Impact of daylight saving time on physical activity patterns

Daylight saving time (DST) remains contentious: some policymakers highlight behavioural benefits, while others emphasize health risks. Here we estimated the behavioural and physiological impacts of DST using longitudinal Fitbit measures from the National Institutes of Health All of Us Research Program. Avoiding strict modelling assumptions, we used a natural difference-in-differences design with Arizona (no DST) as a control against neighbouring Mountain Time states (observing DST). Contrary to common belief, DST transitions produced no net change in total daily steps. Instead, activity was reallocated to other times of day: fall transitions increased morning steps by 202 (confidence interval = [78, 326], P = 0.001) while reducing evening steps by 180 (confidence interval = [−263, −97], P < 0.001); spring transitions showed the opposite. Importantly, these treatment effects varied by demographics and across data-driven activity phenotypes (‘morning walker’, ‘neutral walker’ and ‘evening walker’). These disparities suggest that structural factors (for example, rigid work schedules, perceived safety) may constrain the capacity to flexibly adapt to time shifts for some populations. Physiologically, resting heart rate showed subtle intraday shifts mirroring behavioural changes, although differences were clinically insignificant. Our study provides a large-scale causal analysis of DST’s influence using continuous wearables data, illustrating how observational data can generate real-world evidence to inform health-relevant policies.