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Alternatives to the Concept of a Body Clock
Another major problem that we must consider is the possibility that the continuing rhythmicity is not due to the body clock. Two alternative explanations, and our comments on them, follow.
1. The rhythm is responding to an external influence that has not been controlled in the experimental protocol.
The problem is to decide what such an influence might be. The influences upon humans of the planets, moon, and factors such as magnetic fields, atmospheric pressure, and cosmic rays have been imagined by some, including lovers, astrologers and those who, in the past, have diagnosed types of "lunacy". Unfortunately, when such influences are considered as explanations of the results of free-running experiments, the following problems have never been satisfactorily dealt with.
2. The results are due to a regular structuring of an individual's life-style; they are a reflection of the regularity of our habits rather than of some body clock.
There is much to be said for such a theory. There is no doubt that individuals tend to structure their day around routines of meals, personal hygiene, leisure, work, etc. There are, however, some problems with this theory.
We have stressed the regularity that is observed in free-running experiments and interpreted it as evidence for the body clock. However, we should add that, particularly when experiments lasting several months are performed, certain irregularities do creep into the results. Occasionally sleep is missed, or is twice as long as usual. Sometimes volunteers alternate between normal and long sleeps (lasting 16 rather than 8 hours, for example) or even appear to adopt a sleep/wake cycle that lasts about 50 rather than 25 hours. In such circumstances, rhythms of food intake also can become irregular with missed or extra meals during the course of a single "day". However, the rhythm of body temperature generally retains its regular 25 hour period. These results do not require us to dispense with the idea of an internal body clock, if only because they appear very rarely in experiments lasting only a week or so. (By contrast, the 25-hour rhythms are regularly seen). Instead the results suggest that the system requires an occasional rhythmic input from the outside world to run smoothly. The alternatives to an internal body clock (planetary influences, cosmic rays, etc.) do not offer a ready explanation of these irregularities.
Now try the exercises. Exercise a
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