Accelerometer-measured physical activity timing and risk of incident atrial fibrillation: a UK Biobank cohort study

INTRODUCTION: Although moderate-to-vigorous physical activity (MVPA) is linked to a lower risk of atrial fibrillation (AF), the influence of its timing on AF outcomes remains largely understudied. METHODS: Accelerometer-derived MVPA data from UK Biobank participants free of AF at baseline were used to examine associations between distinct MVPA timing patterns (inactive, morning, afternoon, evening, mixed) and incident AF risk, estimated via multivariable-adjusted Cox proportional hazards models. RESULTS: A total of 88,024 participants (57.4% female) were followed for a median of 7.87 years, during which 3,815 developed incident AF. In the fully adjusted Cox model, engaging in MVPA in the morning, afternoon, or at mixed times was associated with lower AF risk, compared with inactivity. The morning activity group showed the greatest risk reduction (HR = 0.79; 95% confidence interval [CI]: 0.70-0.89; p < 0.001). Stratified analyses revealed that adults < 65 years benefited most from morning exercise. For diabetics, morning and afternoon MVPA were protective (HR 0.66, p = 0.001; HR 0.77, p = 0.025). Among non-diabetics, evening activity offered the greatest benefit (HR 0.76, p = 0.029). In non-hypertensive individuals, the timing of activity made little difference. However, among patients with hypertension, engaging in activity at any time of day was associated with a reduced risk of atrial fibrillation, with the greatest benefit observed in the morning (HR = 0.74, p < 0.001). CONCLUSION: Regular MVPA at any time was associated with lower AF risk, with exploratory evidence suggesting greater benefit in the morning, particularly among younger adults and those with comorbidities.