To the Editor:
Daylight Savings Time (DST) is the practice of setting the clocks forward 1 hour from standard time during the summer months, and back again in the fall, to make better use of natural daylight. It is applied in a large number of countries worldwide affecting 1.5 billion people. Changes in light hours and sleep disruption1,2 occurring during DST changes have been theorized to increase risk of traffic injuries. However, a recent review found inconclusive results on whether DST increased or decreased the risk of road traffic collision, especially in the short term.3 We estimated the risk of daily fatal traffic accidents following summer and fall DST changes in Spain.
We collected all daily deaths caused by road traffic accidents (International Classification of Diseases 9th revision: E810-E819) in all the 52 Spanish provincial capital cities between 1990 and 2014, provided by the Spanish National Institute of Statistics. We also defined dummy variables to identify summer and fall DST changes, in April and October, respectively. We used an ecologic time-series design, where data were analyzed using quasi-Poisson regression with a distributed lag nonlinear model.4 Long-term trend and seasonality were adjusted using a natural cubic spline of time with 10 degrees of freedom per year, and a categorical variable was used to control for day of the week. We modeled lagged effects up to 30 days after DST changes with a natural cubic spline with 3 internal knots, placed at equally spaced values in the log scale. Finally, we derived the fraction of deaths attributable to both, summer and fall DST changes, from the previous model estimates.5
The daily number of deaths caused by road traffic accidents averaged 3.5 (standard deviation = 2.8), ranging from 0 to 33. On the day of a DST change, the average number of deaths raised up to 4.3 (standard deviation = 3.3). Risk of fatal road traffic accident increased by 30% on the same day as summer DST change (risk ratio [RR] = 1.30; 95% CI = [1.04; 1.62] at lag 0), to decrease in the consecutive 1–3 days (Figure). Fall DST change showed a similar distributed lag pattern, but with a smaller risk increase on the same day (RR = 1.16; 95% CI = [0.93; 1.45]) and almost no harvesting effect thereafter (Figure). Over the study period, almost 40 deaths can be attributable to DST changes, 23.4 (95% CI = [2.7; 39.9]) and 14.7 (95% CI = [−7.8; 32.8]) for the summer and fall changes, respectively.
Since the DST change always occurs on a Saturday night, the observed short-term increase in fatal road traffic accidents could be explained by the combination of sleep disruption2,3,6 and regular weekend effects, where traffic injuries might be more often related to drug or alcohol consumption.7 The summer change implies 1 hour less of sleep, potentially resulting in augmented fatigue and impaired driving ability on the day of the change. As for the fall DST change, no hours of sleep are lost, but the loss of 1 light hour in the evening, when collision risk is highest,8 can explain the observed lower excess risk.
Our findings suggest that, on average, DST changes are associated with a cost of 1.5 lives every year due to road traffic accidents in Spain’s capital cities. Considering that 78% of deaths from road traffic accidents in Spain occur on roads outside of cities,9 this could have an additional cost of another 5 lives every year.
Preventive Medicine and Public Health Training Unit
Parc de Salut Mar
Universitat Pompeu Fabra
Agència de Salut Pública de Barcelona
Institute of Environmental Assessment and Water Research
Spanish Council for Scientific Research
GREMPAL Research Group
Idiap Jordi Gol and CIBERFes
Universitat Autonoma de Barcelona and Instituto de Salud Carlos III
Centre for Statistics in Medicine
Botnar Research Centre
University of Oxford
1. Kantermann T, Juda M, Merrow M, Roenneberg T. The human circadian clock’s seasonal adjustment is disrupted by daylight saving time. Curr Biol. 2007;17:1996–2000.
2. Lahti TA, Leppämäki S, Lönnqvist J, Partonen T. Transitions into and out of daylight saving time compromise sleep and the rest-activity cycles. BMC Physiol. 2008;8:3.
3. Carey RN, Sarma KM. Impact of daylight saving time on road traffic collision risk: a systematic review. BMJ Open. 2017;7:e014319.
4. Gasparrini A, Armstrong B, Kenward MG. Distributed lag non-linear models. Stat Med. 2010;29:2224–2234.
5. Gasparrini A, Leone M. Attributable risk from distributed lag models. BMC Med Res Methodol. 2014;14:55.
6. Connor J, Norton R, Ameratunga S, et al. Driver sleepiness and risk of serious injury to car occupants: population based case control study. BMJ. 2002;324:1125.
7. Kanaan A, Huertas P, Santiago A, Sánchez JA, Martínez P. Incidence of different health factors and their influence on traffic accidents in the province of Madrid, Spain. Leg Med (Tokyo). 2009;11:S333–S336.
8. Plainis S, Murray IJ, Pallikaris IG. Road traffic casualties: understanding the night-time death toll. Inj Prev. 2006;12:125–128.