Expert Commentary

Daylight saving time: Research on health, car accidents and energy usage

How does daylight saving time affect our health? We review the research.

(David Trilling)

“Spring forward, fall back.” Like clockwork, we follow this humble command twice a year. But what is it doing to our health and wellbeing?

The idea behind daylight saving time (DST) — when we move our clocks forward by one hour for the warmer months, giving us an extra hour on an autumn Sunday and losing us one in spring — is to make better use of daylight and conserve energy. A later sunset during the long summer days means less need for indoor lighting in the evening. The idea has been around for centuries. Some credit a 1784 essay by Benjamin Franklin.

Germany was the first country to adopt DST, in 1916, to save energy during World War I. Other countries quickly followed. In the 1970s, during the Arab oil embargo, the U.S. Congress approved an emergency yearlong DST. In 2005, the Energy Policy Act extended the observance of DST in the United States by four weeks. Americans (except in Arizona and Hawaii, which do not use DST) now set their clocks forward on the second Sunday in March and back on the first Sunday in November.

Yet academic research shows largely negative impacts on health, which have stoked controversy in recent decades and led some to question if this century-old tradition still merits support. Indeed, some countries, like Argentina and Russia, have dropped DST altogether.

The spring is most dangerous: In the first few days after we lose an hour of sleep, researchers have shown increases in car accidents and heart attacks — the latter by as much as 24 percent. Those phenomena may decrease for a few days after the fall switch, when we are given that extra hour of sleep. Another study reports a mild health boost in the fall, though a 2016 study in Epidemiology found increased depressive episodes in the autumn, when the change means we are suddenly leaving work in the dark.

People with small children report higher levels of unhappiness during the spring transition, when they lose an hour of sleep. Without DST, though, parents worry about children leaving for school on dark winter mornings. Crime drops after the spring change.

As for energy consumption, in a 2008 study carried out shortly after the last federal change to the daylight saving schedule, the U.S. Department of Energy found annual energy usage fell about 0.03 percent. That may not sound like much, but it is enough to power 100,000 homes for a year. Other research found savings higher in regions far from the equator, where the length of the day varies considerably throughout the year. Yet usage near the equator, where the amount of daylight varies little, actually increased after the clocks were switched, they found.

Other resources:

tip sheet from the Department of Energy details more history behind DST and suggests that Benjamin Franklin was joking when he proposed changing our clocks.

The Associated Press style is to use “daylight saving time,” not “savings,” without caps or hyphens. This is often known as summer time. Capitalize when speaking about usage within a particular time zone: Eastern Daylight Time, Pacific Daylight Time. In the winter, we use “standard time” — Eastern Standard Time — sometimes known as “winter time.”

The website lists countries and the dates they change their clocks. Many — including China — do not change at all.

Selected studies:

“Using the Life Satisfaction Approach to Value Daylight Savings Time Transitions: Evidence from Britain and Germany”
Kuehnle, D.; Wunder, C. Journal of Happiness Studies, 2015. doi: 10.1007/s10902-015-9695-8.
Abstract: “Daylight savings time represents a public good with costs and benefits. We provide the first comprehensive examination of the welfare effects of the spring and autumn transitions for the UK and Germany. Using individual-level data and a regression discontinuity design, we estimate the effect of the transitions on life satisfaction. Our results show that individuals in both the UK and Germany experience deteriorations in life satisfaction in the first week after the spring transition. We find no effect of the autumn transition. We attribute the negative effect of the spring transition to the reduction in the time endowment and the process of adjusting to the disruption in circadian rhythms. The effects are particularly strong for individuals with young children in the household. We conclude that the higher the shadow price of time, the more difficult is adjustment. Presumably, an increase in flexibility to reallocate time could reduce the welfare loss for individuals with binding time constraints.”

“Spring Forward at Your Own Risk: Daylight Saving Time and Fatal Vehicle Crashes”
Smith, Austin C. American Economic Journal: Applied Economics, 2016. doi: 10.1257/app.20140100.
Abstract: “Daylight Saving Time (DST) impacts over 1.5 billion people, yet many of its impacts on practicing populations remain uncertain. Exploiting the discrete nature of DST transitions and a 2007 policy change, I estimate the impact of DST on fatal automobile crashes. My results imply that from 2002–2011 the transition into DST caused over 30 deaths at a social cost of $275 million annually. Employing four tests to decompose the aggregate effect into an ambient light or sleep mechanism, I find that shifting ambient light only reallocates fatalities within a day, while sleep deprivation caused by the spring transition increases risk.”

“Daylight Savings Time and Myocardial Infarction”
Sandhu, A; Seth, M; Gurm, H.S. BMJ: Open Heart, 2014. DOI: 10.1136/openhrt-2013-000019.
Results: There was no difference in the total weekly number of PCIs performed for AMI for either the fall or spring time changes in the time period analysed. After adjustment for trend and seasonal effects, the Monday following spring time changes was associated with a 24% increase in daily AMI counts (p=0.011), and the Tuesday following fall changes was conversely associated with a 21% reduction (p=0.044). No other weekdays in the weeks following DST changes demonstrated significant associations.

“Adverse Effects of Daylight Saving Time on Adolescents’ Sleep and Vigilance”
Medina, Diana; et al. Journal of Clinical Sleep Medicine, 2015. doi: 10.5664/jcsm.4938.
Conclusions: “The early March DST onset adversely affected sleep and vigilance in high school students resulting in increased daytime sleepiness. Larger scale evaluations of sleep impairments related to DST are needed to further quantify this problem in the population. If confirmed, measures to attenuate sleep loss post-DST should be implemented.”

“Under the Cover of Darkness: How Ambient Light Influences Criminal Activity”
Doleac, Jennifer L.; Sanders, Nicholas J. Review of Economics and Statistics, 2015. doi: 10.1162/REST_a_00547.
Abstract: “We exploit daylight saving time (DST) as an exogenous shock to daylight, using both the discontinuous nature of the policy and the 2007 extension of DST, to consider the impact of light on criminal activity. Regression discontinuity estimates show a 7 percent decrease in robberies following the shift to DST. As expected, effects are largest during the hours directly affected by the shift in daylight. We discuss our findings within the context of criminal decision making and labor supply, and estimate that the 2007 DST extension resulted in $59 million in annual social cost savings from avoided robberies.”

“Prioritizing Sleep Health: Public Health Policy Recommendations”
Barnes, Christopher M.; Drake, Christopher L. Perspectives on Psychological Science, 2015. doi: 10.1177/1745691615598509.
Abstract: “The schedules that Americans live by are not consistent with healthy sleep patterns. In addition, poor access to educational and treatment aids for sleep leaves people engaging in behavior that is harmful to sleep and forgoing treatment for sleep disorders. This has created a sleep crisis that is a public health issue with broad implications for cognitive outcomes, mental health, physical health, work performance, and safety. New public policies should be formulated to address these issues. We draw from the scientific literature to recommend the following: establishing national standards for middle and high school start times that are later in the day, stronger regulation of work hours and schedules, eliminating daylight saving time, educating the public regarding the impact of electronic media on sleep, and improving access to ambulatory in-home diagnostic testing for sleep disorders.”

“Does Daylight Saving Time Really Make Us Sick?”
Jin, Lawrence; Ziebarth, Nicolas R. IZA Discussion Paper No. 9088. 2015.
Abstract: “This paper comprehensively studies the health effects of Daylight Saving Time (DST) regulation. Relying on up to 3.4 million BRFSS [US Behavioral Risk Factor Surveillance System] respondents from the US and the universe of 160 million hospital admissions from Germany over one decade, we do not find much evidence that population health significantly decreases when clocks are set forth by one hour in spring. However, when clocks are set back by one hour in fall, effectively extending sleep duration for the sleep deprived by one hour, population health slightly improves for about four days. The most likely explanation for the asymmetric effects are behavioral adjustments by marginal people in spring.”

“Incidence of Myocardial Infarction With Shifts to and From Daylight Savings Time”
Jiddou, Monica R.; et al. The American Journal of Cardiology, 2013. doi: 10.1016/j.amjcard.2012.11.010.
Abstract: “Modulators of normal bodily functions such as the duration and quality of sleep might transiently influence cardiovascular risk. The transition to daylight savings time (DST) has been associated with a short-term increased incidence ratio (IR) of acute myocardial infarction (AMI). The present retrospective study examined the IR of AMIs that presented to our hospitals the week after DST and after the autumn switch to standard time, October 2006 to April 2012, with specific reference to the AMI type. Our study population (n=935 patients; 59 percent men, 41 percent women) was obtained from the electronic medical records of the Royal Oak and Troy campuses of the Beaumont Hospitals in Michigan. Overall, the frequency of AMI was similar in the spring and autumn, 463 (49.5 percent) and 472 (50.5 percent), respectively. The IR for the first week after the spring shift was 1.17 (95 percent confidence interval 1.00 to 1.36). After the transition from DST in the autumn, the IR for the same period was lower, but not significantly different, 0.99 (95 percent confidence interval 0.85 to 1.16). Nevertheless, the greatest increase in AMI occurred on the first day (Sunday) after the spring shift to DST (1.71, 95 percent confidence interval 1.09 to 2.02; p <0.05). Also, a significantly greater incidence was found of non ST-segment myocardial infarction after the transition to DST in the study group compared with that in the control group (p=0.022). In conclusion, these data suggest that shifts to and from DST might transiently affect the incidence and type of acute cardiac events, albeit modestly.”

“Transition Into and Out of Daylight Saving Time and Spontaneous Delivery: A Population-Based Study”
László, Krisztina D.; Cnattingius, Sven; Janszky, Imre. Obstetrics and Gynecology, 2016. doi:10.1136/bmjopen-2015-010925.
Results: “The number of deliveries during the week after the transition into or out of DST was similar to that in the comparison period (18,519 observed vs. 18,434 expected in case of the spring shift and 19,073 observed vs. 19,122 expected in case of the autumn shift); the corresponding incidence ratio and 95 percent CIs were 1.005 (0.990 to 1.019) and 0.997 (0.983 to 1.012), respectively. There were no differences in the length of gestation of the deliveries in the exposure and the control periods. Conclusions: Our results do not support the hypothesis that a minor circadian rhythm disruption is associated with an increased short-term risk of spontaneous delivery.”

“Daylight Saving Time Transitions on the Incidence Rate of Unipolar Depressive Episodes”
Hansen, Bertel Teilfeldt; et al. Epidemiology, 2016. doi: 10.1097/EDE.0000000000000580.
Conclusions: “This is the first study to show that the transition from summer time to standard time is associated with an increase in the incidence rate of unipolar depressive episodes. We speculate that the distress associated with the sudden advancement of sunset, which marks the coming of a long period of short days, accounts for this finding.”

“Does Daylight Saving Save Energy? A Meta-Analysis”
Havranek, Tomas; Herman, Dominik; Irsova, Zuzana. Munich Personal RePEc Archive, 2016.
Abstract: “The original rationale for adopting daylight saving time (DST) was energy savings. Modern research studies, however, question the magnitude and even direction of the effect of DST on energy consumption. Representing the first meta-analysis in this literature, we collect 162 estimates from 44 studies and find that the mean reported estimate indicates modest energy savings: 0.34% during the days when DST applies. The literature is not affected by publication bias, but the results vary systematically depending on the exact data and methodology applied. Using Bayesian model averaging we identify the most important factors driving the heterogeneity of the reported effects: data frequency, estimation technique (simulation vs. regression), and, importantly, the latitude of the country considered. Energy savings are larger for countries farther away from the equator, while subtropical regions consume more energy because of DST.”

“Impact of Daylight Saving Time on the Chilean Residential Consumption”
Verdejo, Humberto. Energy Policy, 2016. doi: 10.1016/j.enpol.2015.10.051.
Abstract: “Since 1970 Chile has had a Daylight Saving Time (DST) policy in order to reduce residential electricity consumption in the country. The time change was set for the first time by executive decree in 1970, and since that date it was applied every year without great changes until 2010. Since then, and to date, decrees have been set in order to increase the duration of the DST, arguing that there are reasons associated with energy savings that justify the extension of the measure that has been adopted by the authority in recent years. In the present study the impact of the application of DST in terms of decreased household electricity consumption is analyzed using two complementary methods, one based on a heuristic approach and the other using an econometric model. The results indicate that there is indeed a marginally small reduction in residential electricity consumption, although these results are not homogeneous throughout the country.”


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