“Spring forward in the spring, fall back in the fall.”
Almost everyone has heard this mnemonic device before, a reminder to change their clocks twice a year — once on the second Sunday in March and again on the first Sunday in November. The tradition, known as daylight saving time (DST), recently ended its last cycle on Nov. 2, when most states returned to standard time.
But over the years, this biannual ritual has faced mounting criticism. Many studies have shown that it no longer provides the energy savings or practical benefits it once promised when it was originally implemented during World War I and revived for World War II. Yet despite the evidence and widespread discontent, DST remains in effect in all but two U.S. states, and the debate over its validity continues.
“DST was introduced as a wartime measure. I don’t think it was ever really meant to be permanent. I think it was meant to address the technological limitations of the time,” said Lara Weed, a Stanford Ph.D. candidate in bioengineering who is studying sleep and circadian rhythms. “The thought was mostly around energy, but there are a couple of things that we know now that we didn’t know then, and capabilities that we have now that we didn’t have then.”
The science of the shifts: Circadian rhythms
One of the most widely cited arguments against DST is its negative health effects. However, when people discuss these, they often point to the lack of — or, in the fall, the excess of — sleep as the main culprit.
In reality, it’s the time changes themselves that are harmful. By disrupting the alignment between society’s clock, the position of the sun, and the body’s own internal timekeeper, called the circadian rhythm, these biannual shifts throw human biology off balance.
“Like a sine wave, your circadian rhythm also has a period, and the period of that clock is a little bit longer than 24 hours — it’s about 24.2 hours — so every day, you need to shorten that rhythm so it stays in sync with the outer world and your bedtime doesn’t drift later and later every day,” Weed said.
The way this daily adjustment happens is through sunlight exposure. Morning light acts as a reset signal, helping the body realign with the environment.
“Not only does your circadian rhythm coordinate you with the outer world, but it also has a really important job of coordinating all of your internal world as well. Each of your cells and organ tissues has to coordinate with each other, and they have their own clocks, so where you can run into some health problems is when these clocks become out of sync,” Weed said.
That desynchronization is exactly what happens when clocks jump forward or backward for DST.
“Switching from standard time to DST is a very quick change that can confuse your clock system, and then you have to readapt,” Weed said.
However, this process of readaption doesn’t always go smoothly. It can trigger a range of health issues, which is why the transitions between standard time and DST are often accompanied by spikes in injuries, strokes, and heart attacks.
“The idea is that, in the spring, when we’re transitioning from standard time to DST, you’re getting up an hour earlier, you’re standing up, and your body’s not quite ready for it because it’s anticipating you getting up an hour later, so it’s putting a lot of pressure on your heart,” said Jamie Zeitzer, a Stanford professor of psychiatry and behavioral sciences. “Of course, if you’re a high school student, it’s not a big deal, but if you’re 75 years old, it could be the thing that gives you a heart attack.”
While a single person may feel and see no difference, the cumulative effects become clear when put on a national scale.
“The risk to any individual is quite small. It’s when you have 350 million people doing it on the same day that you can see the risk pop out. So, you can think of it more like a lottery ticket. If everyone’s getting a lottery ticket on the same day, someone’s going to win. It’s just not the kind of lottery you want to win,” Zeitzer said.
The legal stalemate
While the public increasingly agrees that the time change is outdated and even harmful, legislators remain divided over what should replace it. It’s no longer about whether DST is worthwhile, but about which time system should become permanent: standard time or DST.
“When we talk about standard time versus DST, that’s considering where noon is for that time zone — if the sun is perfectly at its highest point at noon, which is what you would see under standard time, or if there’s an offset, and you see that the sun’s at its highest point at 1 p.m. rather than noon, which is DST,” Weed said. “Another way that you can think about that is when society starts compared to when the sun rises. Under DST, society starts one hour earlier, so you’d have to get up earlier for school, and you would have a little bit more light exposure in the afternoon. On the other hand, on standard time, society starts one hour later, so in the morning, you don’t have to get up as early, but when you do go outside, the sun is brighter. But on the flip side, it’s a little bit darker in the afternoon and evening.”
In California, this conversation began years ago. In 2018, Proposition 7 passed with nearly 60% of voter support, authorizing the state legislature to alter the daylight saving schedule or establish DST permanently. However, the measure still required a subsequent two-thirds majority vote in both the California Senate and State Assembly, along with consistent federal policies, so seven years later, nothing has changed.
“We can only change it under certain circumstances, and those circumstances haven’t been met yet. We’d have to agree upon one or the other, and if we agreed on DST to be permanent, then we would need congressional approval too, and that hasn’t happened yet,” said Diane Papan, a California State Assembly member.
Under current federal law, states can opt out of DST entirely, but they cannot independently stay on DST year-round. Most states have not gone this route and continue with the biannual switches, but Hawaii and Arizona are exceptions.
“Right now, it’s a little bit of a free-for-all, at least in terms of whether you’re switching back and forth or on permanent standard time. But the federal government does set the dates that you switch back and forth,” Weed said.
Hawaii abandoned DST just weeks after it began in 1967, reasoning that its position near the equator, where day and night lengths vary little year-round, made the practice unnecessary. Arizona followed the next year, concluding that its hot desert climate would cause more energy use, not less, with an extra hour of daylight during the summer.
California has made several attempts to follow their lead. Most recently, in December 2024, Senate Bill 51 proposed placing the state on permanent standard time and repealing provisions from Proposition 7. The bill stalled in committee, and it remains unclear if or when it will be taken up again.
“I think the public is yearning for permanency, but I don’t think the political will is there at this moment in time. So then what happens is we have DST, and then we fall back into standard time, and everyone gets grumpy, but nothing happens,” Papan said.
This isn’t the first time the country has experimented with a permanent time system since DST was first introduced. In 1974, amid an energy crisis, Congress adopted year-round DST, but the decision was quickly reversed due to national outrage after Florida saw eight schoolchildren hit by cars when they had to walk to school in the dark.
Today, the conversation has returned through the Sunshine Protection Act, which aims to make DST the national standard. However, it has repeatedly failed to pass both the House and Senate, in 2022 and again in October 2025, and has yet to be revisited.
“The Sunshine Protection Act didn’t get very far in Congress, so it doesn’t seem like there’s a movement afoot. It seems like everybody hates having a time change, but the fortitude to go with one or the other, either at the federal level or the state level, doesn’t seem to be there,” Papan said.
The struggle in compromise: Time zones
Every year, the debate over daylight saving policy resurfaces twice, every time the clocks have to shift, but a consensus always remains out of reach. Unbeknownst to many, much of this ongoing disagreement isn’t just a result of varying views; it stems from geographical differences.
“There are many different things that go into time policy, especially in a country as big as the United States,“ Weed said. “The first is how much latitude it covers, such that sunrise on one side of the country is three or four hours different from sunrise on the other side of the country, so that’s where time zones come into play. But then we’re also a pretty tall country, so there are north and south variations, which typically come out as seasonal variations and how much light exposure you’re getting. So it’s pretty complicated to make a one-time policy for the entire United States because it’s so big.”
The United States largely solved this issue with the creation of the four major time zones, aligning most cities’ waking hours with daylight. Still, even within time zones, there’s an imbalance. People on the far east side of a zone experience sunrise and thus start their days earlier than those on the far west, creating a gradient of opinions about how a zone’s time should be decided.
“You might have one line where everybody’s perfectly lined up at noon, but, if you go east, the sun rises a little bit earlier relative to the clock. So, if you’re in the middle of a time zone and have sunrise at 7 a.m., on the east side, people get sunrise at 6:30 a.m., and then the western people get sunrise at roughly 7:30 a.m. So there’s variation there, and research also shows that people who are on the western side of time zones generally have worse health outcomes and more negative consequences, so it’s not a perfect system having these very big time zone lines, and there’s some room for adaptation there,” Weed said.
According to Zeitzer, before the standardization of clocks, such discrepancies didn’t exist.
“The best time scheme is local time. If you think back 200 years, there was a bell tower in the center of town, where, at high noon, when the sun was in the middle of the sky, it rang. That was noon, and that’s how every town had time kept. That’s great because that means your day is always relativized to the town that you’re living in,” Zeitzer said. “But that’s not really going to work for modern society. We can move around too quickly, and we have too much communication, so that’s not going to happen.”
Zeitzer suggests a practical solution to the modern time offset that doesn’t depend on legislation.
“Standard work hours are 9 a.m. to 5 p.m., so maybe we can make them 10 a.m. to 6 p.m., and then, all of a sudden, you would have more light in the morning, or you can make them 8 a.m. to 4 p.m., and then you’d have more light in the afternoon. You can change what is considered to be your clock hours of indoor time, but what you can’t do is change how much light there is,” Zeitzer said. “So that’s what this discussion really is: how do we assign a clock hour to the sun hours?”
Choosing a clock: New research
“I think it’s a little bit of propaganda. DST sounds really good, so when you have all of these politicians trying to sell their policy, they find it’s much easier to sell DST than it is to sell standard time,” Weed said.
Although much of California’s past legislation has leaned toward adopting permanent DST, new circadian research by Weed points in another direction.
“What we found is that our current policy of switching back and forth twice a year has the worst health consequences, and that’s what the status quo is. We found that switching to either permanent time policy, either permanent DST or permanent standard time, would improve health outcomes, but switching to permanent standard time would improve those health outcomes more,” Weed said.
According to Weed’s data, staying on standard time year-round could bring measurable public health benefits, potentially reducing obesity cases in the United States by about 2.7 million people and lowering stroke cases by roughly 300,000 people.
Yet on a national scale, these figures represent only small fractions of the population, leaving room for continued debate.
“I think most people take a very personal approach to it. They think, ‘How does it impact me? What do I want? What do I like?’ And that’s fair enough because the biomedical impact from the circadian system is very small for the individual. For example, there is less than a 1% reduction in the number of people with obesity in the country, and a 0.5% decrease in your lifetime chance of having a stroke. That’s really not a big deal, but then it’s 1% of 350 million Americans. That’s a real number. That’s 3.5 million people,” Zeitzer said. “And that’s what makes it complicated because, to the individual, it’s really a vanishingly small impact on their health, but at a population level, it’s so much bigger.”
Additionally, Weed emphasizes that these effects are not felt evenly across the population. Some groups are more vulnerable than others, a disparity that continues to influence personal opinions on which time standard should prevail.
“There are limitations to this type of modeling and this type of approach, for example, a healthy young person probably wouldn’t have a huge impact from going back and forth between the different time policies. The people who are likely to be most affected are older individuals or people who are already predisposed to something, and then DST, or switching back and forth, pushes them over the edge to moving into a not-so-good health space,” Weed said.
Given that these reductions may only affect certain groups, and that the overall lower risk is relatively small, Zeitzer argues that more discussion is needed before any definitive policy change can be made.
“I don’t think this has been fully discussed yet. The circadian impact, which is just one of the pieces, isn’t so overwhelming as to obviate all other discussion on the topic,” Zeitzer said. “If we found, like, if you switch to this, there’d be a 50% reduction in strokes, that’s just so immense and huge that it doesn’t matter what you like, we have to do it. But we don’t see that. What we’re seeing is much more subtle effects, and when you see these subtle effects, it means that then the conversations should be more meaningful.”
Yet it’ll be a hard clock to settle. While new scientific data may sway future policy, changes in legislation often move far more slowly than changes in understanding.
“Our work basically provided the first data to look at what would happen if we switched to a one-time policy for the entire year,” Weed said. “So I think a lot of the legislation will take some time to incorporate this type of work into it, but it’s hard to backtrack from choosing the wrong pathway with this Sunshine Protection Act. There have been a lot more discussions about it, especially recently, where we’ve seen hundreds of articles come out related to our research, and with this past DST transition. We just have to wait and see what happens.”
For now, the nation remains caught between two clocks. While many states continue to push for reform, progress has been slowed by the need for federal agreement, more research, and conflicting interests. Even as the vast majority of Americans say they want to end the time change, consensus on which time to keep remains elusive.
“Everybody’s going to have their own opinion on this, and you can’t really make everyone happy in this case because you’re not really just relying on the science. There’s also a lot of feelings here, and there’s not really enough information in all of the different dimensions to have a really good picture of everything that factors in. Our study is one of the first looking at circadian health, but then there are also economic factors, there are safety factors, and nobody’s really done this type of study looking at those other pieces,” Weed said.
