Story at-a-glance
- Staying on permanent standard time instead of shifting clocks twice a year could prevent millions of cases of obesity and hundreds of thousands of strokes across the U.S.
- Your circadian rhythm, your body’s internal clock, works best with early morning light, which helps regulate sleep, energy, metabolism, and immunity
- Permanent daylight saving time still offers health benefits, but standard time provides stronger protection for the majority of people
- Research shows only 15% of people, called morning larks, align better with daylight saving time, while the rest of the population benefits more from standard time
- You can strengthen your circadian health by getting outside in the morning, limiting bright evening light, keeping your sleep schedule consistent, and supporting policies that keep clocks on standard time year-round
Every year, millions of Americans move their clocks forward in March and back in November, but the health toll of this routine is rarely considered. The disruption goes beyond grogginess — it unsettles your body’s internal clock in ways that ripple through sleep, metabolism, and cardiovascular health.
Losing alignment with your circadian rhythm doesn’t just leave you tired. It increases the likelihood of serious problems like obesity, heart disease, and stroke, conditions that damage quality of life and shorten lifespan. Even accidents and injuries rise in the days after clock changes, underscoring how deeply light and time shape human biology.
The debate over time policy has dragged on for decades, with arguments about energy savings, school safety, and leisure time. What’s been missing is hard data on long-term health effects. New research now offers that evidence, showing how the choice between daylight saving time, standard time, or biannual switching influences your body at the most basic level.1
The findings point to one simple truth: aligning your schedule with morning light supports a healthier rhythm, while shifting clocks away from that alignment carries measurable risks. This makes the discussion about time policy not just a matter of preference but one of public health.
Standard Time Reduces Obesity and Stroke Risk
For a study published in the Proceedings of the National Academy of Sciences, researchers used computer models that factored in the body’s internal clock to compare three scenarios: permanent standard time, which prioritizes more morning light, permanent daylight saving time, which shifts light later into the evening, and the current system of switching back and forth twice a year.2 Their goal was to see how each time policy affects health outcomes like obesity and stroke.
•Findings showed clear benefits for standard time — The analysis revealed that permanent standard time produced the largest reduction in both obesity and stroke compared to the other policies. Specifically, the model estimated that obesity prevalence would drop by about 0.78%, which equals millions fewer Americans struggling with excess weight. Stroke rates also declined by about 0.09%, preventing hundreds of thousands of new cases each year.
•Daylight saving time offered smaller benefits — Permanent daylight saving time also lowered risks but to a lesser extent. The decreases were about 0.51% for obesity and 0.07% for stroke. While still meaningful, these improvements fell short of the stronger impact predicted under standard time.
•Circadian burden explained the differences — The study introduced the idea of “circadian burden,” meaning how hard your body’s internal clock has to work to stay aligned with the 24-hour day. Permanent standard time provided the lowest circadian burden, giving your body a healthier rhythm to follow. Less burden means your energy, metabolism, and immune system stay more synchronized, lowering risks of chronic conditions.
•Geography influenced results — The benefits weren’t evenly distributed. People’s latitude and their position within a time zone shaped how much they gained from standard time. For example, those living on the western edges of a time zone experienced greater disruption under the current shifting system, so they stood to benefit the most when aligned with standard time.
•Light exposure patterns drove the health improvements — Morning light helped speed up the circadian cycle to match the 24-hour day, while evening light slowed it down. Permanent standard time tilted light exposure toward the morning, which kept people’s biological clocks better synchronized. This alignment supported healthy weight regulation and lowered the risk of stroke by ensuring cardiovascular and metabolic systems worked more efficiently.
Your body relies on light cues to regulate hormones, energy production, and even immune function. When circadian burden increases — like it does during seasonal shifts — systems fall out of sync, leading to higher obesity and stroke rates. By reducing this burden, standard time created conditions for healthier cellular and metabolic function.
Morning Light Proves More Powerful Than Evening Light
In a related press release, Stanford Medicine researchers explained how their county-level modeling revealed the nationwide effects of staying on permanent standard time versus continuing to shift clocks.3 They emphasized that permanent standard time would prevent millions of health problems and argued that the U.S. has been making the “worst choice” by sticking with biannual shifts.
•Researchers quantified the impact with striking numbers — The team estimated that permanent standard time would lead to 2.6 million fewer cases of obesity and 300,000 fewer strokes. Permanent daylight saving time also reduced cases, but only about two-thirds as much. This means that something as simple as choosing the right clock policy could determine whether your body stays in sync or struggles with excess weight and cardiovascular problems.
•More morning light is protective — Lead researcher Jamie Zeitzer explained: “You generally need more morning light and less evening light to keep well synchronized to a 24-hour day.” In other words, morning light speeds up your body’s natural clock to match the day, while evening light slows it down. Standard time prioritizes morning light, keeping your energy, mood, and metabolism running on time. When your rhythms are aligned, your immune system and energy production work better too.
•Not everyone’s body clock ticks the same way — especially if you’re a true morning person — The study showed that about 15% of people — so-called morning larks who naturally wake up early — aligned better with permanent daylight saving time because their body clocks run slightly shorter than 24 hours.
Yet for the vast majority of people, morning light from standard time was far more beneficial. This demonstrates how personalized rhythms influence how you respond to light exposure, but also why a universal policy should favor the largest share of the population.
•The researchers challenged old theories with data — For decades, supporters of standard time argued that it was healthier because it gave more morning light. Critics dismissed this as just theory. “The problem is that it’s a theory without any data. And finally, we have data,” Zeitzer noted. By anchoring light exposure to biological evidence, the team supplied the scientific proof needed to shift the debate from opinion to evidence-based policy.
•Real-world behavior could alter the outcomes — While the models assumed healthy light habits — regular sleep schedules, time outside in the mornings and evenings — the researchers admitted that people’s actual routines are often worse. Zeitzer explained that even in sunny California, most people spend less than 5% of their day outdoors. This means that for you to truly benefit from standard time, you also need to get out into natural light, not just trust that the clock will do all the work.
Simple Steps to Protect Your Body’s Natural Clock
Your body thrives when your internal clock runs smoothly, but clock changes throw it off balance. The research makes it clear that staying aligned with morning light through permanent standard time would lower your risks of obesity and stroke. While you can’t set national policy yourself, you do have control over how much natural light you get and how you support your circadian rhythm day to day. Think of this as a strategy to keep your energy, mood, and metabolism steady.
1.Start your day with real sunlight — Step outside within the first hour after waking, even if it’s just for a short walk or standing on your porch. Morning light is the strongest signal to reset your circadian rhythm, which keeps your sleep, weight, and energy in balance.
2.Limit bright light at night — If you stay up late on screens or under artificial light, your body thinks it’s still daytime. That slows down your clock and makes it harder to sleep. Use dimmer, warmer lights in the evening and avoid scrolling on your phone before bed.
3.Keep your sleep schedule consistent — Your body prefers routine. Going to bed and waking up at roughly the same times every day keeps your rhythm locked in. If you work rotating shifts, focus on anchoring your sleep around as much natural morning light as possible.
4.Support policies that prioritize standard time — The Stanford Medicine analysis showed standard time provides the strongest health benefits.4 If you’re interested in long-term change, reach out to your local representatives and share your support for adopting permanent standard time. Public input matters in these ongoing debates.
5.Spend more time outside during the day — Most people spend far more time indoors than out. Even if you live in a sunny area, you probably aren’t getting enough light exposure to keep your circadian rhythm strong.
Take your breaks outside, eat lunch in a park, and add outdoor daily walks to your routine. These steps put you back in control. You give your body the morning light it needs, protect yourself from disruptive evening light, and align your rhythm so your energy, weight, and long-term health all improve.
FAQs About Daylight Saving Time and Health
Q: Why is switching between standard time and daylight saving time harmful for health?
A: Changing the clocks twice a year disrupts your circadian rhythm — your body’s natural 24-hour clock that regulates sleep, metabolism, and energy. This disruption increases your risk of obesity, stroke, heart attacks, and accidents.
Q: What did the research published in the Proceedings of the National Academy of Sciences find?
A: The study showed that adopting permanent standard time would lead to the largest health improvements, including 2.6 million fewer cases of obesity and 300,000 fewer strokes across the U.S. compared to the current system of shifting clocks.
Q: How does permanent daylight saving time compare to permanent standard time for health?
A: While permanent daylight saving time also reduced obesity and stroke rates, the improvements were smaller — only about two-thirds as strong as the benefits seen with standard time.
Q: Why is morning light more beneficial than evening light?
A: Morning light speeds up your body’s circadian clock so it stays aligned with the 24-hour day, while evening light slows it down. This synchronization keeps your immune system, metabolism, and energy production working properly.
Q: What can I do to support my own circadian health?
A: You can step outside in the morning for natural light, reduce bright light exposure at night, keep a consistent sleep schedule, spend more time outdoors during the day, and support policies that favor permanent standard time.
