Stanford scientists say that American clock changes quietly destroy our health

A new analysis by Stanford Medicine Scientists shows that the change of clocks twice a year disrupts circadian rhythms and is linked to higher risks of stroke and obesity.

Each spring, the Americans advance their clocks for summer time, and every fall, they have given them in the standard time – a routine that little appreciates. Beyond the impractical discomfort, this twice paragage change has proven to be negatively affecting health. The loss of an hour of sleep on the second Sunday in March was associated with an increase in heart attacks and fatal car accidents in the days that follow.

Now, Stanford’s medical researchers report that risks go beyond short -term effects – and that there are healthier alternatives.

The team analyzed how three possible time policies – normal time all year round, time of savings and the current quarter of work – influence circadian rhythms and, in turn, public health in the United States. The circadian pace refers to the natural cycle of 24 hours of the body which regulates many biological functions.

From this point of view, the researchers concluded that the current system is the least favorable. They found that the permanent standard time or the permanent summer time of daylight would both be healthy, the standard time offering the greatest overall advantages.

Using county data on exposure to light, circadian effects and health models, the study estimated that the adoption of permanent standard time could prevent approximately 300,000 strokes per year and reduce the obesity of 2.6 million cases. The permanent summer time of daylight would provide about two thirds of these same advantages.

“We have found that staying in standard time or staying in summer time is certainly better than changing twice a year,” said Jamie Zeitzer, PHD, professor of psychiatry and behavioral sciences and main author of the study published on September 15 in the Proceedings of the National Academy of Sciences. The main author is Lara Weed, a graduate student in bio-engineering.

A missing data theory

Even among those that are suitable that seasonal clock changes should end, there is little consensus on which system should replace them.

“You have passionate people on both sides, and they have very different arguments,” said Zeitzer.

Defenders of the permanent hour of daylight maintain that prolonged evening light can reduce energy consumption, discourage crime and provide more time for leisure after work. Industries such as golf courses and outdoor shopping centers strongly favor this approach, noted Zeitzer. However, when the United States briefly adopted a time of permanent savings in daylight in 1974, it turned out to be so unpopular that the policy was reversed in a year. Many parents, for example, opposed children who head towards school before sunrise.

However, summer time is later extended from six to seven months, and since 2018, legislators have introduced invoices almost every year to make it permanent, although none has passed.

On the other hand, supporters of permanent standard time emphasize that morning light is more beneficial for overall health. Groups such as the American Academy of Sleep Medicine, the National Sleep Foundation and the American Medical Association have all publicly approved the standard time as an option all year round.

“It is based on the theory that light early in the morning is better for our overall health,” said Zeitzer about these endorsements. “The problem is that it is a theory without any data. And finally, we have data.”

Synchronization with 24 hours

The human circadian cycle is not exactly 24 hours – for most people, it is about 12 minutes more – but it can be modulated by light.

“When you have light in the morning, it accelerates the circadian cycle. When you have light in the evening, it slows down,” said Zeitzer. “You generally need more morning light and less evening light to stay well synchronized in a 24 -hour day.”

A circadian cycle excluding synchronization has been associated with a range of poor health results.

“The more you get an exposure to light in bad times, the lower the circadian clock. All these things that are downstream – for example, your immune system, your energy – do not correspond as well,” said Zeitzer.

The researchers used a mathematical model to translate exposure to light under each time policy, based on local sunrise and sunset times, at the circadian load – essentially, how much the innate clock of a person must change to follow the day 24 hours a day.

They found that more than a year, most people would experience the least circadian load in permanent standard time, which prioritizes morning light. The advantages vary somewhat by the location of a person in a time zone and their chronotype – whether they prefer the early mornings, late nights or something between the two.

Contreherently, people who are morning larks, which represent around 15% of the population and tend to have circadian cycles shorter than 24 hours, would suffer the least circadian load under the time of permanent daylight, because more evening light would prolong their circadian cycles closer to 24 hours.

Health implications

To link the circadian charge to specific health results, the researchers have analyzed data at the county of the Centers for Disease Control and Prevention on the prevalence of arthritis, cancer, chronic obstructive disease, coronary diseases, depression, diabetes, obesity and brain vascular accidents.

Their models show that the permanent standard time would reduce the national obesity prevalence of 0.78% and the prevalence of stroke of 0.09%, the conditions influenced by circadian health. These apparently low percentage changes under common conditions would represent 2.6 million less people with obesity and 300,000 strokes of less. Under permanent daylight, the national obesity prevalence would decrease by 0.51%, or 1.7 million people, and a 804%stroke, or 220,000 cases.

As expected, the models have predicted any significant difference under conditions such as arthritis which have no direct link with circadian rhythms.

Not the last word

The study could be the analysis most based on evidence of long -term health implications for different temporal policies, but it is far from the last word, said Zeitzer.

On the one hand, the researchers did not take into account many factors that could influence an exposure to real light, including weather, geography and human behavior.

In their calculations, the researchers assumed coherent and relatively circadian light habits, including a sleep schedule from 10 a.m. to 7 a.m., an exposure to the sun before and after work and the weekend, and an exposure to interior light from 9 a.m. to 5 p.m. and after sunset. But in reality, many people have erratic sleep hours and spend more time inside.

“People ‘luminous habits are probably worse than what we assume in the models,” said Zeitzer. “Even in California, where time is great, people spend less than 5% of their day outside.”

In addition, although circadian health seems to promote permanent standard time, the results are not conclusive enough to overshadow other considerations. Zeitzer hopes that the study will encourage analyzes based on similar evidence of other areas, such as economics and sociology.

He also underlines that Time Policy is simply to choose the clock hours that represent sunrise and sunset, without modifying the total quantity of light. No policy will add light to dark winter months.

“It’s the sun and the position of the earth,” he said. “We can’t do anything about it.”

Reference: “Circadian informed modeling predicts the regional variation of obesity and stroke results in the various permanent time policies of the United States” by Lara Weed and Jamie M. Zeitzer, September 15, 2025, Proceedings of the National Academy of Sciences.
DOI: 10.1073 / PNAS.2508293122

The study received funding from National Health Institutes (Grant F31HL170715).

Never miss a breakthrough: join the Scitechdaily newsletter.