The Hour That Comes Back
When the clocks go back, the extra hour is easier to find on a phone than in bed. What the latest research and sleep guidance actually say.
On the Sunday the clocks go back, the phone on the nightstand hands everyone an extra hour. Not all of it goes to sleep. BBC Future reports that on the autumn change Sunday, people sleep on average only 33 minutes longer than the Sunday before.
It is the rare wellness offer that arrives free and unasked, and still only about half of it gets used.
The mechanism is simpler than the yearly argument suggests. The American Academy of Sleep Medicine (AASM) points out that neither standard time nor daylight saving time provides more light. Only the timing of light and darkness changes. Unlike jet lag, the sunrise and sunset stay where they are. The clock moves; the sun does not.
What that shift does to health is less settled than the headlines that come with it every spring and fall.
Take the heart. In a study published Sept. 9, 2025, in JAMA Network Open, researchers at Duke University School of Medicine looked at nearly 170,000 patients over a decade. They found no significant rise in heart attacks in the weeks around either transition. In spring, 28,678 patients were treated for a heart attack during the change week, against 28,596 the week before and 28,169 the week after.
Three weeks, almost the same count.
Lead author Jennifer Rymer, MD, said "the data simply don't support the idea that changing the clocks causes a surge in heart attacks." The Duke team describes its results as a challenge to earlier, smaller studies.
Not every analysis lands in the same place. BBC Future reports a meta-analysis estimating that heart attacks rise about 4% after the spring change. Both figures are in the published record. The Duke researchers add a caveat of their own: sleep still matters for the heart, and sleeping less than six or more than nine hours has been consistently linked to higher cardiovascular risk.
Sleep itself moves in measurable ways. A 2024 UK study of more than 11,000 people, cited by BBC Future, found sleep was 65 minutes shorter on the spring change Sunday, with signs of catch-up sleep afterward. The autumn turn carries a different finding. A Danish register study covering 1995 to 2012 found that major depressive episodes rose 11% in the 10 weeks after the clocks went back.
Those findings feed a larger question: should the clocks change at all?
The AASM says the U.S. should end seasonal time changes in favor of permanent standard time, which it says aligns best with human circadian biology. Its updated October 2023 position statement was endorsed by 20 medical, scientific and civic organizations. At Oxford, David Ray told BBC Future that most circadian biologists think the winter clock times "are probably better for us."
Stanford Medicine researchers compared three policies in a study published Sept. 15, 2025, in PNAS: permanent standard time, permanent daylight saving time and switching twice a year. Senior author Jamie Zeitzer, PhD, said "staying in standard time or staying in daylight saving time is definitely better than switching twice a year." Their model estimated permanent standard time would lower national obesity prevalence by 0.78 percentage points and stroke prevalence by 0.09.
Small numbers, spread across a whole country.
Zeitzer also cautioned that real light habits are likely worse than the model assumed. "Even in California, where the weather is great, people spend less than 5% of their day outside," he said.
The public leans the same way, if not unanimously. A 2025 Gallup poll, as cited by the AASM, found 54% of Americans oppose daylight saving time; in 1999, 73% supported it. Asked to choose, 48% preferred standard time year-round, 24% permanent daylight saving time and 19% the current twice-yearly changes.
For the change itself, the official guidance stays modest. For the fall, the AASM suggests minimizing light exposure until the target morning wake time. The CDC lists going to bed and getting up at the same time every day among habits that can improve sleep, and says people with sleep problems should talk to a healthcare provider.
None of it touches the larger fact Zeitzer named. "No policy will add light to the dark winter months."
The sun comes up when it was always going to. Only the number on the bedside clock is different.
“No policy will add light to the dark winter months.”
