In the early hours of Sunday 25 October, clocks across the UK will go back an hour as British Summer Time comes to an end. For most people this means an extra hour in bed and lighter mornings, paid for with afternoons that now darken well before the end of the working day. It also revived, as it does every autumn, the argument over whether changing the clocks twice a year is quietly harming us. The research behind that argument has moved on considerably. A long-repeated link to heart attacks has weakened under much larger datasets, the spike in road collisions turns out to be tangled up with the weather, and a 2025 modelling study suggests the real cost may lie less in the switch itself than in months of living out of step with our body clocks.

The Heart Attack Link That Has Not Held Up

The claim that changing the clocks triggers heart attacks rests largely on studies published over the past two decades. In 2019, researchers led by Roberto Manfredini brought seven of them together in the first meta-analysis on the subject, published in the Journal of Clinical Medicine and covering more than 115,000 people. Across the two weeks following either clock change, the odds of an acute heart attack were around 3 per cent higher than in control periods. Split by season, though, the picture changed: the rise was concentrated after the spring change, when an hour of sleep is lost, while the week after the autumn change looked no different from comparison periods. The authors also noted that further studies fully adjusting for confounding factors were needed to confirm the findings.

A much larger study has since cast doubt on even the spring effect. Researchers at Duke University, writing in JAMA Network Open in September 2025, analysed 168,870 patients admitted with a heart attack to 1,124 US hospitals between 2013 and 2022, comparing the week before, the week of and the week after both the spring and autumn transitions. They found no significant differences in heart attack incidence, nor in in-hospital outcomes such as death and stroke, for either change. The one marked increase came in the week after the spring 2020 transition, which coincided with the start of the Covid-19 pandemic. The dataset also included hospitals in Arizona and Hawaii, which do not change their clocks at all. Lead author Jennifer Rymer described it as “the most comprehensive look we’ve had at the relationship between DST and heart health.” It is a US study, but its scale makes it difficult to treat the older heart attack findings as settled.

On the Roads, Darkness and Weather Arrive Together

The road safety evidence is more consistent, but harder to pin on the clocks alone. An RAC Foundation analysis of six years of police data for Great Britain found an average of 278 more personal injury collisions in the two weeks after the autumn clock change than in the two weeks before, roughly 20 extra a day, with three quarters of them happening in the afternoon, the part of the day the change makes darker. The same analysis found, however, that collisions in which the weather was recorded as adverse rose by a similar proportion, and that after the clocks go forward in spring, morning injury collisions actually fall, by an average of 221 over the following fortnight, even though mornings become darker. Steve Gooding, director of the RAC Foundation, summed up the conclusion: “Every year at about this time there are calls to abandon the spring forward, fall-back rhythm of daylight savings time, but our work suggests that it’s darker days and winter weather together that cause the spike in road safety risk.”

That has not ended the debate. The Royal Society for the Prevention of Accidents has campaigned since the 1960s for the autumn change to be scrapped, pointing to a three year experiment that began in 1968, when Britain put its clocks forward in March and did not put them back again until October 1971.

The Case Against the Switch Itself

The most significant recent contribution shifts attention away from the days immediately after a clock change altogether. In a study published in the Proceedings of the National Academy of Sciences on 15 September 2025, Lara Weed and Jamie Zeitzer of Stanford University modelled how three policies, permanent standard time, permanent daylight saving time and twice-yearly switching, would affect circadian rhythms across the contiguous United States, then compared the resulting circadian burden against county-level health data from the Centers for Disease Control and Prevention.

Switching twice a year produced the greatest burden on the body clock. Permanent standard time, which prioritises morning light, came out best, with the model estimating a fall in obesity prevalence of 0.78 percentage points, around 2.6 million fewer people, and in stroke prevalence of 0.09 percentage points, around 300,000 fewer cases. Permanent daylight saving time also improved on the current arrangement, but by about two thirds as much. “We found that staying in standard time or staying in daylight saving time is definitely better than switching twice a year,” Zeitzer said. The estimates come from modelling under idealised light exposure conditions rather than from a trial, and from US data rather than British, but they frame the question differently: not whether one weekend is risky, but whether months of mismatch between the clock and the sun carry a cost. In UK terms, permanent standard time would mean staying on Greenwich Mean Time all year round.

One Hour, Three Different Answers

Put together, these three strands point in noticeably different directions. The acute heart attack risk that once dominated coverage has weakened as datasets have grown, the post-change rise in road collisions is real but shares the blame with autumn weather, and the strongest current argument against changing the clocks concerns long-term circadian misalignment rather than any single weekend. None of this settles what Britain should do, and the evidence on whether lighter mornings or lighter evenings serve people best is still being argued over. What has changed is the quality of the question. The hour went back on Sunday as it always does. The research on what that hour actually costs is considerably more precise than it was.

Sources: Journal of Clinical Medicine (Manfredini et al, 2019); JAMA Network Open (Rymer et al, 2025); Duke University School of Medicine; RAC Foundation; Royal Society for the Prevention of Accidents; Proceedings of the National Academy of Sciences (Weed and Zeitzer, 2025); Stanford Medicine.