Spring Forward at Your Own Risk: Daylight Saving Time and Fatal Vehicle Crashes

Despite mounting evidence that Daylight Saving Time (DST) fails in its pri-mary goal of saving energy, some form of DST is still practiced by over 1.5 billion people in over 60 countries. I demonstrate that DST imposes high social costs on Americans, specifically, an increase in fatal automobile crashes. DST alters fatal crash risk in two ways: disrupting sleep schedules and reallocating ambient light from the morning to the evening. First, I take advantage of the discrete nature of the transitions between Standard Time and DST to measure the impact of DST on fatal crashes in a regression discontinuity design. Then, to measure the duration of the effect, I exploit variation in the coverage of DST created primarily by a 2007 policy change, in a day-of-year fixed effects model. Both models reveal a short-run increase in fatal crashes following the spring transition and no aggregate impact in the fall. Employing three tests, I decompose the aggregate effect into ambient light and sleep mechanisms. I find that shifting ambient light reallocates fatalities within a day, while sleep deprivation caused by the spring transition increases risk. The increased risk persists for the first six days of DST, causing a total of 302 deaths at a social cost of $2.75 billion over the 10-year sample period, underscor-ing the huge costs of even minor disruptions to sleep schedules. JEL Codes: R41