acid), enkephalins, endorphins.
- Specific factors — delta-sleep-inducing peptide.
- Arousal/modulation mediators — acetylcholine, dopamine, adrenaline, histamine.
Glutamate and noradrenaline are also involved in the mechanisms of slow-wave sleep.
What methods are used for objective instrumental recording of sleep in normal physiology?
The main method of objective sleep research is polysomnography. This is a continuous recording of a number of physiological parameters during night or daytime sleep. Within polysomnography, the following are recorded:
- EEG (electroencephalogram).
- EOG (electrooculogram) — recording of eyeball movements.
- EMG (electromyogram) — recording of oral muscle tone.
- ECG (electrocardiogram).
- Electropneumogram — recording of chest respiratory movements and airflow through the upper respiratory tract.
The result of polysomnography analysis is the construction of a hypnogram.
Class and exam questions
What is the traditional theory of the biological significance of sleep?
Historically, sleep was perceived solely as passive rest necessary for restoring the energy reserves of brain cells after active wakefulness. Physiologists even compared this process to the anabiosis of protozoa, when metabolism slows down for survival.
Why is sleep called an active physiological process today?
Modern studies have proven that during sleep, neuronal activity in a number of brain structures does not decrease, and often exceeds daytime levels. Also, during this time, a pronounced activation of many autonomic functions of the body is observed.
What is the main objective sign of sleep distinguished in normal physiology?
The main objective sign is loss of consciousness. This is manifested in the disconnection of a person's active links with the external world and the absence of reactions to environmental stimuli during deep sleep.
What else to study on this topic
- Neurophysiological mechanisms of activation of brain structures during sleep
- Specifics of changes in autonomic nervous system functions in a sleeping person
- Detailed analysis of the concept of sleep as a state of preparation for wakefulness
- Mechanisms of disconnection of the subject's active links with the surrounding world
- Energy metabolism in central nervous system cells during state transitions
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