Daylight Saving Time ends Sunday, November 2, at 2:00 a.m. Eastern. As Americans “fall back,” debate continues over whether switching clocks twice a year benefits health. Most countries do not observe DST, yet parts of North America and Europe still change clocks — a practice begun in World War I to extend summer daylight and conserve energy. Today, many scientists and health experts question its safety.
A new Stanford Medicine study suggests the biannual clock changes do more than disrupt sleep: they disturb circadian rhythms and the physiological processes those rhythms regulate, with measurable effects on public health. Using county-level CDC data and models of light exposure, researchers compared three scenarios: the usual biannual clock shifts, permanent standard time, and permanent daylight time.
Their models estimate that permanent standard time would yield the largest health benefits: about 300,000 fewer stroke cases per year and 2.6 million fewer people with obesity nationwide. Permanent daylight time would produce similar but smaller gains — roughly two-thirds as much — preventing about 220,000 strokes and reducing obesity by about 1.7 million people. Conditions not linked to circadian rhythms, such as arthritis, showed no change in the models.
“We found that staying in standard time or staying in daylight saving time is definitely better than switching twice a year,” said Jamie Zeitzer, PhD, professor of psychiatry and behavioral sciences at Stanford and senior author. He explained that standard time increases early daytime light exposure, which strengthens the central circadian clock and improves synchronization across organ systems involved in metabolism and cardiovascular function.
Other clinicians not involved in the study noted the plausibility of the findings. Mir Ali, MD, a bariatric surgeon, said he was surprised that a one-hour time change could have broad health impacts but that previous studies of night-shift workers support the idea that consistent light schedules matter. He pointed out that circadian disruption affects metabolic and hormonal regulation and can suppress immune function, influencing multiple organs.
Interventional cardiologist Cheng-Han Chen, MD, highlighted how the circadian clock governs blood pressure, heart rate, and metabolism through fluctuations in hormones like cortisol and adrenaline. Disrupting those rhythms can raise risks of high blood pressure, obesity, arrhythmias, heart attack, heart failure, and stroke.
Political efforts to make DST permanent in the U.S. have stalled, meaning the twice-yearly clock changes will continue for now. Zeitzer said the next research steps should bring together experts to evaluate the broader impacts of time-policy choices on exercise, diet, safety, and economics.
In the meantime, people — especially those with cardiovascular or chronic conditions — can take steps to minimize circadian disruption when clocks change. The “spring forward” loss of an hour can meaningfully disturb the body’s clock and may trigger major cardiovascular events in vulnerable individuals. Increasing daytime natural light exposure, maintaining consistent sleep routines, eating a balanced diet, and exercising regularly all support healthy sleep and circadian rhythms. Anyone with chronic health concerns should discuss sleep and timing issues with their doctor or cardiologist.
The Stanford study suggests that eliminating the seasonal clock switches in favor of a consistent time policy — particularly standard time — could modestly but meaningfully improve public health, potentially affecting millions of people.
