Abstract
In spaceflight human circadian rhythms and sleep patterns are likely subject to change, which consequently disturbs human physiology, cognitive abilities and performance efficiency. However, the influence of microgravity on sleep and circadian clock as well as the underlying mechanisms remain largely unknown. Placing volunteers in a prone position, whereby their heads rest at an angle of −6° below horizontal, mimics the microgravity environment in orbital flight. Such positioning is termed head-down bed rest (HDBR). In this work, we analysed the influence of a 45-day HDBR on physiological diurnal rhythms. We examined urinary electrolyte and hormone excretion, and the results show a dramatic elevation of cortisol levels during HDBR and recovery. Increased diuresis, melatonin and testosterone were observed at certain periods during HDBR. In addition, we investigated the changes in urination and defecation frequencies and found that the rhythmicity of urinary frequency during lights-off during and after HDBR was higher than control. The grouped defecation frequency data exhibits rhythmicity before and during HDBR but not after HDBR. Together, these data demonstrate that HDBR can alter a number of physiological processes associated with diurnal rhythms.
Highlights
The simulation models for weightlessness include immersion in water, head-down bed rest (HDBR), drop tower, parabolic flight and clinostat facilities. Of these approaches HDBR is one analogue that can simulate the influence of long-term weightlessness on human physiology [1,2]
The positioning this way can lead to redistribution of the body fluid, which stimulates central volume carotid, aortic and cardiac receptors inducing an increase in diuresis and natriuresis and a decrease in plasma volume
After HDBR, the average urination volume dropped to a lower level comparable to that of the control (Fig. 1 and Table 1)
Summary
The simulation models for weightlessness include immersion in water, head-down bed rest (HDBR), drop tower, parabolic flight and clinostat facilities. We show that during this HDBR, a number of physiological variables were modified, including urinary electrolyte and hormone levels, urination and defecation frequencies, respectively. Change of Urinary Electrolyte and Hormone Profiles We found that the urine volume displays a diurnal oscillation despite the weak rhythmicity (Fig. 1).
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