Every spring and fall, the clock shift throws off sleep and schedules. A new analysis suggests the harm doesn’t stop after a groggy Monday. Modeling light, body clocks, and health points to a simple fix. Pick one time and stick with it – ideally standard time.

Researchers at Stanford Medicine modeled how three policies – permanent standard time, permanent daylight saving time, and the twice-yearly switch – would shape circadian rhythms and downstream health.

The study was led by graduate student Lara Weed in collaboration with Dr. Jamie Zeitzer, the study’s senior author.

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

Why daylight saving hurts health

The team focused on circadian rhythm, the body’s nearly 24-hour clock that orchestrates sleep, metabolism, immune function, and more. This clock is adjusted by light: exposure in the morning shifts it earlier, while light in the evening delays it.

“When you get light in the morning, it speeds up the circadian cycle. When you get light in the evening, it slows things down,” said Dr. Zeitzer. “You generally need more morning light and less evening light to keep well synchronized to a 24-hour day.”

The current policy forces a biannual lurch. That jolt has been tied to spikes in heart attacks and fatal crashes right after the spring jump. The new analysis looks beyond that acute window to the year-round misalignment that comes from toggling the clock.

Standard time aligns with biology

Using county-level sunrise and sunset times, the researchers translated predicted light exposure under each policy into “circadian burden,” the amount the body clock must shift to stay aligned with a 24-hour day.

On average, people would bear the least circadian burden under permanent standard time, which prioritizes morning light. Geography within a time zone matters, as does chronotype.

Early birds – about 15 percent of people – actually fare best under permanent daylight saving time because a bit more evening light stretches their naturally shorter internal day closer to 24 hours. For everyone else, the standard-time bias toward morning light wins out.

New data for a heated debate

The debate, of course, is heated. “You have people who are passionate on both sides of this, and they have very different arguments,” noted Dr. Zeitzer.

Backers of permanent daylight saving time tout brighter evenings for energy, safety, and leisure, though a 1974 trial fizzled within a year as dark winter mornings drew public pushback.

Medical and sleep organizations have rallied behind year-round standard time on theoretical grounds.

“It’s based on the theory that early morning light is better for our overall health,” Dr. Zeitzer said of the endorsements. “The problem is that it’s a theory without any data. And finally, we have data.”

Clock policy shifts disease risk

To link misalignment to real conditions, the team connected circadian burden with county-level prevalence of stroke, obesity, diabetes, depression, coronary heart disease, COPD, cancer, and arthritis.

The model suggests permanent standard time would trim obesity prevalence by 0.78 percent – about 2.6 million fewer people – and reduce stroke prevalence by 0.09 percent – roughly 300,000 fewer cases – each year across the United States.

Permanent daylight saving time would deliver smaller, but still notable, gains: a 0.51 percent drop in obesity (about 1.7 million people) and a 0.04 percent drop in stroke (around 220,000 cases). As expected, conditions with no clear circadian link, such as arthritis, did not budge in the model.

Simulating how light shapes health

The researchers assumed a fixed, circadian-friendly pattern to isolate the time-policy effect: sleep from 10 p.m. to 7 a.m., sunlight before and after work and on weekends, and typical indoor lighting from 9 a.m. to 5 p.m. and after sunset.

That created a clean test bed for comparing policies. But real life is messier. Shift work, late-night screens, and indoor living pull people away from morning light and toward evening glow.

“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.

Health impacts need broader tests

Zeitzer stresses that the model almost certainly paints a best-case scenario for light hygiene. “People’s light habits are probably much worse than what we assume in the models,” Zeitzer said. “Even in California, where the weather is great, people spend less than five percent of their day outside.”

Weather, latitude, building design, commute patterns, and behavior could all blunt or amplify the benefits of a given policy. The study also doesn’t weigh economic, crime, or social outcomes; those need their own evidence-driven analyses.

Standard time is the best fit

Even if lawmakers pick a time and stop switching, winter will still be dark and summer bright. Changing the policy only relabels the sun’s schedule.

“That’s the sun and the position of Earth,” said Dr. Zeitzer. “We can’t do anything about that.”

If the goal is better health, the model’s message is straightforward. Permanent standard time aligns better with human biology for most people, and any permanent choice beats the twice-yearly flip. The path forward is a policy decision, but this time, it comes with data.

The research is published in the journal Proceedings of the National Academy of Sciences.

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