Reacción a "Not changing the time in summer or winter in the US would reduce strokes and obesity, according to a study"

The study published in PNAS by Weed and Zeitzer (2025) adds to the epidemiological evidence linking time changes to an increase in strokes and other morbidity events. Combining mathematical models of circadian response with health data from different regions of the United States, the authors conclude that maintaining standard time (commonly referred to as “winter time”) throughout the year, eliminating biannual changes, could produce a slight decrease in the incidence of obesity and strokes. Maintaining the supposed energy-saving (“summer”) time would also contribute to this reduction, albeit to a lesser extent.

Among the notable aspects of the study is that it uses a rigorous circadian model that includes light exposure, chronotype, latitude, and location within the time zone. It also integrates biological predictions with actual epidemiological data and takes into account socioeconomic and health factors. In addition, it analyses both the acute circadian effects of time changes and the chronic effects of permanently living with one time or another, and its predictions support previous evidence on the immediate negative impact of time changes.

Among its limitations, the authors themselves acknowledge that this is a theoretical study based on modelling, not on experimental trials or actual longitudinal follow-up. Idealised light patterns are used that do not reflect what happens in real life, where work schedules, artificial light and different lifestyles alter light exposure. Furthermore, only intermediate chronotypes have been included, so these results could vary when including people with chronotypes more inclined towards eveningness or morningness. In addition, the analysis focuses solely on the impact of time policy on the circadian system and not on its effects on other aspects of health, the economy or social behaviour.

Nevertheless, the study adds value by systematically comparing three scenarios: the current biannual change and permanent standard and supposed energy-saving schedules, and predicts chronic beneficial effects from maintaining a fixed schedule. In Spain, which also has a biannual change, these results would reinforce the idea that abolishing the time change would be most beneficial to health. However, due to the gap between solar and social time in our country, which exists even with standard time, it is possible that the benefit of maintaining the supposed energy-saving time throughout the year would be even less than in the case of the present study carried out in the US. Furthermore, it should be remembered that the number of hours of natural light varies throughout the year due to the Earth's movement around the sun, regardless of the type of time. In summer, even if we maintained standard time throughout the year, we would still have more hours of daylight than in winter.

In summary, despite its limitations, this is a solid, peer-reviewed study that reinforces the evidence that the biannual change is the least healthy option. It also provides comparative evidence between standard time and daylight saving time, suggesting a slight advantage for permanent standard time, which could help reduce the number of people suffering from obesity and cardiovascular accidents.