Every spring, Americans turn their clocks forward one hour for daylight saving time, and every fall, they turn them back again to standard time.
Most people find this change annoying, and studies show it can even be harmful to our health.
After we lose one hour of sleep in March, there are more heart attacks and deadly car crashes in the following days.
Now, scientists at Stanford University have found that this twice-a-year clock change might cause long-term health problems, too.
Their new study looked at how three different time options—keeping standard time all year, keeping daylight saving time all year, or switching twice a year—affect our internal body clock and health. Our body clock, called the circadian rhythm, helps control things like sleep, energy levels, and how our organs work.
They discovered that the current system of switching twice a year is the worst choice for our health. If we picked one time and stuck with it—either standard or daylight saving—it would be better.
But the best option is to stay on standard time all year. According to their study, doing this could prevent 300,000 cases of stroke every year and help 2.6 million people avoid obesity. Staying on daylight saving time all year would still help, but not as much.
The study’s senior author, Professor Jamie Zeitzer, said that keeping the same time year-round is clearly better than changing it back and forth. But even among people who want to stop the time changes, there’s disagreement about which time to keep.
Some people want permanent daylight saving time because it gives more sunlight in the evening. This can save energy, reduce crime, and give people more time to do outdoor activities after work. Businesses like golf courses and shopping malls support this idea.
However, when the U.S. tried permanent daylight saving time in 1974, it was unpopular and stopped after less than a year. Many parents didn’t like their kids going to school in the dark.
Others argue that permanent standard time is better because it gives more sunlight in the morning, which helps us wake up and stay healthy. Groups like the American Medical Association support this idea, saying early morning light is better for health. But until now, there wasn’t much research to prove it.
Our body clocks don’t run exactly on a 24-hour cycle—most people’s clocks are about 12 minutes longer. Morning sunlight helps speed up our internal clocks to keep us on track. Evening light does the opposite and slows it down.
When we get light at the wrong times, our internal clock gets out of sync. This can lead to health problems like low energy, weak immunity, or even serious conditions like obesity and stroke.
The researchers used computer models to see how each time option affects our health. They looked at sunlight exposure based on where people live and linked this to health problems like obesity, stroke, depression, heart disease, diabetes, and more.
They found that permanent standard time gives most people the healthiest balance of light. This is especially true for people who naturally prefer late nights and have a harder time getting up early.
Even people who are naturally early risers might benefit more from permanent daylight saving time, since evening light helps lengthen their short internal clock cycles. But overall, permanent standard time gave the best results for most people.
The study found that if we kept standard time all year, fewer people would be obese or suffer strokes. For example, it could lower the nationwide rate of obesity by 0.78%—which means 2.6 million fewer people would have obesity.
It could also cut stroke rates by 0.09%, or 300,000 fewer cases. Permanent daylight saving time would still help, but with smaller numbers: about 1.7 million fewer obese people and 220,000 fewer strokes.
Not all conditions are affected by the body clock, though. Things like arthritis didn’t change much under any time policy.
This research is one of the first to show clear long-term health effects of different time rules. However, it’s not perfect. The scientists didn’t include real-world factors like weather, how much time people spend indoors, or irregular sleep habits.
Their model assumed people had a regular sleep schedule, went outside during the day, and had consistent light exposure indoors and outdoors.
In real life, people often spend most of their time inside. Even in sunny California, most people are outdoors for less than 5% of the day. So, the health effects could be even stronger than the model predicts.
Professor Zeitzer says this is a good first step, but more research from other areas—like economics and social sciences—is needed. He also reminds us that no time policy can give us more sunlight in winter. That’s up to nature and the Earth’s orbit around the sun.
Still, this study gives strong evidence that staying on the same time all year—especially standard time—could help millions of people stay healthier.
The study is published in PNAS.
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