Circadian rhythms, clock-mechanisms, are common in animals and insects. Some are exogenous, that is, they are extinguished by blinding animals by darkness or confusing them psychologically with feeding stimuli. Goldfish can be taught to trip levers to release their food, which they learn to do according to a daily rhythm. Other mechanisms are endogenous; they continue for many days, or even weeks, in animals living in perpetual light or perpetual darkness. In the rat, the serotonin concentration in the pineal gland1 rises by day and falls by night. The rhythm continues in continuous darkness, but normally is cued to environmental lighting. It can be destroyed by destruction of sympathetic fibers, or by reserpine feeding to interfere with actions of monamine oxidases.
Similar rhythms in man are recognized but have not been studied in their relationship to disease. In infection, the circadian rise and fall in the temperature, recognized in
CIRCADIAN RHYTHMS. JAMA. 1965;194(3):291–292. doi:10.1001/jama.1965.03090160069021
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