Study suggests most Americans would be healthier without daylight saving time

Skip to main content

Menu

Search

Clear Submit

Research<br>\r\nEducation<br>\r\nPatient Care<br>\r\nGive<br>\r\nAbout<br>\r\n"}}" id="text-5af3feedeb" class="cmp-text"><br>Research

Education

Patient Care

Give

About

Stanford Medicine researchers found that staying on standard time would benefit the most people.

Adobe Stock/zephyr_p

News

Sleep September 15, 2025

Study suggests most Americans would be healthier without daylight saving time

By Nina Bai

According to a new analysis by Stanford Medicine scientists, changing clocks twice a year disrupts circadian rhythms, leading to higher rates of stroke and obesity.

Share Article:<br>Facebook LinkedIn Twitter Email

Every spring, Americans dutifully adjust their clocks forward to daylight saving time, and every fall, back to standard time — but no one seems very happy about it. The biannual time shift is not only inconvenient, it’s also known to be acutely bad for our health. The collective loss of an hour of sleep on the second Sunday in March has been linked to more heart attacks and fatal traffic accidents in the ensuing days.<br>\nNow, a study by Stanford Medicine researchers finds there are longer-term hazards as well — and better alternatives.<br>\nThe 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 throughout the country. Circadian rhythm is the body’s innate, roughly 24-hour clock, which regulates many physiological processes.<br>\nThe 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 benefitting the most people.<br>\nIndeed, by modeling light exposure, circadian impacts and health characteristics county by county, the researchers estimate that permanent standard time would result in some 300,000 fewer people having suffered from a stroke and result in 2.6 million fewer people having obesity. Permanent daylight saving time would achieve about two-thirds of the same effect.<br>\n“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, professor of psychiatry and behavioral sciences and senior author of the study that published Sept. 15 in the Proceedings of the National Academy of Sciences. The lead author is Lara Weed, a graduate student in bioengineering.<br>\nA theory lacking data<br>\nEven among people who want to end seasonal time shifts, there’s disagreement over which time policy to adopt.<br>\n“You have people who are passionate on both sides of this, and they have very different arguments,” Zeitzer said.<br>\nSupporters of permanent daylight saving time say more evening light could save energy, deter crime and give people more leisure time after work. Golf courses and open-air malls are big proponents, Zeitzer said. A trial of permanent daylight saving time begun in 1974, however, was so unpopular it was abandoned after less than a year. Among the objectors were parents worried about their children going to school in the dark.<br>\nNevertheless, the duration of daylight saving time was later increased from six months to seven months. And since 2018, a bill proposing permanent daylight saving time has been introduced in Congress nearly every year, though it has never passed.<br>\nIn the other camp, proponents of permanent standard time contend that more morning light is optimal for health. Organizations such as the American Academy of Sleep Medicine, the National Sleep Foundation and the American Medical Association have endorsed year-round standard time.<br>\n“It’s based on the theory that early morning light is better for our overall health,” Zeitzer said of these endorsements. “The problem is that it’s a theory without any data. And finally, we have data.”<br>\nSyncing to 24 hours<br>\nThe human circadian cycle is not exactly 24 hours — for most people, it’s about 12 minutes longer — but it can be modulated by light.&nbsp;<br>\n“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.”<br>\nAn out-of-sync circadian cycle has been associated with a range of poor health outcomes.<br>\n“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,” Zeitzer said.<br>\nThe 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...