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Academic Word List: Exercise 7

Read the following text, paying particular attention to the highlighted words.

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.

  • Why do individuals have free-running rhythms that differ in length? Even though the average value is 25 hours individuals show values generally within the range 24-26 hours. (As suggested in Chapter 1, "larks" tend to be at the lower end of the range and "owls" towards the top of it.)
  • Where are the sense organs that pick up such external factors? There is evidence for a magnetic sense in some animals, including humans. However, its role in influencing these rhythms - let alone actually being responsible for them - is not at all established. Sense organs for lunar and planetary influences, for atmospheric pressure and cosmic rays are as yet purely hypothetical.

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.

  • It is not clear how the duration of sleep could be controlled as regularly as is observed to be the case. Even though it might be argued that a regular life-style implies a regular degree of fatigue and so will require a regular amount of sleep, in practice, such a degree of regularity is not achieved. Individuals go to bed later or earlier than average on certain occasions, as can be seen by close inspection of Fig. 2.2. For example, an individual might go to bed later than usual on one occasion because a piece of work or some leisure time activity took longer than normal. It is likely that this will make him more tired than usual and so we might guess that he will sleep longer. This would result in his rhythms running more slowly when he was interested and more quickly when he was bored (because he would go to bed earlier due to the lack of something interesting to do and so require less sleep). This idea - that longer activity spans would be followed by longer sleeps and vice versa - does not find experimental support. The reverse is seen: long "daytimes" tend to be followed by shorter sleeps and vice versa. This is a result that is much easier to account for if we suggest that a body clock is responsible for the alternation between sleep and activity. Such a clock will wake an individual when a certain stage of the sleep/wake cycle is reached; if he goes to bed late then that stage will be reached after a shorter period of sleep than usual. Such is the experience of most of us when we go to bed late: we might sleep slightly later than usual but rarely long enough to compensate completely for the late night. Our body clock has woken us for the next day after only a minimal opportunity for extra sleep.
  • When all results from the free-running experiments are considered, the sleep/wake cycle is about 25 hours. If it were due to some "memory" of our life-style then we would predict that about half the population would show a value less than 24 hours and the free-running periods of individuals would be distributed fairly symmetrically about an average value of 24 hours.

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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