The US Congress has considered legislation to make daylight saving time (DST) permanent, and in 2021, the European Parliament postponed the process of ending the seasonal time changes it agreed upon in 2019. DST is a common energy policy around the world. Approximately one-quarter of the world’s population practices the semiannual ritual of adjusting the clock an hour forward in spring (springing forward) and an hour backward in autumn (falling back).

DST, however, has been fraught with controversy and unpopularity. Recent evidence has shown that the policy’s original intention to conserve energy by using daylight in the summer has backfired; DST increases energy consumption because people run air conditioning for longer in the evenings and heating for longer in the mornings. In addition, evidence has shown that it has adverse consequences on economic activities, such as causing traffic accidents and reducing returns in financial markets. But perhaps the most troubling concern over DST is that the seemingly small yet abrupt one-hour changes may cause substantial irregularities in human health, such as drowsiness, headaches, stress, and heart attacks.

Our research examines the effect of DST on the incidence of heart attacks, one of the primary health concerns associated with DST transitions. Sleep deprivation and disrupted biorhythms are two hypothesized explanations for why DST transitions can trigger heart attacks. Our research analyzes whether DST increases the incidence of heart attacks over three time frames: around the transitions, during the overall DST period, and around each transition over several years.

Historical disputes over the adoption of DST in Indiana exempted a large majority of counties from adopting DST until 2006. Our study uses the resulting variation in the practice of DST in the state to analyze its effect on heart attacks. This approach has advantages over prior research. Existing evidence regarding the relationship between DST and heart attacks focuses exclusively on the transitional periods. However, that approach may leave out longer-term effects, distorting the overall cost–benefit analysis of DST. For example, DST may simply hasten heart attacks that would have occurred later even without the time change. Alternatively, the overall effects of DST may be beneficial if people use the longer daylight hours to engage in more physical activity.

Moreover, our research explores whether people subject to DST continue to experience the short-term effects of each DST transition over multiple years and adapt to these effects. People subject to DST may adjust their behaviors over time, for example, by going to bed earlier the night before the spring transition or gradually familiarizing themselves with the time adjustments. These adjustments may lessen the short-term effects of DST transitions over time.

Our research produced several findings using nonpublic administrative records from the Indiana Department of Health containing hospital discharge data that cover all heart attack cases in Indiana from 2002 through 2012. First, spring transitions to DST increased heart attack admissions by 27.2 percent for approximately two weeks after each transition. There was no increase in heart attack admissions in counties that did not observe DST. Additionally, our findings reveal no evidence of changes in heart attack admissions around the autumn transitions.

Furthermore, our research finds no evidence of reductions in heart attack admissions after the transitional periods. Thus, DST transitions increased average daily heart attack admissions overall. These findings suggest that the longer daylight hours during DST do not create any health-related benefits that outweigh the short-term costs.

Finally, our findings suggest that the increase in heart attacks did not decrease during subsequent spring transitions. This indicates that repeated exposure to DST transitions does not significantly enhance people’s resilience or habituation to its impacts. While our findings show that DST transitions create substantial short-term health costs, they reveal no health benefits that might arise from adopting DST permanently. Therefore, while the debate over permanently adopting DST or standard time continues, our findings underscore the urgent need to eliminate the harmful biannual time changes.

Note
This research brief is based on Shinsuke Tanaka and Hideto Koizumi, “Springing Forward and Falling Back on Health: The Effects of Daylight Saving Time on Acute Myocardial Infarction,” Journal of Economic Behavior and Organization 228 (December 2024).