Circadian rhythms in a long-term duration space flight.

In order to maintain cosmonaut health and performance, it is important for the work-rest schedule to follow human circadian rhythms (CR). What happens with CR in space flight? Investigations of CR in mammals revealed, that the circadian phase in flight is less stable, probably due to a displacement of the range of entrainment, resulting from internal period change (the latter was confirmed on insects). The circadian period may be a gravity-dependent parameter. If so, the basic biological requirement for the day length might be different in weightlessness. On this basis, a higher risk of desynchronosis is expected in a long-duration space flight. As a countermeasure, a non-24-hr day length could be suggested, being close to the internal circadian period (in humans about 25 hr). Taking into account a possible displacement of period in weightlessness, it seems reasonable to establish a flexible work-rest schedule, capable to follow the body temperature CR by means of biofeedback.