Stanford study: Daylight Saving Time may be harmful to health

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 clocks each spring and fall is little more than an inconvenience. But researchers at Stanford Medicine say that changing the clocks twice a year carries serious health consequences and that permanent time policies could yield major public health benefits.
The study, published in the Proceedings of the National Academy of Sciences, used advanced circadian rhythm models coupled with data from the Centers for Disease Control and Prevention.
The researchers found that switching to permanent standard time would prevent about 2.6 million obesity diagnoses and about 300,000 strokes each year. Permanent daylight saving time would also reduce health risks, although with about two-thirds of the benefit of standard time.
"We found that staying on standard time or staying on daylight saving time is definitely better than changing time twice a year," said Dr. Jamie Zeitzer, lead author and professor of psychiatry and behavioral sciences.
Lead author Lara Weed, a graduate student in bioengineering, said the findings highlight how the twice-yearly change disrupts circadian rhythms, the body's natural clock that regulates sleep and key biological processes. These rhythms, set by exposure to light and darkness, affect everything from blood pressure to metabolism.
Disrupting this cycle, especially through the delay of evening light during daylight saving time, pushes melatonin production later, leading to sleep deprivation and higher risks for obesity, heart disease, and cognitive decline.
"People's light habits are probably much worse than 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. Senator Rick Scott, Republican of Florida, has re-introduced the Daylight Savings Act, a bill that would make daylight saving time permanent.
"Americans are tired of changing clocks twice a year. It's an unnecessary, decades-old practice that is more of a nuisance to families than a benefit to them," Scott said during the introduction of 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 passed permanent daylight saving time in the 1970s, but public opposition to dark winter mornings led to its repeal. More recently, the Senate passed Scott's proposal in 2022, but the measure failed to pass the House. Similar efforts have been revived this year, though neither has passed both chambers.
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