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

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.