Influence of physical workload on freerunning circadian rhythms of man

While living under constant conditions and complete isolation from environmental time cues for about 4 weeks, 9 male subjects exercised on a bicycle ergometer seven times per 'day' during two weeks and refrained from physical activities during the other 2 weeks. The freerunning circadian rhythms of wakefulness and sleep and of rectal temperature showed, on the average, no difference between the two sections with regard to the autonomous period and the tendency towards internal desynchronization. Even in the one experiment in which the two rhythms became internally desynchronized, the periods of the rhythms remained unchanged during the time the subject worked on the bicycle. Only in one out of the nine subjects, the autonomous period was considerably longer under the influence of work than without it. The hypothesis is advanced that the period of an autonomous rhythm becomes normally independent of physical workload by way of a compensation mechanism.