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Circadian Rhythms |
Every day, as night follows day, we humans unknowingly experience profound changes in our body's physiology. Human body tremperature varies by 2 to 4 degrees Celsius, peaking in the late afternoon, and troughing in early morning (even if the person is confined to bed). Pulse rate and blood pressure peak around the same time as temperature. We experience cycles of hunger, sleep and wakefulness. Urinary output and protection against disease are altered cyclically. Body levels of glycogen are lowest around 3 - 6 A.M., without our knowing it. These cycles or CIRCADIAN RHYTHMS are not passive responses to stimuli, but are pre-adapted and driven by an internal or biological "clock" operating on a 24-hour day. (Here, "circadian" means approximately since the 24 hour period may vary slightly.) Circadian timekeeping is a basic property of all higher forms of life. Hibernation, fur growth and migration in animals are established months in advance by the internal clock as daylight changes in length.
The internal clock can be desyncronized by working night shifts, in jet lag and in certain forms of mental depression. The clock typically takes the better part of a week to adapt to a new pattern.
In taking some medicines circadian rhythms can be a factor, since the metabolism of some drugs can be affected. See my book, "Drugs and the Human Body", 2004 Seventh Edition, Prentice Hall for a discussion.
What are the actual body mechanisms that reset the biological clock? Current theory suggests that as daylight enters our eyes and impinges on the retina (the light-sensitive lining on the back of the eyeball), a protein called MELANOPSIN is produced. It in turn may stimulate the release of a body hormone termed MELATONIN. In this process, the portion of the brain's hypothalamus associated with circadian rhythms is affected.
In summary, circadian rhythms are daily, predictable physiological changes driven by a kind of internal "clock". Photoreceptors in the human retina produce proteins that communicate information about night and day to the body's internal clock.
Interestingly, the human hormone MELATONIN is widely sold in health food stores without governmental control because it is promoted as a food and not a drug. It is touted for treatment of jet lag and insomnia, even though it has no observable hypnotic (sleep-inducing) action. Also, in a contradiction of the ads, a Harvard Medical School study showed that it is not true that older folks have lower body levels of melatonin.
For much more, browse your computer's seach engines (such as Google, Mamma, Lycos, etc.) on the topic "circadian rhythms".. | | |
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