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Circadian Rhythms - 1

Agatha M. Thrash, M.D.
Preventive Medicine

The subject of circadian rhythms has received much attention in recent years because of some degree of understanding of the biologic time clocks, a new area of interest in scientific circles over the past 25 years. Throughout nature, both in the chemical as well as in the biologic realms, we find definite cycles or rhythms.

In biological circles we find definite cycles varying in length from a few seconds to several years in length. Probably the best known of these cycles is the 28-day menstrual cycle in women, and a similar 28-day hormone cycle in men. There is an 18-month hair growth cycle in humans characterized by hair growth for 18 months and then cessation of growth for 18 months. In men during the cessation of growth, the hair usually drops from its follicle, whereas in women the follicle holds onto the hair, enabling women to grow hair of great length. In isolated individuals, this custom may work the other way around with men holding onto the hair and women dropping it. In some men the genetic predisposition is such that the hair follicle does not start growing again during the next 18-month cycle. In such individuals, baldness will result.

Other well-known rhythms include 24-hour fluctuations of various enzymes and hormones that control states of wakefulness, digestion, and a sense of well-being. There are also cycles that involve only parts of a day, some of 90-minute length, and others of varying lengths. There are weekly cycles, annual cycles, and seventh annual cycles known. They have been charted not only in humans but also in other animals, such as fiddler crabs, salmon, and brown bears, to say nothing of a large number of plants such as beans, the sensitive plant, and mimosa. Many other plants have well-known daily cycles in which the leaves are folded together or bowed down depending on the species.

The human being is ideally adapted to be a lark and not an owl. Every morning all human beings, whether or not they are programmed to do so, will make some attempt to wake up its metabolic systems and start the day with a bang. There are some individuals who depress or even partly abolish this cycle by the way they work or stay up late at night. Nighttime workers such as hospital workers and others have been studied and show a flattening of their curves rather than a definite reversal of the up and down cycle of many hormone and enzyme levels in the blood. The loss of a distinct pattern places the body under a strain with a resultant stressful physical condition, if great care is not used to prevent the development of stress.

Normally at about 3 a.m., the hormone and enzyme support to such functions as muscular strength, cheerfulness, proper thinking, digestion, the production of urine, and even respiration is profoundly depressed. If the individual usually arises at around 7 a.m., the pituitary will release ACTH at about 5 a.m. ACTH stimulates the adrenals and adrenaline is secreted in small quantities beginning at about 6 o'clock. This small quantity is sufficient to arouse the thyroid, ovaries, testis, stomach, pancreas, liver, and other organs, so that by 9 a.m., all of one's systems are at their peak. Our various sensory perceptors are also at their peak in the early morning, including taste, smell, hearing, and reaction to noises and noxious stimuli. Pain tolerance is greatly influenced by our rhythms, as are allergies, histamine reactions, and other reactions of the body. We are less tolerant of pain in the afternoon and evening than at any other time during the day, unless it is at around 3 a.m. Protein eaten at 8 a.m., rapidly raises the amino acid levels in the blood, but the same meal taken at 8 p.m., is not so nourishing. Muscular coordination involving fine muscle tasks reaches its peak performance around midafternoon or early evening, probably associated with the peak rise in temperature of the body, which is one or two degrees higher than the morning temperature. The cool temperature of the body is more conducive to strength of muscular activity, but the finer degrees of performance can be done at a higher temperature.

Probably the most unfortunate of all reversions of circadian rhythms occurs in the swing shift type of arrangement, where an individual is required by his job to work evenings one week, mornings the next week, and afternoons the following week. In laboratory animals a significant reduction in average lifespan can be shown by simply inverting the light-dark cycle once each week.

If one wishes for his circadian rhythms to give good support to muscle strength, cheerfulness, good digestion, and proper cleansing of the blood, there are certain things that can be done in the lifestyle to improve these and other functions, even including the memory and concentration. These lifestyle alterations include doing at the same time each day all those things that can be put on a schedule. Meals, bedtime, arising time, nap time, exercise, baths, studying, and as many other activities that are usually done daily should be assigned a definite time. Having a set time for study and other heavy mental functions enables one not only to insure getting these activities accomplished but allows whatever support the circadian rhythm can give to serve its intended function. Have a pattern for daily exercise, which ideally should be performed out-of-doors. Learn to maintain a relaxed attitude about your schedule, but be careful to stay as, close as possible to that which represents the ideal.

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