Should the U.S. Stop Changing Clocks Twice a Year?
Yes. A Stanford Medicine study using circadian-informed modeling found that the current practice of switching clocks twice a year produces the worst health outcomes of any time policy. Permanent standard time could prevent roughly 300,000 strokes and 2.6 million cases of obesity across the country.
Every spring and fall, Americans adjust their clocks by one hour. Most people think of it as a minor inconvenience. But this study, published in PNAS, suggests the health cost of that disruption is far greater than anyone expected. Researchers used county-level health data from the CDC and built models based on how our internal body clocks respond to light and time shifts. The results paint a clear picture: stopping the clock changes would save lives.
Dr. Kumar’s Take
I find this study particularly compelling because it moves beyond the usual “daylight saving time is bad” headlines and actually models the effects across every U.S. county. The fact that even permanent daylight saving time, which many people prefer because of longer evening light, would still deliver about two-thirds of the health benefits compared to permanent standard time tells us something important. Any form of consistency is better than the current system. As someone who counsels patients on sleep health, I have long seen the ripple effects of clock changes on mood, energy, and metabolic function. This data gives real numbers to what many of us have observed in practice.
How Clock Changes Affect Your Body
Your body runs on a master clock called the circadian rhythm. This clock controls when you feel sleepy, when you feel alert, and how your body processes food and repairs itself. It takes its main cue from sunlight. When the clocks shift forward or backward, your internal clock falls out of sync with the external world. Think of it like permanent jet lag that hits the entire population at once.
The Stanford team found that this misalignment does not just cause a few rough mornings. It creates a measurable burden on the body’s metabolic and cardiovascular systems over time. Counties with greater disruption between social time and solar time showed higher rates of obesity and stroke, confirming that the circadian mismatch has real, lasting consequences.
What the Data Show
The researchers analyzed CDC health data at the county level across the United States and applied circadian-informed models to compare three scenarios: keeping the current clock-switching system, adopting permanent standard time, or adopting permanent daylight saving time.
Permanent standard time came out as the clear winner. The models estimated it could prevent approximately 300,000 strokes and 2.6 million cases of obesity nationwide. Permanent daylight saving time also showed significant benefits, achieving roughly two-thirds of the improvements seen with standard time. The current system of switching clocks twice a year performed worst in every measure. This makes sense because the biannual shift forces the body to readjust its circadian rhythm twice each year, compounding the health effects of the misalignment.
Who Would Benefit Most
The study found regional variation in the projected health impacts. Counties at the western edges of time zones, where the gap between solar noon and clock noon is already large, showed the greatest potential gains from eliminating clock changes. These areas already deal with a built-in circadian mismatch, and the biannual shifts make it even worse. People in these regions may experience stronger effects on sleep quality, weight management, and cardiovascular health simply because of where they live within a time zone.
Practical Takeaways
- Keep a consistent sleep and wake schedule year-round, especially during the weeks around clock changes, to minimize circadian disruption.
- Get morning sunlight exposure as early as possible after waking, since natural light is the strongest signal for resetting your internal clock.
- Be aware that the week after a clock change carries higher risk for cardiovascular events, so take extra care with stress management and sleep during that transition.
- If you live on the western edge of your time zone, pay special attention to your sleep habits, as your body already faces a greater gap between solar time and social time.
Related Studies and Research
- Daylight saving time health effects: 157-study review offers a comprehensive look at the evidence linking clock changes to heart attacks, strokes, and other health outcomes.
- Central and peripheral circadian clocks: how your body coordinates time explains the biology behind the master clock and how different organs keep their own time.
- Smoking and heart disease: the life-saving benefits of quitting explores another major modifiable risk factor for stroke and cardiovascular disease.
- Forecasting the burden of cardiovascular disease and stroke provides context on the growing public health challenge of stroke prevention.
FAQs
Why is permanent standard time better than permanent daylight saving time?
Permanent standard time aligns social time more closely with solar time, meaning the sun is highest around noon on the clock. This gives your body the strongest and most consistent light signals to keep your circadian rhythm in sync. Permanent daylight saving time shifts everything an hour later, which means darker mornings and later sunlight. While still better than switching twice a year, this creates a mild but persistent mismatch between your internal clock and your daily schedule. The Stanford study found this difference translates to roughly one-third fewer prevented cases of obesity and stroke compared to standard time.
How does clock-switching cause obesity?
When your circadian rhythm is disrupted, your body’s ability to process food and regulate hunger hormones changes. Studies have shown that even small shifts in sleep timing can increase appetite, reduce insulin sensitivity, and promote fat storage. The biannual clock change forces millions of people into a brief but repeated state of circadian misalignment. Over years and decades, these disruptions add up. The Stanford researchers found this effect was large enough to contribute to 2.6 million cases of obesity when modeled across all U.S. counties.
Are some states already considering permanent time policies?
Yes. Several U.S. states have passed legislation to adopt permanent daylight saving time, though federal approval is still required to implement it. The Sunshine Protection Act, which would make daylight saving time permanent nationwide, has been introduced in Congress multiple times. This Stanford study adds important evidence to that debate by showing that while permanent daylight saving time is better than the current system, permanent standard time would deliver even greater health benefits. Health organizations including the American Academy of Sleep Medicine have endorsed permanent standard time as the healthier option.
Bottom Line
The evidence from this Stanford Medicine study is clear: switching clocks twice a year is the worst option for public health. Permanent standard time could prevent approximately 300,000 strokes and 2.6 million cases of obesity across the United States. Even permanent daylight saving time would capture about two-thirds of those benefits. The science supports what many people already feel in their bodies every spring and fall. It is time to pick a clock setting and stick with it.
