Intrinsic rhythms

Many body functions vary cyclically with a period of about a day; they have a circadian rhythm. Functions regulated in this way involve sleep or wakefulness, core temperature, and the secretion of anterior pituitary hormones. The Humans isolated from all external time cues demonstrate intrinsic circadian rhythms with a period of about 25 hours initially. This decoupling of circadian rhythms to the normal 24-hour period of time is called free running and shows that there exist intrinsic circadian clocks that are usually entrained by environmental cues called zeitgebers (German “time-giver”). Zeitgebers involve exercise, light, social interactions and work schedules. Light is the strongest. A powerful light pulse given during subjective night produces shifts in the circadian rhythm. In humans with normal sleep patterns the nadir in core temperature occurs at about 5 a.m. a light pulse providing during the night before this time causes circadian rhythms to be delayed (phase delay) whereas a light pulse after this time causes phase advance.

After 1–2 weeks free running physiological variables often desynchronize from every other. For instance, typically, secretion of ACTH, fluctuations in core temperature and glucocorticoids and rapid eye movement (REM) sleep continue with a period of about 25 hours but the cycles of secretion of growth and sleep–wakefulness hormone (GH) lengthen to over 30 hours. This suggests in which there are two circadian clocks. Both neither are normally entrained through light–dark cycles of night and day.