Switching to Daylight Saving Time and work injuries in Ontario, Canada: 1993–2007
- 1Institute for Work & Health, Toronto, Canada
- 2Dalla Lana School of Public Health, University of Toronto, Toronto, Canada
- Correspondence to Sara Morassaei, Institute for Work & Health, 481 University Ave, Suite 800, Toronto, ON M5G 2E9, Canada;
- Accepted 18 August 2010
- Published Online First 30 September 2010
Objective To examine whether switching to and from Daylight Saving Time (DST)—1 h shift forward in the spring and 1 h shift back in the autumn—is associated with an increase in work injuries.
Method Data on work-related injuries were obtained from compensation claim records from the Ontario Workplace Safety & Insurance Board for the period 1993–2007. A Poisson regression model was run separately comparing the number of no lost time claims and lost time claims during the week of DST change with the week following DST change, and the week preceding DST change. We also examined if differences in the relationship between DST and work injury claims were present across industry, age, gender and job tenure groups.
Results The results of our regression model did not show an increase in the incidence of work injury claims in the days immediately following the spring shift to DST. There was a significant decrease in the number of claims on Thursday, Friday and Saturday following the spring transition to DST. However, this decline was solely due to the years when Good Friday occurred during DST week (1993, 1998 and 2004) when fewer people are at work. For the autumn transition from DST, no evidence was found that the gain of 1 h sleep results in a decrease or increase in work injury claims.
Conclusion Our findings show that the shift to and from DST had no detrimental effects on the incidence of claims for work injuries in Ontario, Canada.
What this paper adds
Daylight Saving Time (DST) affects millions of people worldwide each year; however, the possible detrimental effects of DST are relatively unknown.
There is some evidence that minor disturbances in sleep duration and sleep cycle can increase the risk of accidents.
Few studies have examined the relationship between the shift to DST and incidence of work-related injuries, although conflicting results have been reported.
In this study, we found no evidence that the shift to and from DST had any detrimental effects on the incidence of claims for work injuries in Ontario from 1993 to 2007.
Daylight saving time (DST) is currently used in Canada and many other countries to improve the match between daylight and people's activities. While DST has obvious energy saving benefits, it is important to explore any possible detrimental effects, which, despite affecting millions of people annually, are relatively unknown. DST results in clocks being moved 1 h forward in the spring and 1 h back in the autumn. Transition to DST has been shown to disrupt circadian rhythms1 and decrease both sleep duration and sleep efficiency.2 From an occupational health and safety perspective, there is some evidence that minor sleep cycle disturbances and sleep deprivation can diminish a person's alertness, memory and mood, which can impact their ability to conduct routine activities, leading to an increased risk of accidents.3 The general public health evidence on the effects of DST are conflicting, with some studies finding an increase in traffic accidents4 5 during the first week of the change to DST in spring, and others finding no detrimental effects on traffic accidents6 7 or hospital treatments due to accidents.8 Other studies have found that DST reduces traffic accidents and fatalities by shifting an extra hour of daylight to the busy traffic period in the evening during the weeks after the start of DST.7 9 10 Limited research has addressed the impact of DST on work injuries. Holland and Hinze11 examined the effects of DST on accidents in a construction setting and found no significant relationship between time changes and accidents. However, a recent study by Barnes and Wagner12 examined the effects of DST on the occurrence of work injuries in miners and found that in comparison with other days, on Mondays directly following the switch to DST, workers sustain more workplace injuries and injuries of greater severity. Given these conflicting results, and their focus only on particular industrial groups, further research on the impact of DST on work injuries is warranted. The aim of this study was to explore the effects of transitions to and from DST on the incidence of work injury claims in Ontario, Canada during the years 1993–2007.
This study used claim reports from the Ontario Workplace Safety & Insurance Board (WSIB) filed between 1993 and 2007. The WSIB is the principal provider of workers' compensation in Ontario and covers approximately 65–72% of labour force participants.13 Information on all claim reports submitted to the WSIB includes the date of the accident, whether the claim resulted in time away from work, the worker's age and gender, and the industry in which the worker was employed. The industry is coded to correspond to the Standard Industrial Classification 1980 and includes agriculture, forestry, fishing, mining, oil, utilities, construction, manufacturing, trade, management, administration, accommodation, food and beverage, public administration, healthcare, social services, education, professional, science and technical. For lost time claims, where time is lost from work (outside of the day of the injury), information on the date employment commenced is also included and can be used to calculate job tenure. For the purpose of this analysis, we selected only those claims occurring on the week of DST change (both forwards and backwards), and the week preceding and following DST change. We examined the total number of no lost time claims (n=130 510) and lost time claims (n=69 336) for the spring shift to DST, and the total number of no lost time claims (n=173 796) and lost time claims (n=95 304) for the autumn shift away from DST. In North America, the shift to DST often occurs during the Easter holiday period. When the Easter holiday period coincided with the week preceding DST change, to ensure comparability between weeks, we instead examined claims from the closest week not including the Easter holiday period. When the Easter holiday period coincided with the week of DST change (1994, 1996 and 1999), we removed these years from our analysis, given the reduced labour market participation on Easter Monday in Ontario.
Models were run using SAS PROC GEMOD14 with the number of claims as the dependent variable, examining increases in the number of claims during the week of DST change, and the week following DST change, relative to the week preceding DST change, assuming a Poisson distribution. We further examined if the relationship between DST and work injury claims differed across industry, age, gender and job tenure groups using an interaction term in the model. Analyses were run separately for claims resulting in time lost from work (lost time claims) and claims that required only healthcare but no time away from work other than the day of the accident (no lost time claims).
In the spring, there were no significant effects on the incidence of work injuries observed in the days immediately following the shift to DST. However, there was a significant decrease in both lost time and no lost time claims towards the end of the week on Thursday, Friday and Saturday following the spring transition to DST, compared to the week before the change occurred (table 1). A similar pattern was not found in the week after the spring shift to DST. Regarding the autumn shift away from DST, there was no evidence of declining work injury claims during the week in which the time change occurred or the week afterwards (figure 1). We also analysed whether there were differential effects of shifting to and from DST across industry, age, gender and job tenure groups. Of the numerous interactions tested, three were found to be significant. These were between gender, industry and job tenure with the Friday after the spring shift to DST (data not shown but available upon request from the authors).
Our findings show that shifting to and from DST in the spring or autumn did not significantly increase the number of work injury claims in Ontario during 1993–2007. Contrary to our hypothesis, there was no evidence that the loss of 1 h sleep due to the spring transition to DST increased the occurrence of work injury claims in the days immediately following the time shift. However, we did observe a significant decrease in the number of claims as the week progressed on Thursday, Friday and Saturday. This finding was solely due to the inclusion of years when Good Friday occurred during the daylight saving week (1993, 1998 and 2004) when fewer people are at work due to the Easter holiday period. When these years are removed from our analysis, the decline in work injury claims at the end of the week disappears (results not shown, but available by request from the authors). Regarding the autumn transition from DST, no evidence was found that the gain of 1 h sleep results in a decrease (or increase) in work injuries.
Our findings differ from those of Barnes and Wagner12 who found that workers sustain more workplace injuries and injuries of greater severity on Mondays directly following the switch to DST in the spring. However, their study only used data on mining injuries from reports to the US Mine Safety and Health Administration, making it difficult to directly compare our results with their study. Our findings support those of Holland and Hinze,11 who reported no increase in construction workplace accidents in the USA due to the effects of DST, and of Lahti et al,8 who found no increase in the incidence of accidents requiring hospital treatment using a nationwide hospital register in Finland.
Our results, however, should be interpreted with the following limitations. We have examined the impact of DST using workers' compensation claims data. Workers' compensation claims are unlikely to represent all work injuries occurring in Ontario, with previous studies suggesting that injury severity is an important determinant of whether a claim is submitted or not.15 As such, increases in relatively minor injuries or near misses associated with DST would not be identified in our data source. However, given that the shift to DST is likely to result in more severe injuries,12 it is unlikely that reporting differences alone would substantively change our findings. Our study also had a number of strengths, including the use of injury claims data from all industries covered by the workers' compensation system in Ontario. We were also able to include measures of gender, age, industry and job tenure for the claimants.
In this study, we found no evidence of detrimental effects due to the shift to or from DST on claims for workplace injuries in Ontario from 1993 to 2007. While previous research has suggested a link between sleep quantity and the risk of accidents,3 based on these results we suggest that the adjustment to DST, and its associated impact on sleep quantity, is too small to increase the incidence of workplace accidents.
Access to the data used for this study was provided by the Ontario Workplace Safety & Insurance Board (WSIB).
Funding Peter Smith is supported by a New Investigator Award from the Canadian Institutes of Health Research.
Competing interests None.
Ethics approval The University of Toronto, Health Sciences Ethics Committee approved the secondary data analyses.
Provenance and peer review Not commissioned; externally peer reviewed.