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TUESDAY, Sept. 16, 2025 (HealthDay News) — Most Americans would be healthier if the nation dropped daylight saving time, a major new study says.

Switching to permanent standard time would prevent about 300,000 cases of per year and result in 2.6 million fewer people with obesity, researchers estimate in the .

Permanent daylight saving time also would improve the nation’s health, but only by about two-thirds of the expected effect from permanent standard time, researchers say.

In other words, the United States has made the worst choice possible for the nation’s health by seasonally waffling between “falling back� and “springing forward,� researchers concluded.

“We found that staying in standard time or staying in daylight saving time is definitely better than switching twice a year,� senior researcher said in a news release. He’s a professor of psychiatry and behavioral sciences at Stanford University in California.

For the study, researchers estimated how different national time policies might affect American’s circadian rhythms – the body’s innate clock that regulates many physiological processes.

The human circadian cycle isn’t exactly 24 hours, researchers noted. It’s about 12 minutes longer for most people, and it can be changed based on a person’s exposure to light.

“When you get light in the morning, it speeds up the circadian cycle. When you get light in the evening, it slows things down,� Zeitzer said. “You generally need more morning light and less evening light to keep well synchronized to a 24-hour day.�

An out-of-sync circadian cycle has been linked with many different poor health outcomes, researchers said.

“The more light exposure you get at the wrong times, the weaker the circadian clock,� Zeitzer said. “All of these things that are downstream — for example, your immune system, your energy — don’t match up quite as well.�

Most people would experience the least circadian burden under permanent standard time, which prioritizes morning light, researchers found.

The research team then linked its analysis of circadian rhythms to county-level data from the U.S. Centers for Disease Control and Prevention (CDC) to see how each time policy might affect people’s health.

Their models showed that permanent standard time would reduce obesity nationwide by 0.78% and stroke by 0.09%. Those seemingly small percentage changes, when played out across the national population, would mean 2.6 million fewer people with obesity and 300,000 fewer cases of stroke.

Permanent daylight savings time would result in a 0.51% drop in obesity — around 1.7 million people — and a 0.04% reduction in strokes, or 220,000 cases.

Either move would help American health. “You have people who are passionate on both sides of this, and they have very different arguments,� Zeitzer said.

Supporters of permanent daylight saving time say more evening light would give people more leisure time to unwind after work, save energy and deter crime. Golf courses and open-air malls are big proponents of this time policy, Zeitzer said.

But permanent daylight saving time already has had a trial run, back in 1974, and officials pulled the plug in less than a year due to its intense unpopularity. Parents worried about their kids going to school in the dark, researchers noted.

Right now, daylight savings time runs seven months each year, with five months of standard time.

A bill proposing permanent daylight savings time has been introduced in Congress nearly every year since 2018, but has never passed.

On the other hand, health groups like the American Medical Association, the American Academy of Sleep Medicine and the National Sleep Foundation have endorsed year-round standard time, researchers noted.

“It’s based on the theory that early morning light is better for our overall health,� Zeitzer said of the groups’ position. “The problem is that it’s a theory without any data. And finally, we have data.�

Earlier studies have linked the “spring forward� move to daylight savings time with increases in heart attacks and car accidents, researchers noted in their paper. The “fall back� shift does not produce the same negative effects.

This new study might be the most comprehensive attempt yet to bring evidence to bear upon the debate, researchers argued. 

However, the team noted that their estimates aren’t perfect, as they assumed that people have relatively consistent light habits in tune with their circadian rhythms. 

That would mean a 10 p.m. to 7 a.m. sleep schedule, sunlight exposure before and after work, and indoor light exposure from 9 a.m. to 5 p.m.

“People’s light habits are probably much worse than what we assume in the models,� Zeitzer said.

Zeitzer also noted that no policy is going to add more light to the dark winter months, regardless of human finagling.

“That’s the sun and the position of Earth,� Zeitzer said. “We can’t do anything about that.�

More information

Harvard Medical School has more on the .

SOURCES: Stanford Medicine, news release, Sept. 15, 2025; Proceedings of the National Academy of Sciences, Sept. 15, 2025

What This Means For You

A move to a permanent time policy could help improve Americans’ lives.