BACKGROUND: Sleep, sedentary behavior (SED), and physical activity (PA) comprise the 24-hour activity cycle (24-HAC). Whether sleep timing or its variability are associated with SED and PA is unknown. This study examined associations between objectively measured sleep timing and variability with objective estimates of the other 24-HAC components (SED, light intensity PA [LPA], and moderate to vigorous intensity PA [MVPA]). METHODS: University students (n=103, 20.2±1.6y, 66% female, 66% white non-Hispanic) wore a 1) ActiGraph GT9X on the wrist, 2) activPAL3 on the thigh, and 3) ActiGraph GT3X on the hip for 7-10 days to capture habitual sleep, SED and LPA/MVPA, respectively. All participants had complete 24-HAC data from at least four weekdays and two weekend days. Times in and out of bed were derived by combining data from sleep logs, primary lying times from the activPAL3, and sleep bouts from the GT9X. The average sleep midpoint was used as an indicator of sleep timing and the standard deviation of the midpoints indexed sleep timing variability. General linear models estimated sex and sleep duration adjusted associations between sleep midpoint and sleep variability with other 24-HAC behaviors. Estimated changes in 24-HAC behaviors per one-minute increase in sleep metric (β), semipartial omega square (ω2) effect sizes, and p-values are presented. RESULTS: Mean±SD values for key measures were: sleep duration (452.4±66.3 min), sleep midpoint (5:00 am±1:27 hr:min), sleep variability (58.4±24.2 min), SED (622.8±94.6 min), LPA (305.5±74.6 min), and MVPA (59.4±35.0 min). Adjusted associations between sleep midpoint and 24-HAC behaviors were significant for SED (β=0.294, ω2 =.063, p=.002) and LPA (β= -0.227, ω2 =.055, p=.001) but not MVPA (β= -.072, ω2 =.022, p=.062). Adjusted associations between sleep variability and 24-HAC behaviors were only significant for MVPA (β= -.379, ω2 =.057, p=.006). These associations persisted when both sleep midpoint and sleep variability were jointly modeled with minimal changes to model estimates. CONCLUSIONS: In a sample of university students, independent of sleep duration, later sleep times were associated with increased SED and decreased LPA while greater variability in sleep timing was associated with decreased MVPA. Future experiments should examine if these relationships are causal.

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