Dynamics of sleep patterns during prolonged simulated weightlessness

Subjects immersed for 6 days in water, simulating weightlessness, exhibited marked changes in the total time spent in sleep and in the percentage of that time spent in each sleep stage. The length of time spent in Stage 4 deep sleep and in REM sleep decreased in the first half of the 6-day immersion period. During the latter half of the immersion period, a clear adaptive response was observed in the sleep patterns: the sequential appearance of the different stages of sleep and the percentage of time spent in the six stages of sleep showed a clear approach to the pre-immersion data. During the 3-day post-immersion period, the patterns approached the normal pre-immersion levels, although there was some overshoot or a negative phase period characterizing this approach. By the sixth day of immersion, the percentage of time spent in each stage of sleep had returned to near normal pre-immersion values, but the total time spent in sleep was still somewhat less than normal. This fact suggests that, once adapted to a condition of weightlessness, the sleep requirement may be less than during normal ambulatory life. During the post-immersion period, the pattern of Stage 4 sleep at early evening, midnight and dawn showed no special reactive characteristics, but rather a clear recovery towards the original pattern. On the other hand, at the duration of REM sleep, it overshot the pre-immersion values at the beginning of the post-immersion period, and then returned to the original pattern. These experiments suggest that some of the changes in sleep patterns which are observed under conditions simulating weightlessness may be adaptive.