Stanford Study Finds America’s Clock Changes Are Hurting Our Health

Every spring and fall, Americans go through the ritual of moving the clock forward or back. For most people, this biannual shift feels like a nuisance. But according to new research from Stanford Medicine, the effects go far beyond inconvenience — they may be quietly damaging public health on a massive scale.

A recent study published in the Proceedings of the National Academy of Sciences (PNAS) explored how different time policies affect our circadian rhythms and, in turn, our risks for major health conditions. The findings are clear: the current system of switching between Daylight Saving Time (DST) and Standard Time is the least healthy choice.

The Three Scenarios Studied

The Stanford team, led by Jamie Zeitzer, a professor of psychiatry and behavioral sciences, and graduate student Lara Weed, compared three different policies:

1. Permanent Standard Time (PST) – No clock changes, year-round standard time.
2. Permanent Daylight Saving Time (PDST) – Staying permanently in daylight saving time.
3. Biannual Switching (Current System) – The twice-yearly clock changes we live with now.

Their analysis used county-level data on light exposure, sunrise and sunset times, and existing health records from the Centers for Disease Control and Prevention (CDC) to model what would happen to rates of stroke, obesity, and other health conditions under each scenario.

Why Circadian Rhythms Matter

The body’s circadian rhythm is its natural 24-hour cycle, regulating processes such as sleep, metabolism, energy levels, immune function, and hormone release. While often referred to as a 24-hour clock, the human circadian cycle is usually slightly longer — about 24 hours and 12 minutes on average.

Light is the most powerful external signal that keeps this clock aligned. Morning light speeds up the cycle, helping us synchronize to the 24-hour day. Evening light, on the other hand, slows it down. Too much light exposure at the wrong time disrupts circadian alignment, which has been linked to a variety of health issues, including obesity, depression, diabetes, cardiovascular disease, and weakened immune function.

Key Findings of the Study

The research team’s modeling produced striking results:

• Permanent Standard Time (PST) provides the greatest health benefits.
  • Could reduce obesity prevalence by 0.78% nationwide, equivalent to 2.6 million fewer cases.
  • Could prevent about 300,000 strokes annually by lowering prevalence by 0.09%.
• Permanent Daylight Saving Time (PDST) is better than the current system but less effective than PST.
  • Could reduce obesity prevalence by 0.51%, or about 1.7 million fewer cases.
  • Could reduce stroke prevalence by 0.04%, translating to about 220,000 fewer cases.
• Biannual Switching, the current system, is the least favorable option, producing the highest circadian burden and the worst health outcomes overall.

Conditions that aren’t directly linked to circadian health, such as arthritis, showed no significant differences between policies.

The Role of Chronotypes

The study also recognized that not everyone responds to time changes the same way. Chronotype — whether you are a morning person (“lark”), evening person (“owl”), or somewhere in between — influences how circadian burden plays out.

• Morning types (about 15% of the population) actually fare slightly better under permanent daylight saving time, since their natural cycle is shorter than 24 hours and evening light helps lengthen it closer to the average.
• For most people, however, permanent standard time results in better alignment and fewer health risks.

The Bigger Picture on Health

To connect circadian burden to real-world outcomes, researchers linked their models with CDC data on conditions such as obesity, stroke, cancer, coronary heart disease, depression, diabetes, and chronic obstructive pulmonary disease (COPD).

Only the conditions most directly tied to circadian disruption — namely obesity and stroke — showed significant reductions under permanent standard or daylight time. Still, because these are both common and serious conditions, even a small percentage shift translates into millions of healthier individuals.

Limitations of the Study

As thorough as this research is, it’s not without caveats:

• Idealized assumptions: The model assumes relatively healthy light habits, such as sleeping from 10 p.m. to 7 a.m., getting sunlight before and after work, and spending time outside on weekends. In reality, many people keep irregular schedules and spend the majority of their day indoors.
• Environmental and geographic differences: Weather patterns, latitude, and behavior all affect how much light exposure people get, and these were not fully accounted for.
• Broader impacts not modeled: Factors like economic outcomes, crime rates, energy usage, and the logistics of children going to school before sunrise weren’t considered in this health-focused study.

Even so, this may be the most evidence-based analysis to date on the health consequences of time policy.

A Debate With Deep Roots

The fight over daylight saving time is hardly new. The U.S. tried permanent daylight saving time in 1974, but public backlash was swift, particularly from parents worried about children traveling to school in the dark winter mornings. The policy was rolled back within a year.

Since then, lawmakers have introduced bills almost every year to make daylight saving permanent, but none have passed. Some industries, such as golf courses and shopping centers, push for more evening light because it encourages recreation and spending. Advocates for standard time, meanwhile, argue that morning light is far more beneficial for long-term health.

Professional organizations like the American Academy of Sleep Medicine, the National Sleep Foundation, and the American Medical Association have all endorsed permanent standard time. Until now, much of their argument was theoretical. This Stanford study provides concrete data to support those recommendations.

The Realities of Light Exposure

One of the most interesting points raised by the researchers is that light habits are worse than assumed. Even in sunny states like California, many people spend less than 5% of their day outdoors. With work and lifestyle shifts keeping people indoors, exposure to natural light is limited — a factor that may actually make circadian health even worse than the models suggest.

This means that the potential benefits of permanent standard time could be underestimated, since the model’s assumptions are more generous than reality.

No Policy Can Change the Sun

The study also reminds us of a simple truth: no matter what policy we adopt, we can’t add daylight to winter months. The Earth’s tilt and the position of the sun determine how much light we get in different seasons. What time policy changes is simply the clock labels we attach to sunrise and sunset, not the actual amount of daylight.

What Comes Next?

The authors emphasize that while their work is significant, it’s not the final word. More evidence-based analyses are needed, especially from fields like economics, sociology, and education, to fully weigh the trade-offs of time policy decisions.

Still, the implications are powerful: by choosing permanent standard time, the U.S. could potentially prevent millions of illnesses and improve circadian health nationwide.

Extra Insight: The Science of Circadian Health

Since circadian rhythms are central to this discussion, here are some additional insights worth knowing:

Circadian Misalignment and Health

• Chronic circadian misalignment has been linked to increased risks of metabolic syndrome, hypertension, and even certain cancers.
• Shift workers, whose schedules often put them at odds with natural light cycles, have higher rates of health issues — a clear demonstration of how damaging circadian disruption can be.

The Importance of Morning Light

• Getting bright light in the morning helps synchronize the circadian clock. That’s why experts often recommend at least 20–30 minutes of outdoor light soon after waking up.
• Evening light, especially from screens and artificial lighting, can delay circadian timing, making it harder to fall asleep and wake up consistently.

Sleep and Productivity

• Even small disruptions in circadian timing can affect sleep quality, alertness, and cognitive performance.
• The switch to daylight saving time each spring has been consistently linked to short-term spikes in heart attacks, strokes, and traffic accidents, all tied to the sudden loss of sleep and circadian disruption.

Conclusion

The Stanford study adds robust evidence to the long-standing debate about America’s clock changes. By showing that permanent standard time could prevent millions of cases of obesity and hundreds of thousands of strokes, the researchers highlight a clear public health opportunity.

Whether lawmakers act on this evidence remains to be seen, but one thing is certain: our biological clocks are not built to handle the twice-yearly time shifts. Staying aligned with natural light — and prioritizing morning exposure — may be one of the most powerful steps toward better long-term health.

Research Reference: Circadian-informed modeling predicts regional variation in obesity and stroke outcomes under different permanent US time policies – PNAS (2025)