Bidirectional Associations Between Habitual Nighttime Sleep and Physical, Physiological, Cognitive, Psycho-perceptual and Injury Outcomes in Athletes and Physically Active Individuals: A Systematic Review

Background: Sleep has been suggested as important for performance and wellbeing in physically active individuals through to athletes.Experimental sleep deprivation studies show that sleep loss can impair performance, but less is known about bidirectional associations between naturally occurring habitual sleep and performance-related outcomes in real-world settings.Objective: To systematically synthetise observational evidence on bidirectional associations between habitual sleep and performance-related outcomes in athletes and physically active individuals.Methods: Following PRISMA 2020 guidelines, this review searched data from six electronic databases (PubMed, Web of Science, SPORTDiscus, ProQuest, CINAHL, and APA PsycINFO).Eligible studies included athletes or physically active individuals and reported associations between naturally occurring habitual nocturnal sleep and physical, physiological, cognitive, psycho-perceptual, injury and competition outcomes.Observational studies examining habitual nocturnal sleep as either an exposure or an outcome were eligible and were assessed using the Newcastle-Ottawa Scale.Results: Sixty-eight observational studies were included.52 studies examined sleep as an exposure, 23 examined sleep as an outcome, and seven examined associations in both directions.In sleep as an exposure direction, the most consistent evidence was observed for psycho-perceptual, competitive, and injury-related outcomes, with better sleep associated with more favourable mood, lower injury risk, and better competitive results.Physical outcomes showed more heterogeneous findings, with associations most apparent for explosive neuromuscular, sport-specific outcomes, while cognitive evidence was mainly related to reaction time, vigilance, and processing speed.In sleep as an outcome direction, psycho-perceptual factors, particularly fatigue, stress, tension, and anxiety were the most consistent predictors of subsequent sleep, while physical and physiological training load showed more variable, context-dependent associations.Conclusion: Habitual sleep is associated with performance-related outcomes, but the evidence supports a dynamic bidirectional relationship rather than a simple one-way pathway.Future research should use longitudinal repeated measures designs that assess sleep and performance simultaneously, alongside factors that may influence both.