Correspondence
Effects of Daylight Savings Time on Collision Rates
N Engl J Med 1998; 339:1167-1168October 15, 1998DOI: 10.1056/NEJM199810153391617
- Article
To the Editor:
The results of a recent Canadian study call into question Coren's findings that motor vehicle crashes increase by 8 percent following the change to daylight savings time and decrease by 7 percent after the change to standard time.1 The study extended Coren's analysis, using the same data source. First, data from the days between the Monday preceding the time change and the Monday one week afterward were analyzed. Second, Coren's hypothesis was statistically tested with data from the years 1984 to 1993, to evaluate the significance of any differences obtained.
A graphical analysis (Figure 1Figure 1
Effect of the Change to or from Daylight Savings Time (DST) on the Mean (+SE) Collision Rates.) indicated that there were no peaks on the Mondays after the change to daylight savings time or troughs on the Mondays after the return to standard time, and no corresponding return to base-line values after each transition. The results of a paired t-test with pooled national data failed to reach significance (P=0.5), with a mean rate of 129.8 for the Monday one week before and for the Monday immediately after the change to daylight savings time (95 percent confidence interval for the difference between the means, –12.12 to +12.43). An analogous paired t-test with pooled national data found no difference, with the mean rate one week following the change equal to 130.1 (95 percent confidence interval for the difference between the means, –13.12 to +13.84).Next, the mean motor vehicle crash rates for the Monday one week before the change to standard time were compared with those for the Monday immediately after the change and showed a significant increase (160.8 vs. 188.5; 95 percent confidence interval for the difference between the means, 6.6 to 48.4; t=2.66; P<0.01). This result is inconsistent with Coren's hypothesis. A paired t-test showed that the mean rate of 188.5 for the Monday immediately after the change was not significantly different from the mean rate of 186.5 for the Monday one week after the change (95 percent confidence interval for the difference between the means, –14.6 to +12.7; t=0.16; P=0.4).
Thus, the results of both a graphical analysis and the variability estimation of 10 years of data failed, as had an earlier study,2 to support Coren's hypothesis. The effects reported by Coren may stem from the increased number of vehicles on the road and the increased number of kilometers traveled in the extra daylight hour in the spring, rather than from the minor disruption in circadian rhythm induced by the loss of one hour of sleep.
2 ReferencesAlex Vincent, Ph.D.
Transport Canada, Montreal, QC H3B 1X9, Canada1
Coren S
Free Full Text | Web of Science | Medline2
Stewart DE. The implications of an early return to day-light saving time in Canada: impact on road safety. Technical memorandum. Road Safety and Motor Vehicle Regulation TMSE 8503. Ottawa, Ont.: Transport Canada, 1985.
Dr. Coren replies:
To the Editor: In my study of the effects of daylight savings time on traffic accidents, I found increased accident rates on the Monday after the spring shift in time and decreased rates in the fall. I interpreted this in terms of sleep time lost or gained. Vincent uses a larger data base than that available to me and fails to replicate these results. Unfortunately, Vincent's analyses are based on t-tests of annual counts, rather than more sensitive pooled relative-risk measures. More important, analysis of recent data from larger data banks gives me reason still to believe that the shift to daylight savings time in the spring is associated with an increased risk of accidents, although the rebound reduction in accidents in the fall may be more problematic.
In an extension of the original study, I obtained data from the National Highway Traffic Safety Administration on all (366,910) deaths in the United States due to traffic accidents for the years 1986 through August 1995.1 Data were cumulated over the 10-year period. Contrasting traffic fatalities for the Monday immediately after the spring shift to daylight savings time with the pooled frequency for the Mondays preceding and following that date shows the expected significant increase, with a relative risk of 1.17 (95 percent confidence interval, 1.07 to 1.29; ξ2=10.83, 1 df; P<0.001). The magnitude of this shift is larger than in my first study, amounting to 17.2 percent. The fall time shift, however, was associated with an insignificant reduction in traffic deaths (2.6 percent), with a relative risk of 0.97 (95 percent confidence interval, 0.89 to 1.07; ξ2=0.29, 1 df; P not significant).
This result is similar to that of studies based on accidental deaths not related to traffic accidents.2,3 For example, I looked at every accidental death in the United States that was reported to the National Center for Health Statistics for the years 1986 through 1988.4 Since over 80 percent of accident-related deaths occur within four days after the accident, data for the analysis were restricted to the first four workdays immediately following the change to daylight savings time and the first four workdays in the week preceding and the week after the change. There were 8429 accidental deaths in the spring-shift analysis and 8771 in the fall. The interval immediately following the spring shift showed a 6.6 percent increase in accidental deaths (relative risk, 1.07; 95 percent confidence interval, 1.01 to 1.11; ξ2=5.52, 1 df; P<0.05). The fall shift, however, was associated with a nonsignificant 1.5 percent decrease (relative risk, 0.99; 95 percent confidence interval, 0.922 to 1.021; ξ2=1.34, 1 df; P not significant).
These data are consistent with the hypothesis that a small decrease in the duration of sleep can increase one's susceptibility to accidents. Although work schedules accentuate the loss of sleep after the spring shift to daylight savings time, the absence of a reduction in accidents in the fall may reflect the fact that many people do not take advantage of the hour gained to extend their sleep.
4 ReferencesStanley Coren, Ph.D.
University of British Columbia, Vancouver, BC V6T 1Z4, Canada1
National Center for Statistics and Analysis. Fatal accident reporting system, 1986–1995. (Computer files.) Washington, D.C.: National Highway Traffic Safety Administration, 1986–1995.
2
Coren S. Sleep thieves. New York: Free Press, 1996.
3
Coren S. Accidental death and the shift to daylight savings time. Percept Motor Skills 1996;83:921-2.
4
National Center for Health Statistics. Multiple cause of death, 1986, 1987, 1988. (Computer files.) Hyattsville, Md.: Department of Health and Human Services, 1986–1988.
- Citing Articles (6)
Citing Articles
1
Daniel Kuehnle, Christoph Wunder. . (2016) Using the Life Satisfaction Approach to Value Daylight Savings Time Transitions: Evidence from Britain and Germany. Journal of Happiness Studies 17:6, 2293-2323.
CrossRef2
Austin C. Smith. . (2016) Spring Forward at Your Own Risk: Daylight Saving Time and Fatal Vehicle Crashes. American Economic Journal: Applied Economics 8:2, 65-91.
CrossRef3
Yvonne Harrison. . (2013) The impact of daylight saving time on sleep and related behaviours. Sleep Medicine Reviews 17, 285-292.
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Tatjana Meschede. . (2010) Accessing Housing: Exploring the Impact of Medical and Substance Abuse Services on Housing Attainment for Chronically Homeless Street Dwellers. Journal of Human Behavior in the Social Environment 20, 153-169.
CrossRef5
Andrew Worthington. . (2004) Business Expectations and Preferences Regarding the Introduction of Daylight Saving in Queensland. Economic Analysis and Policy 34, 145-162.
CrossRef6
Mats Lambe, Peter Cummings. . (2000) The shift to and from daylight savings time and motor vehicle crashes. Accident Analysis & Prevention 32, 609-611.
CrossRef
