The surprising science behind why daylight saving time is good for wildlife

The surprising science behind why daylight saving time is good for wildlife

By Jackie Flynn Mogensen edited by Claire Cameron

Most people in the US have a love-hate relationship with daylight saving time (DST). On the one hand, we lose an hour of sleep, which can disrupt circadian rhythms, make us cranky and even potentially lead to deeper health problems. On the other hand, it also means longer, sunnier evenings and more daylight for outdoor activities. But what about other animals? Surprisingly, science suggests that daylight saving time may be good for creatures that encounter humans—and especially those that may encounter our cars.

In the United States alone, drivers may collide with deer as often as more than a million times a year, based on estimates compiled by the Federal Highway Administration, and other large animals—usually elk, moose, and other ungulates—are often hit as well. These crashes often kill the animals and cost hundreds of lives.

Animals’ risk of becoming roadkill depends on several factors, including how many vehicles are on the road, how many animals are on the road, and how animals and human drivers behave, explains Tom Langen, a professor of biology at Clarkson University who studies animal-vehicle collisions. However, daylight savings time can minimize these collisions.

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Human-animal crashes usually happen on Fridays because people leave town for the weekend; during full moons because deer are more likely to be on the move; during the fall deer mating season in North America; and at dusk.

“The animals become active just after dusk and start moving around, including crossing roads or surfing and grazing along roads, and that’s when they get hit by vehicles,” says Langen.

Setting the clocks back in autumn – pushing the commute time on high evenings closer to dusk or after the sun goes down – also increases the chances of cars hitting animals. In a 2021 analysis of more than 35,000 deer-vehicle collisions in New York state, Langen and a co-author concluded that reverting to DST contributed to “far higher” crash rates, with the greatest increase on workdays.

A 2022 study by researchers at the University of Washington concluded that permanent daylight saving time could save about 33 lives per year, prevent about 36,000 deer deaths from car crashes and reduce some of the $1 billion in costs associated with those collisions. Such costs include property damage and medical expenses.

“We were surprised to find that the increase in collisions that you get when it’s darker in the evening during standard time was not offset by reduced collisions in the morning,” said Laura Prugh, senior author of the 2022 paper and professor of quantitative wildlife science at the University of Washington.

In fact, the rate of deer collisions was 14 times higher just after dark than before sunset — resulting in a “net increase” in deer collisions during the standard-time months, she says.

Setting the clocks forward in the spring means darker morning commutes, but not much extra risk to deer and other ungulates. This is partly because deer tend to be less active in the spring, says Langen. But it is also because the evening commutes will mostly take place before dusk.

In other words, from a human perspective, daylight saving time is a hit or miss, depending on who you ask. (And opinion polls indicate that opinions on that are mixed.) But for animals like deer, a switch to permanent daylight saving time in North America will almost certainly reduce roadkill, says Langen.

And it’s not just deer and ungulates that are at risk – other mammals, including raccoons, skunks and foxes, are also active at dusk, says Langen. In Australia, research shows that even koalas can see the benefits of a switch to permanent daylight saving time.

Ultimately, Langen advises drivers to stay safe by paying attention to peak collision times year-round and keeping in mind that animals often cross the road in groups. Being on call near dusk is also a good practice, adds Prugh.

“It is important for drivers to be aware that encounters with animals can happen at any time, but there are certain times when you are particularly vulnerable,” says Langen.

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