Abstract Introduction Studies consistently find that humans average approximately six hours of sleep per night in space, which is less than they sleep on Earth. It is unclear whether the sleep loss observed during spaceflight is related to modifiable factors, such as irregular scheduling and inadequate sleep environment or due to features of spaceflight that may alter physiology, such as microgravity. Recent missions have afforded crew better, more stable sleep schedules, and an improved sleep environment. Hence, the evaluation of sleep under these improved conditions should provide insight into the causes of sleep deficiency observed in space thus far. Methods Crewmembers (n = 19) wore actiwatches for two bouts of data collection lasting two weeks each before flight (at approximately L-270 and L-180), either continuously (n = 9) or for two weeks every two months while in space (n = 10), and for seven days postflight, immediately upon return to Earth. We compared sleep outcomes (sleep duration, wake after sleep onset [WASO], sleep efficiency, and sleep latency) between phases of flight using mixed effects models, with participant and intercept included as a random effect. Results Crewmembers averaged 7.3 +/- 0.4 hours of sleep per night in space. Though this was less than they achieved preflight (7.9 +/- 0.5) or postflight (7.8 +/- 0.8; F (2, 2866) = 86.26, p < .0001, ηp2 = .06), this duration of sleep meets the recommended amount for optimal human health and well-being. Sleep efficiency (%) also increased inflight (88.7 +/- 2.7) compared to preflight (84.7 +/- 5.5) and postflight (84.4 +/- 6.1; F (2, 2630) = 124.08, p < .0001, ηp2 = .09), while WASO (min) decreased inflight (29.6 +/- 8.2) compared to preflight (46.4 +/- 15.5) and postflight (48.7 +/- 16.0); F (2, 2629) = 285.90, p < .0001, ηp2 = .18). There were no changes across these variables over time inflight. Conclusion We conclude that humans are capable of achieving sufficient sleep in space. Going forward, it is imperative that crewmembers are provided with stable schedules, with moderate workload, and environments that are conducive to sleep. Support (if any) NASA Human Factors and Behavioral Health Element, NASA Human Research Program