Winding clocks an hour back this fall, when daylight saving time ends for the year across much of the U.S., might do more than just disrupt sleep: evidence suggests such time changes could damage health in the long run. A new study published on Monday in Proceedings of the National Academy of Sciences USA indicates that eliminating these repeated time shifts and keeping standard time permanently might help lower rates of obesity and stroke.
“Our work reveals that there may be greater health benefits on a population level if we switch to a permanent standard time,” says study co-author Lara Weed, a bioengineering Ph.D. candidate at Stanford University. “We hope that policymakers take a peek at [the rates of disease in] their individual states when they’re making assessments on how to incorporate the information that we provided.”
Past research has shown that switching to standard time every fall and daylight saving time every spring brings various kinds of negative consequences, including a higher risk of heart attacks and car accidents, along with disruptions to sleep, alertness and productivity. Several international scientific associations, including the American Medical Association and the American Academy of Sleep Medicine, have spoken out against time shifting, arguing that the greater morning light exposure provided by standard time could help reduce many of these problems. Few studies have examined the long-term health effects of sticking with a single time policy, however.
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To investigate this question, Weed and her graduate adviser, Stanford sleep medicine researcher Jamie M. Zeitzer, looked at how these two time observations, which are used in most U.S. states, affect the body’s circadian system—the internal biological “clock” that regulates our sleep, metabolism and other essential functions.
This natural timing system is “responsible for keeping you synchronized with the outer world and keeping your organ systems synchronized,” Weed says. “When you have changes in your light exposure pattern, especially ones that might affect the timing of the circadian clock, you might also have impacts on these different organ systems.”
Weed and Zeitzer built computer models to simulate how light exposure affects people’s circadian cycles—and subsequent health—under permanent standard time, permanent daylight saving time and the current system of biannual switching. They then combined these exposure simulations with a mathematical model of the circadian clock, adjusting for geographical factors such as latitude and longitude. Finally, they linked their estimates of yearly “circadian burden”—the amount of time the body would take to adjust itself to stay on track with the time observed in the outside world across each year—to county-level health data from the U.S. Centers for Disease Control and Prevention.
The results showed that, over the course of a year, most people would experience fewer abrupt changes to their circadian rhythms under permanent standard time, in which morning light exposure is greater. With whole-year standard time, the study estimates U.S. obesity prevalence would drop by nearly 0.8 percent—meaning roughly 2.6 million fewer people with obesity nationwide. Stroke prevalence would also fall by about 0.09 percent, representing more than 300,000 fewer cases. Permanent daylight saving time would also help but at a lesser rate, leading to about 1.7 million fewer cases of obesity and 220,000 fewer strokes.
“That, health-care-wise, is definitely significant,” says neuroscientist Roxanne Prichard of the University of St. Thomas in Minnesota, who was not involved in the new research. She adds, however, that the study assumes most people work between 9 A.M. and 5 P.M., whereas a 2024 survey found 16 percent of the U.S. population has nonstandard schedules—a factor that should be taken into account in future studies. Another limitation is that the new study’s model didn’t include other health data, such as car accidents, suicides or drunk driving rates, which could also shift with different time policies. The study authors found no significant effects on other chronic conditions such as diabetes, cancer, arthritis or depression.
Although the results are only computer modeled projections, Weed says they are consistent with scientific organizations’ stance that permanent standard time could help prevent some public health problems and safety risks. Still, Weed notes, more studies are needed to examine the potential economic and social effects of adopting permanent standard time in the U.S. The U.S. government enacted a trial of permanent daylight saving time amid a severe energy crisis in the 1970s. But it ended less than two years later because of public backlash and concerns over children commuting to school in the dark and an increase in car accidents. Other countries have made the move to permanent standard time. Mexico, for instance, eliminated time shifting in most areas in 2022. Lawmakers argued that permanent standard time would improve safety, boost productivity and reduce energy consumption by providing more daylight. “A lot of people don’t realize what a big deal it is when we see light and how that impacts our body,” Prichard says. “The data is clear: permanent standard time is the healthier option.”
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