Every year, people in the United States change their clocks twice—for daylight saving time in spring and back to standard time in fall.
Many people find this shift annoying, but new research shows it may also harm our health more than we thought.
A study from Stanford Medicine looked at how different time setups—permanent standard time, permanent daylight saving time, and switching twice a year—affect our body’s internal clock, known as the circadian rhythm.
This rhythm controls many body functions like sleep, mood, energy, and even the immune system.
When our body gets light at the wrong time of day, like too much in the evening and not enough in the morning, it can make our circadian rhythm go out of sync. This can lead to poor health over time.
The researchers found that the current practice of changing clocks twice a year is the worst for our circadian health.
They used computer models to predict how much people’s internal clocks would need to adjust under each time policy. Then they connected these predictions to real health problems like obesity and stroke.
Their results showed that keeping permanent standard time (which gives people more morning light) would be the best for most people’s health.
It could help prevent about 2.6 million cases of obesity and 300,000 strokes each year in the U.S. Permanent daylight saving time also showed benefits, but not as much—about two-thirds of the effect.
Some people support permanent daylight saving time because it gives more light in the evening, which may be good for leisure or shopping.
But others worry about kids going to school in the dark in winter, which happened when permanent daylight time was tried in 1974. It didn’t last long because people disliked it.
Medical organizations like the American Academy of Sleep Medicine support permanent standard time, saying that morning light is better for sleep and health. But until now, these ideas were based mostly on theory, not solid data. This new study provides real evidence to support those claims.
The human circadian rhythm isn’t exactly 24 hours—it’s usually a bit longer. Morning light helps to “reset” the clock each day. Evening light, on the other hand, can delay the rhythm and make it harder to fall asleep.
When the body’s internal clock doesn’t match the 24-hour day, it can affect energy levels, mood, and even the risk of diseases.
Interestingly, the study also found that people who naturally wake up very early (called “morning larks”) might do better with permanent daylight saving time because their internal clocks are shorter and evening light helps balance them out.
To check the health effects, the researchers looked at public health data from the CDC. They found that people’s health could improve if we stopped changing the clocks and picked one system—especially standard time. The changes in obesity and stroke rates may seem small in percentage, but they mean millions of people would be healthier.
Of course, the study has some limits. It assumed that people had a regular sleep schedule and good light exposure habits, like going outside in the morning. But in real life, many people don’t get enough sunlight and spend most of their time indoors.
The researchers also noted that this study focuses on circadian health, but other factors like economics or social habits may affect the best time policy for a community. In the end, no time policy can change how much sunlight we get in the winter—that depends on Earth’s position around the sun.
Still, this study adds valuable evidence to the debate and shows that choosing the right time policy could have big effects on public health.
The study is published in PNAS.
Copyright © 2025 Knowridge Science Report. All rights reserved.
