Biological Rhythms: The Circadian Rhythm

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-circadian rhythms last about 24 hours.. two best known are sleep-wake cycle and the body temperature cycle.

-Youre sleep-wake cycle is governed by what time of the day it is (external cues). There is also an internal endogenous 'clock' that is free running. This works without any external cues. Yet under normal circumstances the internal clock does not work alone, main external cue is daylight so they help adjust your clock to where you live.

-Core body temp is a great indicator of the circadian rhythm.. lowest at 4:30am and highest around 6:00pm. There is a trough after lunch.

-Hormone production also follows a circadian rhythm. Cortisol is high at midnight and peaks aroud 6:00am. Melatonin and growth hormone both follow a clear circadian rhythm.


-Michael Siffre studied the human internal clock and spent long periods of time living underground to study his own biological rhythms. Underground he had no external cues. First underground stay of 61 days he resurfaced on…


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