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Abstract. The sleep-wake cycle and the circadian rhythm of rectal temperature were recorded in subjects who lived sing[y in an isolation unit. In 10 subjects, the freerunning rhythms remained internally synchronized, 10 other subjects showed internal desynchronization. Times of onset and end of bedrest ("sleep") were determined in each cycle and referred to the phase of the temperature rhythm. In the synchronized subjects, onset of sleep occurred, on the average, 1.34 h before the minimum of temperature, and end of sleep 6.94h thereafter, with narrow distributions. The desynchronized subjects had a broad bimodal distribution of sleep onsets (peaks 6.3 and 1.3 h before the minimum); the duration of sleep varied between more than 15 h when sleep began about 10 h before the temperature minimum, and less than 4 h when sleep began several hours after the minimum. The dependence of sleep duration on body temperature is interpreted as a continuing action of the coupling forces between the two rhythms after mutual synchronization is lost.

In man as well as in animals, diurnal rhythms are based on endogenous, periodic processes which can be considered self-sustained oscillations in a technical sense2). Evidence for this is given by the observation that, underconstant conditions, the rhythm continues a) undamped and b) with a frequency which deviates from that of the earth's rotation6). It is this deviation of the free-running 'circadian '11) period from 24 hours which excludes external cues as possible causes of the rhythm. In many aspects, the whole organism behaves like a single oscillator which obeys the law of oscillation theory. Some findings, however, suggest that the circadian system consists of a multiplicity of oscillators which normally are synchronized with each other but which can become desynchronized under special circumstances13,14). Thishypothesis seems to be supported by recent results from experiments with human subjects which will be discussed below. METHODS Single subjects are kept in isolation and without watches in an underground soundproof bunker which is continuously illuminated. Next to the bed-sitting-room is a toilet and a small kitchen. The subject prepares his own meals. His rectal temperature is measured continuously by means of an electric thermometer and recorded outside the experimental chamber. The subject is asked to sample his urine in intervals of his own choice. The samples are stored in an ice box between the two lock-gates of the entrance from where they are removed for analysis. The subject can not perceive the time of removal. The records of several system of electric contacts, activated by the subject either consciously or unconsciously, serve to survey the subjects general activity. Communication between experimenter and subject is restricted to letters. Other details of the arrangements are given in two earlier publications4,8). RESULTS A subject who is kept in isolation and without a time-telling device con

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