Hour of need

All but two U.S. states, Hawaii and Arizona, set clocks forward an hour just before spring and back again in the fall. These biannual time shifts are not only inconvenient but also, research shows, unhealthy: The second Sunday in March, when daylight saving time begins, has been linked to more heart attacks and fatal traffic accidents in the ensuing days.

A study by Stanford Medicine researchers found there are longer-term hazards — and better alternatives.

In a national study, the researchers compared how three different time policies — permanent standard time, permanent daylight saving time and biannual shifting — could affect people’s circadian rhythms and, in turn, their health.

The team found that, from a circadian perspective, we’ve made the worst choice. Either permanent standard time or permanent daylight saving time would be healthier than our seasonal waffling, with permanent standard time benefiting the most people.

Indeed, by modeling light exposure, circadian impacts and health characteristics county by county, the researchers estimate that permanent standard time would prevent some 300,000 cases of stroke per year and result in 2.6 million fewer people having obesity. Permanent daylight saving time would achieve about two-thirds of the same effect.

“We found that staying in standard time or staying in daylight saving time is definitely better than switching twice a year,” said Jamie Zeitzer, PhD, a professor of psychiatry and behavioral sciences and senior author of the study, which was published Sept. 15, 2025, in Proceedings of the National Academy of Sciences.

The human circadian cycle is not exactly 24 hours — for most people, it’s about 12 minutes longer — but it can be modulated by 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.”

The researchers used a mathematical model to translate light exposure under each time policy, based on local sunrise and sunset times, to circadian burden — essentially, how much a person’s innate clock has to shift to keep up with the 24-hour day.  They found that over a year, most people would experience the least circadian burden under permanent standard time, which prioritizes morning light.