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The shift to and from daylight savings time and motor vehicle crashes

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Abstract

The objective of the study was to examine whether the shifts to and from daylight savings time in Sweden have short-term effects on the incidence of traffic crashes. A database maintained by the Swedish National Road Administration was used to examine crashes from 1984 through 1995, that occurred on state roads the Monday preceding, the Monday immediately after (index Monday), and the Monday 1 week after the change to daylight savings time in the spring and for the corresponding three Mondays in the autumn. The Mondays 1 week before and after the time changes were taken as representing the expected incidence of crashes. Crash incidence was calculated per 1000 person-years using population estimates for each year of the study. The association between 1 h of possible sleep loss and crash incidence was estimated by the incidence rate ratio from negative binomial regression. The incidence rate ratio was 1.04 (95% CI, 0.92–1.16) for a Monday on which drivers were expected to have had 1 h less sleep, compared with other Mondays. In the spring, the incidence rate ratio for crashes was 1.11 (95% CI, 0.93–1.31) for Mondays after the time change compared to other spring Mondays. The corresponding rate ratio for the fall was 0.98 (95% CI, 0.84–1.15) It was concluded that the shift to and from daylight savings time did not have measurable important immediate effects on crash incidence in Sweden.

Introduction

Daylight savings time may have beneficial effects on traffic safety by reducing overall exposure to night-time driving (Ferguson et al., 1995). However, it has been suggested that the adjustment to time changes is not immediate and may have short-term detrimental effects on alertness and performance (Mark and Folkard, 1976). It is well recognized that cognitive function and performance efficiency are adversely affected by severe disruptions in circadian rhythm and insufficient sleep (Graeber, 1994). There is some evidence that minor sleep cycle disturbances also can diminish a person’s ability to conduct routine activities; the risk of accidental death may be adversely affected by the shift of time keeping by as little as 1 h (Mitler et al., 1988, Coren, 1996a). Similarly, in an analysis of over 20 000 traffic crashes in Canada for the years 1991 and 1992, Coren reported that the shift to and from daylight savings had measurable effects on the incidence of automobile crashes (Coren, 1996b). In that study, the crash incidence, compared with the previous and following Mondays, was greater on the Monday directly after the spring shift to daylight savings time (rate ratio 1.09, 95% confidence interval (CI), 1.03–1.15) and less on the Monday immediately following the shift back to standard time in the autumn (rate ratio 0.94, 95% CI, 0.90–0.98).

The purpose of this study was to examine whether the shifts to and from daylight savings time in Sweden had short-term effects on the incidence of traffic crashes. We hypothesized that crash incidence would be greater on the Monday immediately after the spring shift (the equivalent of travelling one time zone to the east) as drivers might have lost up to 1 h of sleep due to the time change. Conversely, the Monday immediately after the fall time change was expected to be associated with a lesser incidence as drivers could have gained up to 1 h of sleep.

Section snippets

Methods

We used a database maintained by the Swedish National Road Administration to examine crashes from 1984 through 1995, that occurred on state roads the Monday preceding, the Monday immediately after (index Monday), and the Monday 1 week after the change to daylight savings time in the spring and for the corresponding three Mondays in the autumn. The Mondays 1 week before and after the time changes were taken as representing the expected incidence of crashes. Crash incidence was calculated per

Results

Overall, there were more crashes during the autumn Mondays (n=4194) compared with spring Mondays (n=2650). In the spring, the crash incidence per 1000 person-years on the index Monday showed a small elevation compared with the preceding and following Mondays. In the autumn, however, crash rates on the index Monday were intermediate between the preceding and following Mondays (Fig. 1). We used negative binomial regression to estimate the incidence rate ratio associated with a 1 h loss in average

Discussion

We found little evidence that the shift to or from daylight savings time had immediate effects on crash incidence in Sweden. While the pattern in the spring showed some consistency with the theory that the spring shift transiently increases the incidence of crashes, there was almost no reciprocal autumn decrease in crash incidence. We deliberately used methods similar to those Coren used to analyze Canadian crash data, but were unable to confirm his finding that there were consistent changes in

Acknowledgements

We are indebted to Ingegard Bryngelsson of the Swedish National Road Administration for providing data for this study. This work was supported in part by the Swedish Society of Medicine and Wahlmarks Fund of Uppsala City Council, Sweden.

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