Processes during sleeping

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Processes during sleeping

Sleep is a global state, the control mechanisms of which are manifested at every level of biological organization, from genes and intracellular mechanisms to networks of cell populations, and to all central neuronal systems at the organismic level, including those that control movement, arousal, autonomic functions, behaviour and cognition. Many brain loci have specific functions in sleep at each of these organizational levels.

One of the main sleep regulators is circadian clock system. The genetically controlled molecular circadian clock is synchronously expressed collectively and individually by each of the ~20,000 cells of the mammalian suprachiasmatic nucleus (SCN), which is situated bilaterally in the hypothalamus, just above the optic chiasm. The SCN contains the ‘master clock’ mechanism, the entrainment of which by the daily light–dark cycle sets the 24-h rhythm for all other physiological rhythms in the organism. The SCN does this, in part, by controlling molecularly related peripheral cellular oscillators that exert local control over physiological rhythms at sites closer to the expression of these rhythms. Mammalian circadian rhythms are maintained intracellularly by interlocking positive- and negative-feedback control of the transcription (and subsequent translation to protein) of three period genes (Per1–3), two cryptochrome genes (Cry1,2), and the Clock and Bmal1 (brain and muscle ARNT-like 1) genes. SCN neurons communicate circadian time to other brain structures primarily by action potentials that are mediated by sodium channels, and most SCN cells contain the inhibitory transmitter GABA (γ-aminobutyric acid).

The hypothalamo–pituitary–adrenocortical (HPA) system mediates the reaction to acute physical and psychological stress. The stress reaction is a prerequisite for the individual survival. It starts with the release of corticotropin-releasing hormone (CRH) from the parvonuclear cells of the hypothalamus. This results in the secretion of corticotropin (ACTH) from the anterior pituitary and finally in the secretion of cortisol from the adrenocortex. Research during the last three decades has revealed a bidirectional interaction between the sleep EEG and the HPA system. During sleep, both the nadir and the major portion of the secretion of ACTH and cortisol occur. It is well established that during ageing, sleep quality deteriorates continuously, particularly due to between decreases in SWS and SWA. Furthermore, levels of most hormones, including GH, decrease. Controversial reports exist however, on the effect of age on HPA hormones. Elevated and unchanged cortisol levels have also been reported in the elderly. The study including the largest sample of normal human adults over a lifetime reported age-dependent increases of mean cortisol levels and of the nadir and, selectively in women, of the acrophase. The rhythmicity of cortisol was preserved in the elderly, but the amplitude was dampened and the morning rise was advanced in time.

Notable qualitative and quantitative changes in sleep occur with age. Moreover, many sleep-related disorders occur with increasing frequency among elderly people. Typically, there is a phase advance in the normal circadian sleep cycle: older people tend to go to sleep earlier in the evening but also to wake earlier. They may also wake more frequently during the night and experience fragmented sleep. The prevalence of many sleep disorders increases with age. Insomnia, whether primary or secondary to coexistant illness or medication use, is very common among elderly people. Rapid eye movement (REM) sleep behaviour disorder and narcolepsy, although less common, are frequently not considered for this population. Periodic leg-movement disorder, a frequent cause of interrupted sleep, can be easily diagnosed with electromyography during nocturnal polysomnography. Snoring is a common sleeprelated respiratory disorder; so is obstructive sleep apnea, which is increasingly seen among older people and is significantly associated with cardio- and cerebrovascular disease as well as cognitive impairment.