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.

The article published by PNAS has undergone a peer review process that guarantees its soundness and quality, backed by the prestige of the journal itself. It also provides insight into a little-studied aspect, namely the chronic effect of DST/SDT (daylight saving time and standard or winter time) time changes, as opposed to the more widely known acute effects. The results may help decision-makers in both Europe and Spain to decide whether or not to maintain the time change. In this regard, the data support the elimination of the time change and the adoption of SDT over DST (albeit with little difference). The study is elegant and well designed.

Its limitation is that it uses models (on the other hand, one of the few possible approaches for this type of study) and, as such, modelling involves assumptions that are not always reproduced in outpatient conditions. Thus, in Spain, a sleep schedule from 10:00 pm to 7:00 am during working days is unlikely. The difference in geographical extent could also mean that the results obtained are not as conclusive in our country, as the differences in latitude between east and west are much smaller than in the US, but this does not detract from the validity of the data obtained or the conclusions.

The article contrasts health data at the county level in the United States with a circadian model developed from a typical, fixed daily routine. The title itself is not very suggestive. One would expect differences when comparing regional data. From the title, it can be inferred that the authors do not find any noteworthy trends in these differences.

The study tackles the complex task of comparing different daylight saving time policies. There are not many natural experiments for this type of study. The authors compensate for this lack with modelling. The health data is quite good and provides a very detailed granularity that would allow for the analysis of differences at the American county level.

However, in my opinion, the model used has limitations that are worth noting when discussing time regulations. First, I would like to point out that the policy of seasonal time change is related to the fact that the typical working day does not actually exist: there is a distribution of working days and, in particular, those who get up early and those who do not. It is not just a matter of preferences (chronotype) but, in many cases, of the type of activity. Seasonal time change cushions these differences: early risers do not rise as early in winter because the time is set back in autumn, and those who are more active later in the day do not rise as late in summer because the time is set forward in spring.

Secondly, the study and its conclusions are based on current behaviour, with the current time regulation. It is impossible to predict how society will respond to a possible change in time policy. If, for example, the time change is eliminated and permanent winter time is adopted, dawn will come earlier in spring and summer, and some people will find it advantageous to start their day earlier. This social component is very difficult to include in these studies and is a significant limitation.

In other words, the authors start from a typical day that is valid for the whole year, probably based on social knowledge. The question is to what extent that typical annual day is the result of using the seasonal time change. As we pointed out in a recent study (reference 19 of the paper), socially a regular rhythm is very welcome, physiologically a regular rhythm is also optimal, but these stable preferences interfere with the unavoidable fact that, at certain latitudes, dawn comes much earlier in summer than in winter. Given that morning light activates human physiology, those who live at a certain latitude may be inclined to prefer to become active earlier in summer and later in winter, something that is not taken into account in this work.

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.

This Stanford study in PNAS conveys a very important message: maintaining a stable schedule that is aligned with natural light has clear health benefits. Using models applied to the entire US population, the authors show that reducing the “circadian load” — that is, the desynchronisation between our biological clock and the environment — is associated with less obesity and fewer strokes. This goes a step beyond what we already knew: not only do the spring and autumn time changes cause acute sleep problems, accidents and heart attacks, but the schedule we maintain on a permanent basis also influences our long-term health.

In Spain, this discussion carries even more weight. For decades, our country has been living in a time zone that does not correspond to its geographical position: we are about an hour ahead of the sun. This encourages more evening habits, we go to bed later and accumulate sleep debt. Therefore, it is not enough to decide whether or not to maintain the time changes: we should also opt for the healthiest time zone, which would be the one that corresponds to us naturally — GMT0 — or at least GMT+1, but not remain in GMT+2, which further exacerbates the gap between sunlight and our social schedules. The underlying message is clear: the more aligned we are with the sun, the better it is for our circadian, metabolic and cardiovascular health.

For me, a common denominator in many of these studies advocating the elimination of seasonal time changes in countries outside the tropics is that they seem to lack an understanding of the fundamentals of seasonal time changes.

Let me explain: in tropical areas, the sun rises and sets at times that vary little throughout the year. These countries do not have winter and summer time; they do not change their clocks simply because they do not have seasons.

But outside these areas, this is not the case; we have seasons. For example, at the latitude of Spain (which coincides with that of the northern United States), the sun rises three hours later in December than in June, and sets three hours earlier in December than in June. In other words, the day is six hours (or more) longer in June than in December.

Therefore, it is not possible to stick to a fixed time as in tropical countries. If, for example, in Spain or much of the US, you lock the official time in winter, in the middle months of the year it will dawn very early, and the average person's activation will be out of sync and will take place when the sun is already very high on the horizon. The opposite occurs if you lock the official time to summer time.

And finally, two more objections:

• On the quantitative side: the authors are theorising variations that, in many cases, would be less than 1%! (In many of these studies, the claimed quantitative benefit data is less than the data's margin of error). Some time ago, José María Martín Olalla and I published this paper in which we quantitatively defined the margins within which some of the items studied in that paper could move, which are lower than one might think.
• The authors talk about eliminating the seasonal time change to obtain minimal variations, without taking into account what the social reaction and consequences of such a measure would be (for example, that many people could be forced to change their schedules to adapt to the fact that they live in an area with seasons). To give a very exaggerated example: it is as if someone told you that the solution to ending tooth decay is to pull out the teeth of the entire population.

Is the study of good quality?

‘Methodologically, it is consistent and reasonable. If we base ourselves on pure epidemiological theory, we are looking at an ecological study that provides estimates but cannot establish causality. However, these studies are necessary for further research and provide us with essential information on a very large population group that could not be obtained with other types of methodological designs.’

What does it add to previous research?

"The authors use light exposure simulation models based on a mathematical model to estimate the possible impact of different time policies in the US. Although this is an issue that has been discussed for several years due to the potential health problems associated with this time change due to circadian misalignment, this study provides valuable information through mathematical models of the possible consequences in relation to obesity and cardiovascular disease. It also indicates the best time schedule for the US population. These results are additional evidence to be taken into account by politicians."

What are the implications for the time change in Spain?

"The exact data cannot be extrapolated to Spain due to differences in latitude, time zone, time patterns, socio-demographic and health data, among others. However, a similar model could be applied to analyse what would happen in our country. One of the main differences with Spain in relation to this study is that in our country, the best and least popular time for the population in general is winter time. It should be remembered that our time zone is not the correct one and winter time would be the one that most closely resembles the time zone in which we find ourselves. The health findings seen in this study could be similar to those we might have in our country if a similar study were conducted due to the circadian disruption caused by biannual time changes.

What are its limitations?

"These factors are difficult to integrate into a study of this type, but some of the limitations would be:

• Use of an idealised light exposure scenario with a fixed pattern that does not incorporate seasonal changes.
• Possible variations in subjects' schedules, such as shift work, differences between weekdays and weekends, and other behavioural patterns (difficult to integrate these individualities) are not included, and how this may affect light use (natural or artificial).
• To determine health data, they only use intermediate chronotype data, when evening types have the most associated problems.
• Factors such as age, rural or urban environments, disease patterns, etc. are not taken into account.
• Methodological design (indicated above).