Abandoning daylight saving time could prevent over 300,000 stroke cases a year in the US, study claims

Abandoning the biannual switch to daylight saving time could prevent more than 300,000 strokes and slash over 2 million obesity cases a year, a new model using data from over 300 million Americans suggests.

Adopting permanent standard time (ST) takes less toll on our circadian rhythm — the body's rough 24-hour pacemaker — than daylight saving time (DST) or flicking between the time policies twice a year, the researchers found.

As a result of this reduced burden on our body clock, staying in ST year round could prevent almost 900,000 more cases of obesity and around 85,000 more strokes a year than remaining in DST year-round, the scientists concluded in a study published Sept. 15 in the journal PNAS.

"We can't propose public policy without data," study co-author Jamie Zeitzer, a professor of psychiatry and behavioral sciences at Stanford University, told Live Science. The data from this research marks "the beginning of a conversation," he said.

Time for a change?

DST, which makes clocks wind forward by an hour every spring, was first introduced in the U.S. in 1918 to save fuel during World War I. It was later made federal law by the Uniform Time Act of 1966.

Our circadian rhythm regulates and coordinates the functioning of each cell in the body. This internal pacemaker is highly sensitive to light exposure, and disruptions to it — for instance, when people must wake up or stay awake during dark hours — creates a "circadian burden" that has been associated with a measurable increase in negative health outcomes, including heart attacks, strokes and car accidents around the time of the switchover.

Insufficient sleep and a circadian burden are also metabolic stressors associated with reduced energy expenditure and excess food intake — risk factors for weight gain and obesity. As such, the position of the American Academy of Sleep Medicine is to scrap DST altogether and permanently stick to ST. Although the health risks of biannual switching are low for individuals, the effects are noticeable at the population level, Zeitzer said.

"You're really looking at lottery ticket kind of risk. But if 350 million people are doing it on the same day, someone's going to win the lottery," Zeitzer told Live Science. "[It's] just not one that you want to win."

To model the health impacts of a mismatch between circadian rhythm and the environment, Zeitzer and colleague Lara Weed, a bioengineer at Stanford University, calculated the circadian burden associated with each time policy: permanent ST, permanent DST and biannual shifting.

To do this, they simulated artificial light and sunlight exposure for the people in every U.S. county across a year under all three time policies. They assumed regular sleep routines (10 p.m. to 7 a.m.), and regular work schedules in a well-lit office (9 a.m. to 5 p.m., Monday to Friday).

The team found that biannually flicking between time policies created the largest disruptions to circadian clocks and therefore the largest circadian burden.

The researchers then used data on county-level prevalence of chronic diseases to determine how the different circadian burdens predicted eight health outcomes: arthritis, cancer, chronic obstructive pulmonary disease, coronary heart disease, depression, diabetes, obesity and stroke. They controlled for various socioeconomic and health factors that could also determine disease prevalence, such as high blood pressure, health insurance status and unemployment.

Sticking to ST would prevent around 2,602,866 cases of obesity and 306,988 strokes compared to a biannual shift. Permanent DST prevented obesity and strokes to a lesser degree, with 1,705,437 fewer predicted obesity cases and 220,092 fewer strokes on average, compared to the biannual switch.

Their model found neither policy led a statistically meaningful reduction in the six other health outcomes.

However, the research is based on simulations which deliberately included unrealistic assumptions, including universal regular light exposure and sleep, and it also did not consider seasonal variations in behavior. Moreover, the health outcomes dataset relied on self-reported information, such as for body mass index (BMI) and whether they'd had a past stroke or not.

Also, they did not include race in their model, despite there being persistent racial and ethnic disparities in sleep in the U.S. This exclusion "doesn't necessarily change the results of the study," said Karin Johnson, a professor of neurology at UMass Chan School of Medicine-Baystate and a spokesperson for the American Academy of Sleep Medicine, who was not involved in the research.

It does mean, however, that the increased risks of stroke and obesity are likely to be felt most keenly by people [namely, Black and Hispanic or Latino populations] that are "already at risk for sleep disparities from other causes," Johnson told Live Science in an email.