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Astronauts in space often experience sleep loss. In the AGBRESA (Artificial Gravity Bed Rest) study, we examined 24 participants (mean age ± SD, 33 ± 9 years) during two months of 6 o head-down tilt (HDT) bed rest, which is a well-established spaceflight analogue. Polysomnography was recorded during baseline (BDC-9), HDT (nights 1, 8, 30 and 58) and recovery (R, nights 1 and 12). Mixed ANOVAs with post-hoc step-down Bonferroni adjustment indicated that compared to BDC-9, arousals were increased, while sleep duration, N3, and sleep efficiency were all decreased during HDT. Significant quadratic associations between sleep duration and quality with time into HDT did not indicate adaptive improvements during the course of HDT. While sleep duration recovered quickly after the end of bed rest, participants still displayed protracted sleep fragmentation. We conclude that physiological changes caused by exposure to microgravity may contribute to persistent sleep deficits experienced during real space missions., Competing Interests: Competing interests: The authors declare no competing interests., (© 2024. The Author(s).)