Stanford study links clock changes to stroke risk; permanent time policies could curb hundreds of thousands of strokes

A Stanford Medicine study warns that changing clocks twice a year could put about 300,000 Americans at risk of a potentially fatal stroke each year by disrupting the body's circadian clock. The next clock change is scheduled for November 2, 2025, at 2 a.m. local time, when clocks are set back one hour to Standard Time. The researchers say regular bidirectional shifts between light and darkness can misalign the body's natural rhythms and place added stress on the brain's circadian system.

Published in the Proceedings of the National Academy of Sciences, the study used simulations to compare three time policies: ongoing biannual clock changes, permanent Standard Time, and permanent Daylight Saving Time. Lead author Dr. Jamie Zeitzer, a professor of psychiatry and behavioral sciences at Stanford, and co-author Lara Weed, a bioengineering graduate student, quantify a metric called circadian burden—the total hours per year the internal clock must adjust to stay synchronized with daily life. The biannual switching produced the largest burden, about 21 hours per year, while permanent Standard Time yielded about 18.5 hours and permanent Daylight Saving Time about 19.6 hours.

Those shifts in circadian alignment can influence blood pressure regulation and inflammation, factors linked to stroke risk. The researchers caution that the projections rely on modeling assumptions, but the pattern aligns with what is known about sleep disruption and cardiovascular risk. The study notes that the morning and evening light exposure patterns under different policies drive the measured circadian burden.

Under their scenarios, ending clock changes could substantially reduce stroke incidence compared with today. The simulations estimate that moving to permanent Standard Time would prevent about 300,000 strokes annually relative to continued switching, while adopting permanent Daylight Saving Time would prevent about 220,000 strokes in the same comparison. The authors stress these are population-level projections rather than observed outcomes.

Beyond strokes, the analysis suggests fewer obesity cases under both permanent policies. They project about 2.6 million fewer obesity cases if the nation stayed on Standard Time year-round and about 1.7 million fewer if it kept Daylight Saving Time year-round. The researchers found no statistically significant link between time policy changes and arthritis, cancer, heart disease, or depression in their models.

Zeitzer said that staying on standard time or daylight saving time is better than switching twice a year. He noted that the goal is to reduce the circadian burden, which can ripple through immune function and energy, affecting overall health.

Context matters: Earlier this year, researchers in the United Kingdom urged ending daylight saving time amid concerns about cancer risk, traffic accidents, and suicidal behavior. The British Sleep Society highlighted evidence that the spring shift can leave people sleepier and may be associated with an uptick in cardiovascular events and mortality in the days immediately after the transition.

Public policy discussions about permanent time arrangements continue as lawmakers weigh options. The current clock change remains scheduled for November 2025 in most of the United States, but the study’s authors say the health implications deserve careful consideration as states and the federal government evaluate daylight saving policies.