Sleep is regulated by homeostasis – the balance of chemical reactions in the body – and circadian rhythms, which depend on the time and season. Homeostatic regulation occurs due to the fact that certain neurotransmitters accumulate in the central nervous system, which subsequently activates the process of falling asleep. Meanwhile, the melatonin hormone is involved in the circadian rhythm as it provides an impetus for falling asleep in the evening, after sunset.
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It is customary to divide sleep into two main phases: slow or NREM – non-rapid eye movement – and fast or REM – rapid eye movement. The slow phase takes up about 75% of the total sleep time and is divided into three stages – N1, N2, and N3. N1 is the intermediate stage between falling asleep and waking, the so-called “shallow sleep.”During it, various manifestations of hypnagogia occur: for example, a hypnagogic jerk – the sensation of falling and involuntary startle when falling asleep. Subsequently, the N2 stage takes up half of the total sleep time. Passing into it, a person ceases to be aware of their surroundings, and the body temperature begins to decrease. Finally, N3 is the deep slow-wave sleep during which blood pressure drops and breathing becomes slower; the muscles are relaxed, and blood flow to them increases. According to Fultz et al. (2019), “during NREM sleep, low-frequency oscillations in neural activity support memory consolidation and neuronal computation” (p. 628).
REM sleep takes about a quarter of the total sleep time. This phase, for the first time, occurs about an hour and a half after falling asleep and repeats every hour and a half – each time becoming longer. During this phase, eyes move quickly back and forth, dreams occur, and the body is motionless: all skeletal muscles, except for the oculomotor muscles, are as relaxed as possible.
Sleep disturbances and lack of sleep on a regular basis can cause a number of health problems. First of all, the brain and nervous system suffer those who sleep little often have mood changes and problems with memory, thinking, and concentration. Nollet et al. (2020) state that “animals and humans experiencing partial sleep restriction usually exhibit detrimental physiological responses, while total and prolonged sleep loss could lead to death” (p. 1). Thus, sleep is crucial for mental health and the normal functioning of the brain.
Fultz, N. E., Bonmassar, G., Setsompop, K., Stickgold, R. A., Rosen, B. R., Polimeni, J. R., & Lewis, L. D. (2019). Coupled electrophysiological, hemodynamic, and cerebrospinal fluid oscillations in human sleep. Science, 366(6465), 628–631. Web.
Nollet, M., Wisden, W., & Franks, N. P. (2020). Sleep deprivation and stress: A reciprocal relationship. Interface Focus, 10(3), 20190092. Web.