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WHAT IS SLEEP: MAN'S BIOLOGICAL CLOCK

If persons are so shut off from the outer world that they have no external time-markers such as light, sound, or other changes that admit inferences to social life (as, for example, in the bunker experiments made by Ashoff), the surprising result among a majority of test subjects is that their biological clocks are oriented toward a 24-hour rhythm over long periods of time.

The 24-hour rhythm is caused by the rotation of the earth and the resulting alternation of light and darkness. But, surprisingly enough, the biological periodicity of animals and humans is not precisely fitted to this cycle. Actually the periodicity mostly shows a somewhat longer rhythmic pattern lasting about 25 hours, or occasionally even a shorter rhythmic period lasting about 23.5 hours. Only rarely is it shorter than 23.5 hours. In this context,

Halberg spoke of circadian periodicity, which is derived from "circa," meaning "approximately," and “dian,” meaning "corresponding to the day." In other words, circadian means related to the 24-hour cycle. Biological periodicity, which is oriented toward a 24-hour cycle, is a fixed rhythmic pattern. The cycle is in fact frequently longer than 24 hours, only rarely shorter, but it determines our biological feeling for time.

There are species of animals whose inner clocks are oriented toward rhythms other than the alternation between darkness and light. For example, the time-marker among many crabs and mussels is the ocean s tidal change, a 12 94-hour cycle, as the tide is a more important factor than light in the life of these species.

Hence it may be stated that biological clocks vary to some extent according to species. Birds, for example, have a differently oriented biological clock than fish, and fish in turn have a differently oriented biological clock than man. Within the species, however, there are early risers and late sleepers. Experiments with chaffinches (Aschoff) confirm this observation. Comparable to early risers among humans, birds used in the experiment started flying 1.5 hours before the light stimulus was turned on to wake them up. Other birds in the experiment only began to fly .5 hours after the light signal. In other words, they had a longer circadian cycle than the early risers, and although there is a biological clock peculiar to the species chaffinch, even among them there are individual short sleepers and long sleepers.

The sleep-wakefulness mechanism is not the only function controlled by the biological clock. All other vegetatively controlled bodily functions and many aspects of psychic behavior are also subject to this rhythm. The functioning of our own circadian periodicity can be observed even by the medical layman whenever there are any sudden, short-term changes in the accustomed time system such as occurs in flying east or west. Our biological clock requires several 24-hour cycles to regain its normal course and adjust to a new time system. Readjusting (resynchronization) usually takes longer on flights to the west than to the east. In addition, individual vegetatively controlled organic systems require varying periods of time for resynchronization. If the biological clock has not yet completely adjusted to the new time, both mental and physical efficiency is significantly impaired. For this reason, after longer east-west flights people should immediately allow for a rest of at least several hours to give their biological clocks some opportunity to adjust to the local time in the new setting.

The discrepancy between our biological clock, which has a rhythm of approximately 25 hours, and the 24-hour rhythm of our day and night cycle constantly compels us to reconcile the difference. Our sleep rhythm must also actively participate in this ongoing process of adjustment. The necessity of the sleeper to learn to adjust provides biological proof that life means, among other things, learning to overcome discrepancies or ostensible deviations.

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