What is the best time to start the day in view of the variation in when the sun rises? This is the problem analyzed by Jorge Mira Pérez and José María Martín-Olalla, lecturers at the University of Santiago de Compostela (USC) and the University of Seville (US), in a study that has just been published in the journal Royal Society Open Science. In it, they analyze the physiological and social foundations of the practice of seasonal time change and review its impact on health.
The study takes as an example the cities of Bogotá and New York, which are located on the same meridian but at different latitudes, to point out that in winter the sunrise is delayed by an hour-and-a-half in the latter city. “This delays life in New York during the winter, but in spring the delay in sunrise has disappeared and activity can start earlier. Putting the clocks forward in spring facilitates this adaptation,” says Mira.
The study includes several current and past examples of societies with delayed activity in winter and earlier activity in summer, in line with the synchronizing role of morning light for our bodies. “Modern societies have several synchronization mechanisms. For example, the use of a standard time in a large region, or the use of pre-set schedules. Time shifting is another synchronizing mechanism, which adapts human activity to the corresponding season,” says Martín-Olalla. The authors suggest that the first weekend in April and the first weekend in October would be the most appropriate time for the clocks to change.
The study reviews the impact of the seasonal time change on human health, considering two types of effects: those associated with the change itself, and those associated with the period during which daylight-saving time is in effect. In the first case, the authors point out that published studies have not analyzed the problem epidemiologically and that the evidence suggests that the impact is very weak.
“A very comprehensive study in the United States reports a 5% increase in traffic accidents in the week following the clocks going forward in spring but overlooks the fact that from one year to the next, weekly traffic accidents fluctuate by 15%. Changing the clocks has an impact, but it is very weak compared to the other factors influencing the problem,” Mira points out.
“Changing the clocks has worked for a hundred years without serious disruption. The problem is that in recent years it has been associated only with energy saving when, in fact, it is a natural adaptation mechanism,” says Martín-Olalla.
In the second case, the authors point out that the current controversy stems from an erroneous interpretation of the seasonal time change. According to Martín-Olalla and Mira, changing the clocks is not a time zone jump, nor does it cause the population to live adjusted to the sun in another place, nor does it cause their rhythm of life to be misaligned with respect to the sun.
“In a way it is the other way round, changing the clocks aligns the start of activity with the sunrise,” Mira points out. “In 1810, the Spanish National Assembly had already made this kind of seasonal adaptation and there were no time zones or anything like that. Social life is simply reorganized because the length of the day in summer makes it possible to do things in the morning earlier than in winter,” says Martín-Olalla.
Mira and Martin-Olalla are highly critical of studies that report long-term effects of seasonal time change and associate it with increased risk of cancer, sleep loss, obesity, etc. They point out that these studies analyze data within the same time zone in the US or Russia, but that says nothing about the seasonal time change.
“They study what happens, for example, in New York and Detroit, at the extremes of the US East Coast time zone, but on the basis of annual cumulative incidences. The time difference between the two cities is always the same throughout the year. Therefore, it cannot be inferred that the effects they report are associated with changing the clocks, which is seasonal in nature.”
The study concludes by analyzing why changing the clocks succeeded in the 20th century and what difficulties medical associations now face as they seek to eliminate the practice and adopt permanent winter time. They point out that the main effect of the measure was to increase daylight hours during the leisure segment in spring and summer.
“People agreed that they would go to work earlier in spring and summer if it meant they could leave earlier and could enjoy the evening more by going for a walk, going to the park or the beach—it seems obvious, but it has to be said,” Mira says.
By contrast, the elimination of changing the clocks faces a difficult dilemma when it comes to choosing which time to keep: to bring the start time forward in winter, when the sun rises the latest, or to bring the start time back in summer, when the sun rises the earliest. “Either choice plays against human physiology, which is why ending clock changes has proven difficult in both Europe and America” says Martín-Olalla.
Mira says, “They have opened a Pandora’s box, they have exacerbated criticism of a natural practice that works reasonably well and now fear that the remedy is worse than the disease.”
More information:
José María Martín-Olalla et al, Assessing the best hour to start the day: an appraisal of seasonal daylight saving time, Royal Society Open Science (2025). DOI: 10.1098/rsos.240727
Provided by
University of Seville
Citation:
The naturalness of the seasonal time change: Researchers question basis of modern criticism (2025, March 19)
