Should We Keep Changing the Clock? Characterizing Causal Effects of Daylight Saving Time on Behavior and Physiology

Daylight Saving Time (DST) remains contentious: some policymakers highlight behavioral benefits, while others emphasize health risks. Existing evidence relies largely on aggregated data and strict modeling assumptions, obscuring individual-level patterns and yielding associational rather than causal insights. We address these limitations using objective, longitudinal Fitbit measures from the NIH All of Us Research Program. Avoiding strict modeling assumptions, we employed a natural difference-in-differences design using Arizona (no DST) as a control against neighboring Mountain Time states (observing DST). Contrary to the 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 while reducing evening steps by 180; spring showed the opposite. Importantly, these treatment effects varied by demographics: older adults (65+) and lower-income individuals showed greater morning reallocation during fall DST, while younger and higher-income adults showed limited adaptation. We also demonstrate that activity reallocation patterns differed across data-driven activity phenotypes ("Morning-," "Neutral-," and "Evening Walker"). These disparities suggest that structural factors (e.g., 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 behavioral changes, though differences were clinically insignificant. Our study provides the first 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.