Most Americans Would Be Healthier Without Daylight Saving, Study Finds

The ritual is as predictable as it is annoying. Twice a year, Americans collectively fumble with their clocks, springing forward in March and falling back in November. But what if this biannual dance isn’t just inconvenient, what if it’s slowly killing us?

New research from Stanford Medicine suggests our current time policy may be the worst possible choice for public health. By analyzing circadian rhythms across the entire continental United States, scientists have quantified something sleep experts have long suspected: the constant switching between standard and daylight saving time creates a chronic burden on our biological clocks that translates into real health consequences.

The numbers are startling. If America permanently adopted standard time, the researchers estimate we could prevent 300,000 strokes annually and see 2.6 million fewer people struggling with obesity. Even permanent daylight saving time would achieve about two-thirds of these benefits.

The Science of Sync

“We found that staying in standard time or staying in daylight saving time is definitely better than switching twice a year.”

Jamie Zeitzer, a Stanford professor of psychiatry and behavioral sciences who led the study, explains that the human circadian cycle naturally runs about 12 minutes longer than 24 hours, requiring daily reset signals to stay synchronized. Light serves as the primary timekeeper, with morning light speeding up our internal clocks and evening light slowing them down. When this delicate system falls out of alignment, the consequences ripple through our physiology.

Zeitzer’s team used mathematical models to calculate “circadian burden,” essentially how hard our internal clocks must work to keep pace with a 24-hour day under different time policies. They found that most Americans experience the least circadian stress under permanent standard time, which prioritizes morning light exposure.

The research, published in the Proceedings of the National Academy of Sciences, represents the first data-driven analysis of long-term health impacts from different time policies. Previous advocacy for permanent standard time from medical organizations like the American Academy of Sleep Medicine had been based largely on theory.

Beyond the Numbers

The study’s methodology was notably comprehensive, modeling light exposure patterns county by county and accounting for individual chronotypes, whether people are natural early birds, night owls, or somewhere between. Counterintuitively, the roughly 15% of Americans who are extreme morning larks would actually fare better under permanent daylight saving time, as extra evening light would help extend their naturally short circadian cycles.

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

The health implications emerge from this circadian disruption. Using CDC data on disease prevalence, the researchers linked circadian burden to specific conditions known to be influenced by biological clock function. Their models predicted meaningful reductions in obesity and stroke under either permanent time policy, while showing no effect on conditions like arthritis that lack circadian connections.

Despite mounting scientific evidence, America’s time policy remains mired in political and economic debates. Since 2018, Congress has repeatedly considered bills proposing permanent daylight saving time, driven partly by business interests. Golf courses and retailers favor extended evening light, while parents worry about children walking to school in darkness.

The 1974 experiment with year-round daylight saving time was abandoned after less than 10 months due to public opposition. Yet the duration of daylight saving time has gradually expanded from six to eight months annually.

Zeitzer acknowledges his study’s limitations. The models assume relatively healthy light habits, consistent sleep schedules, outdoor time before and after work, that don’t reflect reality for many Americans. “People’s light habits are probably much worse than what we assume in the models,” he noted, pointing out that even in sunny California, people spend less than 5% of their day outside.

The research also can’t account for numerous factors that influence real-world light exposure, from weather patterns to individual behavior variations. But it provides the first quantitative framework for evaluating time policy’s health consequences.

As the debate continues, one thing seems clear: our current approach of switching twice yearly appears to be the worst of all options. Whether America chooses permanent standard or daylight saving time, millions of people could benefit from simply stopping the clock shuffle that has defined our springs and falls for over a century.

Proceedings of the National Academy of Sciences: 10.1073/pnas.2508293122