Reallocation of 24-hour physical behaviour composition and mortality: exploring effect modification by sleep characteristics

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ABSTRACT Background Time compositions spent in physical behaviours (sleep, physical activity, posture) is associated with premature mortality, but the moderating role of sleep remains unclear. This study investigates the associations of time reallocations between physical behaviours with all-cause, cardiovascular disease (CVD) and physical activity-related cancer mortality, and the potential effect modification by sleep duration and regularity. Methods Population-based prospective cohort study of 58,149 adults from the UK Biobank accelerometry subsample, collected between 2013 and 2016, followed up to 2022. Daily sleep duration, sleep regularity index (SRI), sedentary behaviour (SB), standing, light-intensity (LPA) and moderate-to-vigorous physical activity (MVPA) were measured from wrist-worn accelerometers. Participants were categorized based on age-specific sleep duration recommendations, and sample-based sleep regularity index. Associations of behavioural reallocations with all-cause, major CVD, and PA-related cancer mortality were examined using compositional Cox regression. Results Over a mean follow-up of 8.0 years, 2,209 mortality events occurred (345 CVD and 997 cancer). Displacing sleep with MVPA was associated with lower all-cause and CVD mortality risk in the whole sample, regardless of sleep duration and regularity. Among participants who meet sleep duration guidelines, reallocating 30 minutes from sleep to standing, LPA or MVPA was favourably associated with all-cause mortality with HRs of 0.86 (95%CI 0.79, 0.93), 0.87 (0.80, 0.95), and 0.80 (0.73, 0.87), respectively. Reallocating 30 minutes from sleep to SB, standing, or LPA was adversely associated with CVD risk (HRs 1.08 (1.02, 1.15), 1.10 (1.01, 1.20), and 1.11 (1.03, 1.20)) among those not meeting guidelines. Beneficial associations of reallocating SB to sleep were evident only amongst short (<7h/day) or regular (SRI>87.8) sleepers across all-cause, CVD and cancer mortality. Conclusions Sleep duration and regularity may modify the theoretical replacement effects of sleep and other physical behaviours on mortality. Replacing sleep with equivalent amounts of MVPA appears largely protective; however, replacing sleep with lower intensity of activity (LPA and standing) is only protective for those who are already meeting sleep duration guidelines. Health effects of increasing sleep may depend on initial sleep regularity and duration. Future personalised behavioural interventions could be optimised by incorporating baseline sleep characteristics in their design and behavioural targets.