Every spring, we dutifully set our clocks forward to daylight saving time, and every fall, we set them back to standard time—but no one seems particularly happy about it. The biannual time change is not only inconvenient, but also known to be very bad for our health. The collective loss of one hour of sleep when the clocks change in March is associated with an increase in heart attacks and fatal traffic accidents in the days that follow.

Permanent Standard Time Better than Time Change

A study by researchers at Stanford Medicine has now found that there are also longer-term dangers – and better alternatives. The researchers compared how three different time systems—permanent standard time, permanent daylight saving time, and biannual time changes—could affect people’s circadian rhythms and thus their health across the country. The circadian rhythm is the body’s innate, approximately 24-hour clock that regulates many physiological processes.

The team found that, from a circadian perspective, we have made the worst choice. Both permanent standard time and permanent daylight saving time would be healthier than our seasonal back-and-forth, with permanent standard time benefiting most people. By modeling light exposure, circadian effects, and health characteristics in each county, the researchers estimate that permanent standard time would prevent about 300,000 strokes per year and result in 2.6 million fewer people with obesity. Permanent daylight saving time would achieve about two-thirds of the same effect. “We found that it is definitely better to stick with standard time or daylight saving time than to change twice a year,” said Jamie Zeitzer, PhD, professor of psychiatry and behavioral sciences and lead author of the study, which is published in the Proceedings of the National Academy of Sciences. The lead author is Lara Weed, a doctoral student in bioengineering.

Synchronization With 24 Hours

Even among people who want to end seasonal time changes, there is disagreement about what time policy to pursue. “There are people who are passionate about both sides, and they have very different arguments,” Zeitzer said. Proponents of permanent daylight saving time say that more light in the evening could save energy, prevent crime, and give people more leisure time after work. In the other camp, proponents of permanent standard time argue that more morning light is optimal for health. Organizations such as the American Academy of Sleep Medicine, the National Sleep Foundation, and the American Medical Association have come out in favor of year-round standard time. “That’s based on the theory that early morning light is better for our overall health,” Zeitzer said of these endorsements. “The problem is that it’s a theory without data. And ultimately, we have data.”

The human circadian cycle is not exactly 24 hours—for most people, it is about 12 minutes longer—but it can be modulated by light. “Getting light in the morning speeds up the circadian cycle. Getting light in the evening slows down the cycle,” Zeitzer said. “In general, you need more morning light and less evening light to stay well synchronized with a 24-hour day.” A desynchronized circadian cycle is associated with a range of health problems. “The more light you get at the wrong time, the weaker your circadian clock becomes. All the things that depend on it—for example, the immune system and energy—then don’t function as well,” Zeitzer said.

The researchers used a mathematical model to convert light exposure under each time regime, based on local sunrise and sunset times, into circadian load—essentially, how much a person’s internal clock needs to shift to keep pace with the 24-hour day. They found that most people would experience the least circadian load over the course of a year under permanent standard time, which prioritizes morning light. The benefits vary somewhat depending on a person’s location within a time zone and their chronotype—whether they prefer early mornings, late evenings, or something in between. Counterintuitively, people who are early risers, who make up about 15% of the population and tend to have a circadian cycle of less than 24 hours, would experience the least circadian strain under permanent daylight saving time, as more evening light would extend their circadian cycle closer to 24 hours.

Health Impacts

To establish the link between circadian strain and specific health outcomes, the researchers analyzed county-level data from the Centers for Disease Control and Prevention on the prevalence of arthritis, cancer, chronic obstructive pulmonary disease, coronary heart disease, depression, diabetes, obesity, and stroke.

Their models show that permanent standard time would reduce the nationwide prevalence of obesity by 0.78% and the prevalence of stroke by 0.09%, both conditions that are influenced by circadian health. These seemingly small percentage changes in common diseases would result in 2.6 million fewer people with obesity and 300,000 fewer strokes. With permanent daylight saving time, the nationwide prevalence of obesity would decrease by 0.51% or 1.7 million people, and that of strokes by 0.04% or 220,000 cases. As expected, the models did not predict a significant difference in diseases such as arthritis, which are not directly related to the circadian rhythm.

Results Not Conclusive Enough

While the study is arguably the most thorough analysis of the long-term health effects of different time policies, it is far from the final word, according to Zeitzer. For one thing, the researchers did not take into account many factors that could influence light exposure in real life, including weather, geography, and human behavior. In their calculations, the researchers assumed consistent and relatively circadian-friendly light habits, including a sleep schedule from 10 p.m. to 7 a.m., sun exposure before and after work and on weekends, and indoor light exposure from 9 a.m. to 5 p.m. and after sunset. In reality, however, many people have irregular sleep habits and spend more time indoors.

“People’s light habits are probably much worse than we assume in the models,” Zeitzer said. “Even in California, where the weather is great, people spend less than 5% of their day outdoors.” Although circadian health seems to favor a permanent standard time, the results are not conclusive enough to push other considerations into the background. Zeitzer hopes the study will inspire similar evidence-based analyses from other fields, such as economics and sociology. He also points out that time policy is simply a matter of deciding what times represent sunrise and sunset, not changing the total amount of light available.