Evolution of sleep and wakefulness organization in Macaca mulatta during Spacelab flight simulation

NASA and CNES agencies planned to perform neurophysiological studies aboard Spacelab using rhesus monkeys as human models. The present study was conducted to assess the influence of restraint and confinement conditions on the circadian sleep-wake rhythm before an actual flight. Three experimental 18-day flight simulations were performed on 6 adult Macaca mulatta, chair-restrained in a high-fidelity mock-up of the Spacelab compartment. Wakefulness, non-REM sleep and REM-sleep were determined by analysis of animal behavior from video recordings. As the experiment progressed significant changes were observed, characterized by a progressive increase of non-REM sleep of around one hour, mainly during the light period (7 am - 11 pm) at the expense of wakefulness. Duration and number of sleepiness episodes and phases of inactivity increased. A clear, new, pattern of sleep-wake cycles occurred, mainly in the last 3 days with a sharp rise in total non-REM sleep. The observed disturbances were determined to be a consequence of the confinement, the limited space and the reduced motor activity as well as of restraint. The evolution of sleep-wake cycles in 3 stages reflected the fact that the organism appeared to have adapted to the new situation. These results suggest that future studies of monkeys during orbital flight should take into account the effects of external factors such as the conditions of confinement and of restraint as well as the effects of microgravity.