A new Stanford Medicine study suggested that Americans could avoid millions of cases of obesity and hundreds of thousands of strokes each year by ending the practice of changing clocks for daylight saving time, The Hill reported.
For many Americans, resetting the clock each spring and fall is little more than an inconvenience. But researchers at Stanford Medicine say the biannual shift carries serious health consequences and that permanent time policies could deliver major public health benefits.
The study, published in the Proceedings of the National Academy of Sciences, used advanced circadian rhythm models paired with Centers for Disease Control and Prevention data.
Researchers found that moving to permanent standard time would prevent an estimated 2.6 million obesity diagnoses and about 300,000 stroke cases annually. Permanent daylight saving time would also reduce health risks, though with roughly two-thirds the benefit of standard time.
“We found that staying in standard time or staying in daylight saving time is definitely better than switching twice a year,” said Dr. Jamie Zeitzer, senior author and professor of psychiatry and behavioral sciences.
Lead author Lara Weed, a graduate student in bioengineering, said the findings underscore how the twice-yearly shift disrupts circadian rhythms, the body’s natural clock that regulates sleep and key biological processes. Those rhythms, set by exposure to light and darkness, influence everything from blood pressure to metabolism.
Disrupting that cycle, especially through delayed evening light under daylight saving time, pushes melatonin production later, leading to sleep deprivation and higher risks of obesity, heart disease, and cognitive decline.
“People’s light habits are probably much worse than what we assume in the models,” Zeitzer said. “Even in California, where the weather is great, people spend less than 5% of their day outside.”
The health findings add to a long-running debate in Washington and across the country. Sen. Rick Scott, R-Fla., has again proposed the Sunshine Protection Act, a bill that would make daylight saving time permanent.
“Americans are sick and tired of changing their clocks twice a year. It’s an unnecessary, decades-old practice that’s more of an annoyance to families than a benefit to them,” Scott said while introducing the measure.
But many sleep experts argue that permanent standard time is healthier. The National Sleep Foundation said in a statement that daylight saving time “essentially fights the natural order of our circadian rhythms” and leaves Americans out of sync with their biological clocks.
Congress briefly enacted permanent daylight saving time in the 1970s, but public opposition over dark winter mornings led to its repeal. More recently, the Senate passed Scott’s proposal in 2022, but the measure died in the House. Similar efforts have been reintroduced this year, though none have cleared both chambers.
The next time change is scheduled for Nov. 2, when Americans will again “fall back” an hour.
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