A study appearing January 30 in the journal Current Biology puts forth evidence of one downside of Daylight Saving Time (DST): it increases the risk of fatal car accidents in the United States for about a week each year.
The evidence shows about a 6% increase in the risk of fatal traffic accidents in the week after the time change each spring. In other words, more than 28 fatal accidents could be prevented yearly in the United States if the DST transition were abolished. The effect is especially pronounced in the morning hours and in locations further west within a time zone.
“The acute adverse effects of DST on fatal traffic accident risk are real, and can be prevented,” says senior author Céline Vetter, Dr. Phil, a circadian sleep scientist at the University of Colorado Boulder, in a release. “Although the observed effects are of moderate size and are not long-lasting, we must not forget that DST transition affects billions of people every year, and thus small changes in risk can have a substantial public health impact.”
The findings come at a time when numerous states, including Oregon, Washington, California, and Florida, are considering doing away with the switch entirely, and mounting research is showing spikes in heart attacks, strokes, workplace injuries, and other problems in the days following the time change.
But, Vetter explains, the findings on car accidents weren’t so clear-cut. In the new study, she and her colleagues, including first author Josef Fritz, set out to look deeper.
They took advantage of the federal Fatality Analysis Reporting System, which recorded 732,835 fatal car accidents across all US states observing DST from 1996 to 2017. After controlling for factors like year, season and day of the week, they found a consistent rise in fatal accidents in the week following the spring time change. Notably, that spike moved in 2007, when the Energy Policy Act extended daylight saving time to begin on the second Sunday of March instead of the first Sunday in April.
“Prior to 2007, we saw the risk increase in April, and when daylight saving time moved to March, so did the risk increase,” says Vetter. “That gave us even more confidence that the risk increase we observe is indeed attributable to the daylight saving time switch, and not something else.”
In absolute numbers, they report, this risk increase translated to an additional 5.7 fatal accidents per day from Monday to Friday after the spring DST transition in the United States. That’s more than 28 deaths during the workweek. Over the course of the 22 years, they report, that’s more than 626 out of 8,958 fatal accidents that might have been prevented.
Those on the western edge of their time zone, in places like Amarillo, Texas, and St. George, Utah, already get less sleep on average than their counterparts in the east—about 19 minutes less per day, research shows—because the sun rises and sets later but they still have to be at work when everyone else does.
“They already tend to be more misaligned and sleep-deprived, and when you transition to daylight saving time it makes things worse,” says Fritz, a postdoctoral researcher in the Department of Integrative Physiology. In such western regions, the spike in fatal accidents was more than 8%, the study found.
The increase kicks in right away, on the Sunday when the clocks spring forward, and the bulk of the additional fatal accidents that week occur in the morning.
Changes in accident patterns also occur after the “fall back” time change, the study showed, with a decline in morning accidents and a spike in the evening, when darkness comes sooner.
Because they balance each other out, there is no overall change in accidents during the “fall back” week.
“The public health impact of the DST transition regarding fatal traffic accident risk is clear from our data,” Vetter says. “Because our data only included the most severe accidents, namely where a fatality was recorded, this estimation is likely an underestimation of the true risk.”
The findings offer yet another reason to consider getting rid of the switch to DST, the researchers say. They now hope to understand and better characterize the effects of DST on individuals and the physiology and health outcomes associated with the “mini-jetlag” DST causes. With such an understanding, they say they’ll be in a better position to identify the people who are most vulnerable and affected by the time change.