Every year, twice a year, we change our clocks. They are moved forward in March, and back in November. But beyond the hassle of adjusting sleep schedules, scientists say this practice could be taking a significant toll on our long-term health.

For years scientists have warned about the immediate dangers of the time change. In the days following the March switch, rates of heart attacks and fatal traffic accidents rise. A new study goes further. It shows that beyond the short-term risks, the twice-yearly clock changes may also carry significant long-term health consequences and that adopting a permanent time policy could improve our health nationwide.

The worst option? Our current back-and-forth system.

At the heart of the findings is our circadian rhythm, the body's internal clock that governs everything from hormone production to metabolism and immune response. “When you get light in the morning, it speeds up the circadian cycle. When you get light in the evening, it slows things down,” Jamie Zeitzer, a professor of psychiatry and behavioral sciences at Stanford University and senior author of the study, said in a media release. “You generally need more morning light and less light at night to keep well synchronized to a 24-hour day.”

When our circadian rhythms are out-of-sync, as they often are during and after clock changes, the result can be fatigue, metabolic disruption and increased risk for serious conditions like obesity and stroke.

To test how different policies might affect Americans' health, researchers compared three scenarios: 1) permanent standard time; 2) permanent daylight savings time; 3) and the current system of switching twice a year. To do this, they modeled light exposure in every U.S. county and then connected those patterns to health data from the Centers for Disease Control and Prevention.

The results were striking. Permanent standard time, the schedule that is most in line with natural sunlight, would prevent an estimated 2.6 million cases of obesity and 300,000 causes of stroke nationwide.

Permanent daylight saving time also offered benefits, though to a lesser degree. It would potentially prevent 1.7 million cases of obesity and 220,000 strokes. The worst option? Our current back-and-forth system.

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

If it's clear that the clock changes are bad for us, the bigger question may be which time should we stick with? Here, the debate intensifies.

Supporters of permanent daylight saving time argue that more evening light saves energy, deters crime and provides additional after-work leisure. Businesses like golf courses and shopping malls have historically been in favor. But parents object to darker mornings when children head to school and history shows Americans don't always embrace the change. A trial run of permanent daylight saving time in 1974 was abandoned in less than a year after widespread complaints.

Spending more time outside in the morning, maintaining a regular sleep schedule and minimizing bright light in the night can all help strengthen circadian alignment.

On the other hand, health organizations, including the American Academy of Sleep Medicine, the National Sleep Foundation and the American Medical Association, support permanent standard time, which aligns better with the body's need for morning light. “The problem is that it's a theory without any data. And finally, we have data,” notes Zeitzer.

While Congress continues to debate bills that resurface almost every year to lock in daylight saving time, this study may tip the scales by providing some of the strongest health-based evidence to date.

For the average American, the findings are a reminder that light exposure is not just about mood; it's about health. Spending more time outside in the morning, maintaining a regular sleep schedule and minimizing bright light in the night can all help strengthen circadian alignment, regardless of which clock the government sets.

And while no policy can create more daylight in the winter, science suggests that at least dropping the disruptive time changes would make us all healthier. The study's results are evidence for the idea that changing to permanent standard time would be better from a circadian health perspective than permanent daylight time, though researchers believe either would be better than the current biannual shift.

The science may be complex, but the takeaway is simple: when we align our clocks with our bodies rather than the other way around, we stand to gain more energy, better health and perhaps a little more peace of mind each spring and fall.

The study is published in PNAS.