Sleep disruption, use of sleep-promoting medication and circadian desynchronization in spaceflight crewmembers: Evidence in low-Earth orbit and concerns for future deep-space exploration missions.

Introduction: The spaceflight environment presents unique demands on human physiology; among those demands, is sleep. Sleep loss and circadian desynchronization is a major concern for future deep- exploration plans, including long-term crewed missions to the Moon and Mars. Aims: Analyze evidence of sleep disruption in crewmembers during low-Earth orbit missions, identify the use of sleep-promoting medication among crewmembers and deepen the comprehension of challenges to sleep physiology for future missions to the Moon and Mars. Results: Evidence consistently indicates a loss of sleep and circadian rhythm disruption during low-Earth orbit missions. Sleep duration is shortened especially the night before a critical operation and during circadian-misaligned sleep episodes. The prevalence of sleep-promoting medication ranges between 71% and 78%; medication is more frequently taken on circadian-misaligned sleep episodes. Regarding the Moon, Apollo astronauts had variable sleep duration. For some, sleep was restful while others had poor-quality sleep. Many reported fatigue and errors due to the lack of rest. A loss of the 24-h light/dark might be expected due to the Moon's complex illumination characteristics. Regarding Mars, one main challenge will consist in synchronizing the circadian clock to a Martian day (24.65 h).