Physical activity participation in young adulthood is typically explained by motivational and environmental determinants; however, regulatory models of daily behaviour suggest that transient fluctuations in sleep quality, circadian preference, and stimulant use may also be associated with how individuals appraise effort-related demands. Within this behavioural-temporal regulatory perspective, perceived barriers to physical activity may be related to variations in functional energy, alertness, and temporal alignment rather than solely stable contextual constraints. The present cross-sectional study examined whether insomnia symptoms (sleep initiation and awakening problems), chronotype, and daily caffeine intake were concurrently related to perceived personal, social, and environmental physical activity barriers in 788 university students (18-27 years). Standardized self-report measures were administered under controlled assessment conditions. Pearson correlations and theory-informed hierarchical regression models were applied. Sleep initiation problems demonstrated very weak positive correlations with total and domain-specific barriers (r = 0.12-0.17), whereas awakening problems showed very weak inverse correlations (r = -0.10 to -0.14,≤ 0.005). Chronotype was weakly associated only with personal barriers (β ≈ -0.09,= 0.013). Daily caffeine intake showed a weak negative association with environmental barriers (β ≈ -0.15,< 0.001). Across models, explained variance remained limited (adjusted R= 0.040-0.053), indicating that these variables explained only a very small proportion of variance in perceived physical activity barriers. These findings suggest that sleep-related and chronobiological characteristics are not meaningful independent predictors of perceived physical activity barriers in this population and demonstrate only weak, domain-specific, and non-directionally consistent associations. Accordingly, the findings should be interpreted cautiously as exploratory rather than practically predictive. Given the cross-sectional design and low explained variance, the results primarily highlight the limited explanatory utility of these psychobiological factors relative to broader unmeasured contextual determinants. Longitudinal and time-sensitive designs incorporating objective behavioural assessments are required to clarify temporal ordering and potential regulatory mechanisms. p p p2
