The Cost of Daylight Savings Time

Morning Light, Healthier Life: Why Permanent Standard Time Benefits Our Circadian Clock

Every spring, Americans adjust their clocks forward to daylight saving time, and every fall back to standard time, but these shifts are more than just an inconvenience. The shift each March has been linked to spikes in heart attacks and traffic accidents in the days that follow. Now, new research suggests the health impacts of clock-changing go well beyond those acute effects. In this piece we will look at how a single time policy year-round could make millions of Americans healthier, reducing prevalence of obesity and stroke across the nation.

Why Light Timing Matters for Your Circadian Clock

Humans evolved with a natural circadian rhythm of about 24 hours, synchronized to the solar day-night cycle. Light is the main signal that entrains this internal clock each day. Morning light sends a powerful cue to advance or “speed up” the circadian cycle, helping your central clock stay aligned with local time. Evening light, by contrast, tends to delay or “slow down” the clock. In simple terms, we generally need more light early in the day and less light at night to remain well-synchronized to a 24-hour schedule. If your light exposure is shifted (for example, you miss morning sunlight or get too much bright light late at night), your internal clock can drift out of sync with the world. An out-of-sync circadian rhythm has been linked to a host of health issues, from poor sleep and mood changes to metabolic and cardiovascular problems.

Standard Time vs. Daylight Time: A Healthy Clock for the Body

Given the importance of light timing, the question arises: what happens when we shift the clock itself? In the U.S., we currently switch clocks twice a year. Come March, daylight saving time (DST) jumps sunrise an hour later; in November, standard time (ST) returns sunrise to an earlier hour. This seesaw means months of altered light schedules, and experts have long suspected it’s not ideal for our biology. Leading medical organizations such as the American Medical Association and the American Academy of Sleep Medicine have even called for ending the seasonal time changes, advocating for permanent standard time to maximize morning light. Until now, however, solid data backing one policy over another had been limited.

That changed with a new Stanford-led study by Lara Weed, published in Proceedings of the National Academy of Sciences in 2025. The researchers used mathematical models to compare three scenarios: our current biannual switching (BAS), permanent DST, and permanent ST. They simulated how each policy would influence people’s light exposure throughout the year (based on local sunrise and sunset times) and, in turn, how much circadian disruption or “burden” that would create. The verdict was striking: our habit of switching back and forth is the worst option for circadian health. In contrast, keeping one fixed time year-round leads to significantly less circadian burden, and standard time came out as the healthiest option overall.

“We found that staying in standard time or staying in daylight saving time is definitely better than switching twice a year,” notes Dr. Zeitzer, “with permanent standard time benefiting the most people.” This means aligning our clocks with natural solar time, as standard time does, provides the most robust circadian stability for the majority of people.

Geography and Chronotype: One Size Doesn’t Fit All

Interestingly, the benefits of one time policy over another aren’t uniform across everyone. The study showed that where you live and your chronotype (morning lark versus night owl) influence how much you’d gain from permanent standard time. Geography matters because the United States spans multiple time zones and a wide range of latitudes. People living on the western edge of a time zone currently have considerably later sunrises by the clock (for example, it stays dark later in the morning). These regions suffer more from the lack of morning light, especially under extended daylight saving time, and thus stand to benefit the most from a switch to permanent standard time’s earlier sunrises. In fact, the models predicted greater health improvements for residents in western parts of each time zone under permanent ST compared to those farther east.

Chronotype differences add another twist. If you’re a night owl (an evening person who naturally stays up late and sleeps in later), you tend to have a slightly longer-than-24-hour internal cycle. This makes it harder for you to catch enough morning light under daylight saving time schedules, leaving you more burdened. Owls, who make up a large segment of the population, would therefore see major circadian gains from permanent standard time’s extra morning light. Morning larks, on the other hand, are the minority (about 15 percent of people) with naturally shorter circadian cycles. These early birds often wake up before the rest and feel sleepy sooner in the evening. Counterintuitively, the study found that extreme larks might experience slightly less circadian disruption under permanent DST, since the added evening light helps stretch their short internal day a bit longer. However, for the vast majority of individuals who are not extreme larks, year-round standard time provided the lowest circadian “burden” (the easiest alignment) compared to DST or switching. In summary, permanent ST appears to be the most universally beneficial, while permanent DST might only be preferable for a small slice of very early risers.

A Nationwide Health Impact: Millions Fewer Obesity and Stroke Cases

How do these circadian advantages translate into real health outcomes? To answer that, Weed’s team linked their circadian disruption models to U.S. health data for chronic conditions that have known circadian influences. They focused on two big ones: obesity (metabolic health) and stroke (cardiovascular health), among other conditions. The differences were eye-opening. Under a permanent standard time scenario, the nationwide prevalence of obesity was predicted to drop by about 0.78 percentage points, and stroke prevalence by about 0.09 percentage points, relative to the current switching system. Those percentage changes sound small, but across a country as large as the U.S. they are huge in absolute numbers. A 0.78 percent reduction in obesity equates to roughly 2.6 million fewer Americans classified as obese, and 0.09 percent fewer strokes means about 300,000 fewer stroke cases. In public health terms, that’s a massive improvement potentially preventing numerous deaths and diseases.

Importantly, the study found no significant differences between time policies for illnesses unrelated to circadian rhythms (for example, rates of arthritis did not change). This detail bolsters the case that the observed obesity and stroke improvements are indeed tied to circadian health, not some unrelated factor. While the study didn’t explicitly model other outcomes like mental health, accident rates, or overall mortality, it’s plausible that keeping a steady, sun-aligned schedule year-round would have positive effects there too, given what we know from other research.

Takeaway

This new research provides compelling evidence that how we set our clocks can profoundly influence public health. Of course, adopting permanent standard time won’t magically solve every health problem. People will still need healthy habits and good sleep routines. But as the study by Weed demonstrates, a seemingly small policy choice can have a surprisingly large impact when scaled to a nation of 330 million people. On the order of millions fewer cases of disease, the circadian science is telling us that aligning our social clock with our biological clock matters. As policymakers debate the future of daylight saving time, the message from this research is clear: prioritizing morning sunshine and a stable schedule is a data-backed strategy for a healthier society.

1. References:
   Weed L, Zeitzer JM. Circadian-informed modeling predicts regional variation in obesity and stroke outcomes under different permanent US time policies. Proc Natl Acad Sci U S A. 2025;122(38):e2508293122. doi:10.1073/pnas.2508293122