March 10, 2014 Celeste Monforton, DrPH, MPH 3Comment

It started with a yawn. Then a conversation about whether daylight savings time (DST) begins too early in the year. “On Monday, kids will be going to school in the dark and with one hour less sleep,” said my mom. My brother remarked: “There are more accidents in the days immediately following the time change.”  I was skeptical about his car accident remark, but didn’t want to open my mouth without some facts. Here’s some of what I learned with just a minute of searching on PubMed.

Researchers with Stanford University School of Humanities and Sciences assembled data from U.S. fatal automobile accidents over a 21 year period. Varughese and Allen examined the mean number of accidents on the weekend of the time change, and compared it to the weekends immediately before and after the time shift. The authors reported a significant increase in fatal motor vehicle accidents on the Monday immediately following the springtime switch to DST (t = 1.92 (p = 0.034)).

Researchers with the Karolinska Institute in Stockholm also examined the rate of motor vehicle accidents on the first Monday of DST, and the Monday’s immediately preceding and following the switch. Lambe and Cummings used data from the Swedish National Road Administration for crashes occurring from 1984 through 1995. The rate was calculated as the number of incidents per 1000 person-years for each of the study years. In the springtime switch, they reported an accident rate ratio of 1.11 (95% CI, 0.93-1.31) for the Monday immediately following DST implementation. The rate ratio in the fall declined to 0.98 (95% CI, 0.84-1.15) when individuals potentially gained one hour of sleep.

Cathleen Zick with the University of Utah examined whether DST led to greater engagement by adults in physical activity. She used data from BLS’s American Time Use Survey for the periods 2003-2009. Zick compared responses to the survey’s “moderate-to-vigorous physical activity” (MVPA) variable from respondents in Colorado, New Mexico and Utah. She compared that data to responses from Arizona residents, a state that does not observe DST. She found that despite the additional evening hour of daylight, adults aged 18-64 did not affect time spent on MVPA.

Researchers with William Beaumont Hospital in suburban Detroit examined the incidence of acute myocardial infarction (AMI) over one-week periods of time from the switches to and from DST. Jiddou and colleagues reported an incident rate of 1.17 (95% CI 1.00 to 1.36) for the week following the time change. They indicated that the greatest increase in AMI occurred on the Sunday immediately after the spring shift to DST (1.71, 95% confidence interval 1.09 to 2.02; p <0.05).

Other studies in the literature include evaluations of a DTS effect on the sleep/wake cycle of college students (here), diurnal pattern of urban PM10 (here), cyber loafing (here), and work-related injuries (here, here), including those among coal miners (here).

I was skeptical of my brother’s assertion of a daylight saving effect. I shouldn’t have been.

3 thoughts on “Daylight saving time and public health consequences

  1. We need to get rid of the twice-yearly clock changes altogether: pick one “time” and keep it all year, permanently.

    The original rationale for DST was to conserve candles, and more recently, conserve energy spent on lighting at one or the other end of the day. In the end that’s nonsense because lighting is used either in the morning or in the evening, and the difference is probably insignificant including in the all-important metric of CO2 output (which of course wouldn’t be an issue if we were on renewables + nuclear instead of 19th century energy sources).

    When DST started, people typically had two clocks: a main clock in the house, and an alarm clock. Optionally a pocket watch or wrist watch. Changing the time setting on an analog clock is easy: pull out the knob and twist.

    Today there are clocks on every conceivable device, and changing most of them is a royal pain in the arse that involves dealing with contextual menus and other garbage foisted upon us by idiots who know nothing about user-interface design. The sheer hassle factor in and of itself should be sufficient to get rid of the yearly clock-changes.

    But if there are public health consequences such as an increase in heart attacks and auto accidents, then there’s no excuse for sticking to the present system.

    Question is, who is pestering Congress about this?

  2. I love daylight savings time. When I get out of work and have that extra hour of sunlight, it is a boon. It may not encourage me to exercise, but I sure seem to live more life!

  3. Are Americans (an people inthe industrialized world in general) so sleep deprived that missing a single extra hour on Saturday night can cause them to be more at risk for distraction-related accidents? Why not just go to bed earlier on Sunday to make up the hour? Or is it the fact that it’s darking in the morning again all of the sudden that makes people sluggish?

Leave a Reply

Your email address will not be published.

This site uses Akismet to reduce spam. Learn how your comment data is processed.