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.
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