According to new research, maintaining a consistent standard time throughout the year may be the healthiest option for most people. (Image courtesy of Peter Cade via Getty Images)
A new model using data from more than 300 million Americans suggests that eliminating daylight saving time twice a year could prevent more than 300,000 strokes and reduce more than 2 million cases of obesity each year.
Researchers have found that switching to a permanent standard time (ST) has less of an impact on our circadian rhythms—the body's rough 24-hour pacemaker—than switching to daylight saving time (DST) or switching between time zones twice a year.
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“We can't make public policy without data,” study co-author Jamie Seitzer, a professor of psychiatry and behavioral sciences at Stanford University, told Live Science. He called the study's findings “the beginning of a dialogue.”
Is it time for a change?
Daylight saving time, which moves clocks forward an hour each spring, was first introduced in the United States in 1918 to save fuel during World War I. It later became federal law with the Uniform Time Act of 1966.
Our circadian rhythm regulates and coordinates the function of every cell in the body. This internal pacemaker is extremely sensitive to light, and disruption to it, such as when people are forced to wake up or stay awake during darkness, creates “circadian stress,” which is associated with a significant increase in adverse health outcomes, including heart attacks, strokes, and car accidents, during the transition to a new cycle.
Sleep deprivation and circadian stress are also metabolic stressors associated with decreased energy expenditure and excess food intake—risk factors for weight gain and obesity. Therefore, the American Academy of Sleep Medicine advocates for the complete abolition of daylight saving time and the permanent use of daylight saving time. Although the health risks of switching to daylight saving time twice a year are small for individuals, the effects are significant at the population level, noted Seitzer.
“You're really looking at a risk comparable to a lottery ticket. But if 350 million people do it on the same day, someone's bound to win,” Seitzer told Live Science. “It's just not a situation where you want to win.”
To model the health impact of circadian-environment misalignment, Seitzer and his colleague Lara Weed, a bioengineer at Stanford University, calculated the circadian load associated with each time policy: permanent standard time, permanent daylight saving time, and a biennial daylight saving time switch.
To do this, they simulated the exposure of residents of each US county to artificial light and sunlight over the course of a year, across all three time-of-day scenarios. They assumed regular sleep hours (10:00 PM to 7:00 AM) and work in a well-lit office (9:00 AM to 5:00 PM, Monday through Friday).
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A group of scientists found that switching time zones twice a year results in the greatest disruption of circadian rhythms and, therefore, the greatest circadian load.
The researchers then used county-level chronic disease prevalence data to determine how various circadian factors predicted eight disease outcomes: arthritis, cancer, chronic obstructive pulmonary disease, coronary heart disease, depression, diabetes, obesity, and stroke. They controlled for various socioeconomic and health factors that can also determine disease prevalence, such as high blood pressure, health insurance, and unemployment.
Adherence to daylight saving time (DST) would have prevented approximately 2,602,866 cases of obesity and 306,988 strokes compared to a biennial DST. Permanent DST would have prevented obesity and stroke to a lesser extent: 1,705,437 fewer predicted cases of obesity and 220,092 fewer strokes than a biennial DST.
Their model showed that none of the policies resulted in statistically significant reductions in six other health indicators.
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However, the study relied on modeling that intentionally included unrealistic assumptions, including consistent light exposure and sleep, and did not account for seasonal behavioral changes. Furthermore, the treatment outcome dataset relied on self-reported information, such as body mass index (BMI) and history of stroke.
Furthermore, they did not include race in their model, despite persistent racial and ethnic sleep disparities in the United States. This exclusion “doesn't necessarily change the study's findings,” said Karin Johnson, a professor of neurology at the University of Massachusetts Chan School of Medicine at Baystate and a spokesperson for the American Academy of Sleep Medicine, who was not involved in the study.
However, this means that the increased risk of stroke and obesity is likely to be felt most strongly by people [namely Black and Hispanic people] who are “already at risk for sleep disturbances for other reasons,” Johnson told Live Science in an email.
TOPICS: Daylight Saving Time
Sophie Berdugo, Social Link Navigator, Live Science Contributor
Sophie is a UK-based staff writer for Live Science. She covers a wide range of topics, having previously covered research ranging from bonobo communication to the first water in the universe. Her work has also appeared in publications such as New Scientist, The Observer, and BBC Wildlife, and her freelance work for New Scientist was shortlisted for the 2025 Association of British Science Journalists' Newcomer of the Year Award. Before becoming a science journalist, she earned a PhD in evolutionary anthropology from Oxford University, where she spent four years studying why some chimpanzees are better tool users than others.
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A study claims that eliminating daylight saving time could prevent more than 300,000 strokes per year in the United States.
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1Research claims that eliminating daylight saving time could prevent more than 300,000 strokes per year in the United States.
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