Introduction: Microgravity is known to alter various physiological variables in humans. However, changes in neuroanatomy, cognition, and sleep have not been significantly studied in the literature. The aim of the present study was to determine the effect of seven-days of supine unloading on a supersaturated saline-filled water bed (hyper-buoyancy floatation, HBF), a novel Earth-based microgravity analogue, on cognition and sleep.

Materials and methods: Twelve healthy male subjects were subject to seven continuous days of HBF, during which subjects' (floatonauts') major cognitive domains (psychomotor speed, attention, memory, executive function, and social cognition) and sleep were monitored by CANTAB testing batteries and 64-lead EEG respectively. Ten control subjects underwent identical cognitive testing regime protocol for comparison.

Results: The unloading period of seven days resulted in a significant (p< 0.05) impairment in psychomotor speed and spatial working memory. However, a significant improvement was recorded in several domains of verbal memory. Various physiological changes, including the altered sleep architecture were recorded in floatonauts. Seven days of exposure lead to overall decreased sleep time, decreased non-rapid eye movement 2 sleep stage (N2) duration and decrease in overall NREM duration. Conversely, an increase in mean REM duration, sleep latency, along with increases in height and spine length were demonstrated. Postural changes appeared negatively related to changes in NREM duration, and verbal memory performance.

Conclusions: Overall, the results suggest several cognitive and physiological changes induced by an intervention period of only seven days of HBF. These are hypothesized to occur due to the changes in neuroanatomical homeostasis, cephalic body fluid shift, physical activity, and sleep architecture. Further studies are required to investigate potential mechanisms underlying the observed multiple domain cognitive changes and their possible reversal.