Effects of a 45-day head-down bed rest on the diurnal rhythms of activity, sleep, and heart rate

In space, circadian rhythms are prone to alteration. Head-down (–6°) bed rest (HDBR) is an approach simulating the weightlessness condition, which allows effects of weightlessness on physiological variables to be studied on the ground. In this work, we analyzed the changes in diurnal rhythms of activity, sleep-wake cycles, and cardiovascular responses in a 45-day HDBR. All subjects exhibited increased daytime sleep, whereas the total sleep time was comparable. The results of wrist activity monitoring showed a significant reduction during HDBR and recovery after HDBR. Moreover, changes in the average frequency, amplitude, and phase of heart rate were observed. Analysis of heart rate variability showed a decrease in both high-frequency and low-frequency power after HDBR. Together, these data demonstrate that HDBR can alter the diurnal rhythms of a number of physiological processes, some of which might be attributed to reduction in activity of the autonomic nervous system.