The science behind why people hate Daylight Saving Time so much

In the summer of 2017, when communication professor Jeffery Gentry moved from Oklahoma to accept a position at Eastern New Mexico University, he was pleasantly surprised to find it easier to get up in the morning. The difference, he realized, was early morning light. On September mornings in Portales, New Mexico, Gentry rose with the sun at around 6:30 am, but at that time of day in Oklahoma, it was still dark.

As the Earth rotates, the sun reaches the eastern edge of a time zone first, with sunrise and sunset occurring progressively later as you move west. Gentry’s move had taken him from the western side of Central Time in Oklahoma to the eastern edge of Mountain Time. Following his curiosity into the scientific literature, he discovered the field of chronobiology, the study of biological rhythms, such as how cycles of daylight and dark affect living things. “I really just stumbled upon it from being a guinea pig in my own experiment,” he said.

In 2022, Gentry and an interdisciplinary team of colleagues added to that body of research, publishing a study in the journal Time & Society that showed the rate of fatal motor-vehicle accidents was highest for people living in the far west of a time zone, where the sun rises and sets at least an hour later than on the eastern side. Chronobiology research shows that longer evening light can keep people up later and that, as Gentry found, morning darkness can make it harder to get going for work or school. Western-edge folks may suffer more deadly car wrecks, the team theorized, because they are commuting in the dark while sleep deprived and not fully alert.

With all the hullabaloo over the health and safety of setting clocks forward an hour in the spring for Daylight Saving Time (DST) and back in the fall with Standard Time (ST), could where you live in a time zone actually have a more profound effect? I asked Gentry. “That’s very possible,” he said.

Time researchers make this point, and research results and public opinion polls reflect it: Something is awry about the way we mark time. Those problems start with the annual toggle between DST and ST. In these days of sharp division, poll after poll finds most people unified in their dislike of switching clocks back and forth with the season. However, the question of whether to stick with ST or DST year-round once again sends people to different camps.

Scientists generally advocate for permanent ST, or “natural time,” as Gentry calls it because it better aligns people’s schedules with the sun year-round. “People who study the issue are all in agreement,” he said. On the other hand, public opinion on both sides of the Atlantic tends to favor permanent DST—and many politicians agree—perhaps because of the positive associations with summer sunshine. (A bill to make that switch passed the US Senate unanimously in 2022 but then stalled in the House; a new version was recently reintroduced.)

Scientists generally advocate for permanent standard time because it better aligns people’s schedules with the sun year-round.

Some scientists have fired back that such a move would be a grave mistake: The German newspaper Die Welt quoted pioneering chronobiologist and sleep researcher Till Roenneberg warning that permanent DST would make Europeans “dicker, dümmer und grantiger” (fatter, dumber, and grumpier).

The conflict over DST versus ST makes for grabby headlines and engaging social media posts. But focusing on the clash misses the bigger questions about how we choose to mark time. A close look at the research reveals not only uncertainties about the effects of DST, but also about other factors, such as how time zones are drawn and, possibly most important, how structuring our schedules around light and dark could have a profound impact on health and safety.

“We absolutely need to think about our time,” said Beth Malow, a neurologist and director of the sleep division at Vanderbilt University Medical Center. “And how are we going to actually figure this out as a country?”

Internal clocks are ticking

The 24-hour cycle of light and dark created by the Earth’s rotation is the force that rules our lives. Homer’s rosy-fingered dawn is what chronobiologists call a zeitgeber, German for “time giver”—a natural signal that touches off cyclical processes in the body governing our internal clocks. Morning light, for example, cues our bodies to ramp up production of cortisol, a hormone that helps us feel awake and alert. Meanwhile, as cortisol dwindles through the evening, darkness triggers the sleep-promoting hormone melatonin.

In the language of chronobiologists, the biological clock rhythms of humans and other animals are entrained, or synchronized, to the solar clock.

Humans have devised schemes such as time zones and Daylight Saving Time to optimize their interactions with these natural cycles of light and dark. But the match between time policy and the zeitgeber is often imperfect.

When we set clocks forward with DST in the spring, many people suddenly have to get up for school or work before the light has jumpstarted physiological processes associated with wakefulness. Cortisol levels peak about an hour later during DST according to a 2014 Australian study. Then, at the other end of the day, people have to go to bed before hours of darkness have signaled to their body that it’s time to sleep.

Humans have devised schemes such as time zones and Daylight Saving Time to optimize their interactions with the natural cycles of light and dark.

The abrupt change, especially to DST in the spring, can wreak havoc on health and safety. In a 2020 commentary for JAMA Neurology, Beth Malow and colleagues outline evidence for negative health effects during the DST transition, including less and poorer quality sleep, an increased risk of stroke and heart attack, and a decreased sense of well-being, particularly for men who work full time.

In addition, although the research on road safety is mixed, some studies find an uptick in traffic accidents and fatalities in the days after the DST switch.

However, those bad effects are fleeting. The longer-term impact of DST is hard to research because the amount of sunlight changes with the seasons. Only one study has directly compared permanent DST to permanent ST: a seven-year study of students aged 10 to 24 living in northwestern Russia when the government mandated a switch from seasonal DST to year-around DST in 2011—and then switched again, to permanent ST, in 2014.

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