Rhythm of the rectal temperature during a 6-month free-running experiment.

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Abstract
A volunteer subject was placed free running for 6 months in a cave. Among other parameters, his deep rectal temperature was continuously recorded by periods of up to a month and covering a total of 3 months. A circadian rhythm of this temperature was maintained during the whole of the isolation, the period progressively lengthening to reach an average value of 24 hr., 44 min. during the last 4 months with, nevertheless, relatively high fluctuations always between 18 and 31 hr. (SD: 2 hr., 25 min.). There was no noticeable change either in the extreme values or in the amplitude of the temperature variations. The rhythm of activity was, on the other hand, much more irregular and unable to impose its rhythm on the temperature. In spite of this, close relationships were found between the 2 rhythms which had an almost synchronous development throughout the whole experiment. These facts are not contradictory if one accepts the hypothesis of an internal influencer which controls the temperature, the activity, and the psychological estimation of time. But if the rhythm of the temperature reflects faithfully the functioning of the influencer, the rhythm of activity may elude it but only by becoming bi- or demicircadian, which would enable it to remain phased with the rectal temperature.