Endocrine activity during sleep
Almost all functions of humans are subject to cyclic changes and are governed by the nervous system. Most rhythms are driven by an internal biological clock located in the hypothalamic suprachiasmatic nucleus (SCN) and can be synchronized by external signals such as light-dark cycles. Homeostatic activities such as body temperature, blood volume, water balance and sleep, are rhythmic. Likewise, most hormones are secreted in a rhythmic fashion. Both sleep and circadian effects interact to produce the overall rhythmic pattern of the pituitary and pituitary-dependent hormones. Some of the 24-h hormonal rhythms depend on the circadian clock (ACTH, cortisol and melatonin), or are sleep related (prolactin and TSH). GH secretion is influenced by the first slow wave sleep (SWS) episode at the beginning of the night. Pulses of prolactin and GH are positively linked to increases in delta wave activity, i.e. deepest phases of sleep, occurring primarily during the first third of the night. Pulses of TSH and cortisol are related to superficial phases of sleep. As a result of the consolidation of the sleep period in humans, the wake-sleep transition is associated with physiological changes with the endocrine system being part of the adaptive mechanism to reduce physical activity during sleep.
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