Circadian sleepwake rhythms

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The timing of sleep (but not its amount) is regulated by a circadian 'clock' in the suprachiasmatic nucleus of the hypothalamus. Without an environmental cue to time of day (or zeitgeber), the duration of the endogenous 'free running' sleep-wake cycle in humans has generally been considered to be about 25h, although doubt has been cast on this recently with the suggestion that the intrinsic circadian pacemaker is close to 24h in human adults consistent with other species. (10) From an early age the individual's sleep-wake rhythm has to synchronize with the 24-h day-night rhythm. The main zeitgeber by which this is achieved ('entrainment') is sunlight, but social cues, such as meal times and social activities, are also important, as well as ambient temperature or noise level, and internal body signals such as hunger, temperature, and hormonal changes.

The suprachiasmatic nucleus also controls other biological rhythms including body temperature and cortisol production with which the sleep-wake rhythm is normally synchronized. In contrast, growth hormone in adults is locked to the sleep-wake cycle and is released with the onset of SWS, whatever its timing.

Melatonin is related to the light-dark cycle rather than the sleep- wake cycle. It is secreted by the pineal gland during darkness and suppressed by exposure to bright light. It influences circadian rhythms via the suprachiasmatic nucleus pacemaker which, in turn, regulates melatonin secretion by relaying light information to the pineal gland. Its widespread popularity as a sleep-promoting agent is not justified by what little is known about its action and clinical effectiveness. (1.1)

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