Stanford Medicine study suggests permanent standard time could maximize public health benefits

Stanford Medicine researchers conducted a nationwide comparison of three clock policies—permanent standard time, permanent daylight saving time, and biannual clock shifts—and found that keeping standard time year-round would benefit the most people. Using a mathematical model of light exposure, circadian biology, and county-level health data, the study estimates that permanent standard time could prevent about 300,000 strokes annually and reduce obesity by around 2.6 million people, with permanent daylight saving time delivering about two-thirds of these benefits. The work also notes that curtailing clock changes generally improves circadian alignment, though individual timing preferences (chronotypes) influence who benefits most. The study was published in the Proceedings of the National Academy of Sciences and funded by the NIH. Lead author Lara Weed and senior author Jamie Zeitzer caution that real-world light exposure varies with weather, behavior, and geography.

Overview of time policy options

Researchers from Stanford Medicine explore three possible national time policies: permanent standard time, permanent daylight saving time, and the current biannual clock-shifting tradition. The goal is to understand how these choices align with the body's circadian rhythm, a roughly 24-hour cycle that governs many physiological processes. The team concludes that both permanent standard time and permanent daylight saving time outperform switching clocks twice a year, with standard time offering the broadest health benefits across most of the population. The analysis emphasizes that more morning light and less evening light generally supports better synchronization to a 24-hour day, vital for immune function, energy, and cardiovascular health.

"We found that staying in standard time or staying in daylight saving time is definitely better than switching twice a year." - Jamie Zeitzer, PhD, professor of psychiatry and behavioral sciences.

Modeling approach and key findings

The study uses a mathematical model linking local sunrise and sunset times to circadian burden, a proxy for how much the innate clock must shift to stay aligned with a 24-hour day. The model accounts for light exposure patterns and health characteristics at the county level, then projects potential health outcomes across the United States. Across most regions, permanent standard time minimizes circadian burden, while fixed daylight saving time helps morning exposure for some chronotypes but increases exposure to light in the evening for others. Lead author Lara Weed notes that chronotype diversity means some individuals (early birds) may fare differently, yet standard time generally yields the greatest net benefit overall.

"You generally need more morning light and less evening light to keep well synchronized to a 24-hour day." - Jamie Zeitzer, PhD.

Health implications by policy

The authors link circadian health to common conditions using CDC county data on obesity, stroke, and other diseases. Under permanent standard time, the nationwide prevalence of obesity could drop by about 0.78 percentage points and stroke by about 0.09 percentage points, translating to approximately 2.6 million fewer people with obesity and 300,000 fewer stroke cases. Permanent daylight saving time would achieve roughly two-thirds of these gains. The researchers emphasize that conditions unlikely to be circadian-driven, such as arthritis, show no meaningful difference under any policy. The work suggests that circadian-aligned light exposure can substantially affect population health, even if individual effects vary by location and chronotype.

Limitations and future directions

The authors acknowledge that their models simplify real-world light exposure, assuming predictable habits and schedules. Weather, geography, indoor lighting, and irregular sleep patterns could shift the outcomes. Zeitzer stresses that the study is a starting point for evidence-based policy analysis and encourages similar research across other fields such as economics and sociology. He also notes that policy decisions adjust the timing of sunrise and sunset without changing total daylight available in winter, which remains determined by Earth's rotation and tilt.

Policy context and takeaway

Although there is ongoing political interest in permanent daylight saving time, public sentiment about darker winter mornings remains a concern for some parents and educators. The Stanford Medicine study provides a rigorous quantitative framework indicating that avoiding clock shifts would likely benefit a large portion of the population, particularly regarding obesity and stroke risk. The NIH-funded research invites further multidisciplinary assessment to inform legislative debates on time policy.

"We can’t do anything about the sun or Earth's rotation, but we can choose how to align our clocks with it for better health." - Jamie Zeitzer, PhD.