A time dependent biological rhythm roughly synchronised to the earths day-light night time 24 hour pattern. Light is used in man to provide better synchronisation via for example the hypothalamic suprachiasmatic nuclei but the imperfect feedback leads to significant conditions such as jet-lag.
- Secretion of melatonin which induces sleepiness. The secretion is maximal at night and is involved in the regulation of sleep and wake.
- Secretion of ACTH
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