[Changing gears in the human circadian mechanism]

Subjects who live in isolation and without information on time of day, develop, in all their functions, free-running circadian rhythms whose common periods usually scatter around 25 h. That means that the circadian system, though desynchronized from the 24-h day, remains internally synchronized. However, if the sleep-wake cycle gets longer than 28 h, or shorter than 22 h, the rhythms of autonomic functions uncouple from the sleep-wake cycle, resulting in a state of internal desynchronization. In internally synchronized as well as in desynchronized systems, the intervals between meals change in strong proportionality to the duration of wake time, nevertheless, neither the caloric intake per meal nor body weight are altered. Hence, there must be a saving of energy with increasing wake time. This is accomplished by a reduction of muscular work, as becomes evident from the negative correlation between locomotor activity per hour and wake time. On the occurrence of internal desynchronization, the regression lines describing the correlations mentioned above are shifted against each other in such a way that the consumption of energy adapts to the needs economically. This mechanism is comparable to the effects of a variable geer.