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

Quality of the study.

The study was conducted using a very rigorous and transparent methodological design. It uses previously validated mathematical models of the circadian system (modified Jewett–Forger–Kronauer) and reliable official data on socioeconomic and health indicators (US Census, CDC PLACES dataset). The use of mathematical models based on idealised light exposure allows us to identify what the effect of eliminating daylight saving time and adopting a stable schedule throughout the year might be.

Strengths of the article.

• It addresses the effect of the official schedule throughout the year and not just during short periods before and after the changes, as has been the norm until now.
• Multidisciplinary approach combining specialists in biomedical engineering, chronobiology, and epidemiology.
• Uses county-level data, weighting the impact of the time schedule according to the resident population, which gives greater relevance to the conclusions in relation to public health.
• Total transparency in the analysis procedures, sources, and availability of data.

Weaknesses of the article.

• The model assumes an idealised pattern of light exposure and sleep schedules for the entire population, which may underestimate the real impact of maintaining a stable schedule in populations with irregular schedules, night work, or limited access to natural light.
• It does not incorporate behavioural factors (physical activity, time outdoors) or meteorological factors, which could modify light exposure and, consequently, potential chronodisruption.
• It does not analyse or weigh the heterogeneity of chronotypes at the population level, as it uses the intermediate chronotype (the most common) as a general reference.

Originality of the approach.

The spring and autumn time changes have been questioned for years because of their negative effects on health in the days immediately following the changes, to such an extent that all national and international sleep and chronobiology societies propose their abolition.

This study has gone a step further and analysed what would happen if the United States adopted a fixed schedule throughout the year, estimating the impact of maintaining standard time (SDT) or daylight saving time (DST) permanently.

Main results.

Mathematical simulations clearly show that maintaining standard time (SDT) throughout the year would significantly reduce the burden on the circadian system (chronodisruption) and, with it, the prevalence of obesity and strokes. The benefit would be somewhat less if permanent daylight saving time (DST) were adopted, but in any case, both options would improve health compared to continuing to change the time twice a year. The authors also show that the impact is not the same across the country: geographical position within the time zone (east-west) and chronotype (morning or evening people) influence the degree of benefit. In general, the benefits would be greater for people living in the westernmost areas of each time zone and for evening people.

General conclusion.

This work is pioneering because it quantifies the chronic effects of time policy on health, something that has hardly been studied until now. Its conclusions support the idea of abandoning seasonal time changes and opting for permanent standard time (closest to solar time) as the healthiest option for the majority of the population. Although the model assumes ideal light conditions and does not include all real-life factors, such as irregular sleep schedules or time spent outdoors, it provides a solid scientific basis for the debate on the future of time policy.

These results could also be extrapolated to other countries with large differences between official and solar time, such as Spain.