What’s “better”, summertime or wintertime? —On translating scientific ambiguity into public trust

Christine Blume
sci five | University of Basel
6 min readFeb 18, 2021


Photo by Jon Tyson on Unsplash.

Late summer sunsets and plenty of daylight after work — doesn’t that sound appealing? Well, it likely does to most of us. And the fact that daylight saving or ‘summertime’ comes with one more hour of daylight in the evening might explain the outcome of the 2018 survey by the European Commission, where the majority of participants expressed their preference for ‘summertime’ all year round rather than standard or ‘wintertime’. But don’t get your hopes up too high, for some researchers have raised a warning finger: mood and metabolic disorders, even higher cancer rates may be what the additional hour costs! Is this doom-mongering or a realistic prognosis? The truth is: we don’t know.

“The risk of negative effects is larger with perennial daylight saving time. However, the magnitude and significance of the potential consequences is unknown.” — current scientific consensus

Among the general public, such insecurities, contradictory findings and diverging interpretations can result in the perception that science is not trustworthy. However, insecurities and doubt are at the very core of scientific knowledge gain. Especially during the past year, this discrepancy became overly evident as scientists saw themselves confronted with pressure to provide definite and simple answers and recommendations on how to keep the spreading of the Coronavirus at bay. But this is not the task of scientists. Rather, it is their task to objectively inform about relevant topics by providing a well-balanced analysis of existing as well as missing knowledge, particularly in cases where evidence is not solid. And I’m not saying this is easy, especially when a debate is emotionally charged.

I am a postdoctoral researcher at the University of Basel’s Centre for Chronobiology. In the debate about perennial summer- vs. wintertime, I have repeatedly argued for a balanced view and against fanning fear. Here, I give three reasons why this is not a black-and-white case — and thus requires careful communication.

1. It’s tricky: What scientific studies (not) tell us

Science is often not straightforward. Usually, it rather resembles a maze: The way to the answer is long and crooked. And sometimes, you may not even be able to reach it directly. Photo by Ashley Batz on Unsplash.

It is actually very difficult to directly compare the effects of perennial summer- and wintertime. Why? Because theoretically, this would require two populations that only differ in the time zone they live in, that is, summer- or wintertime.

In such a situation, one option is to try and learn from findings gained in studies originally designed to answer a different question. For example, one study reported that 30 minutes of light exposure in the morning are more effective at improving symptoms of winter depression than light later during the day. While this is of course a very specific finding, the results suggest that a later sunrise, as under perennial summertime, might negatively affect mood. However, such interpretations obviously suffer from constraints and therefore must be handled with the necessary caution. For example, it is unclear to what extent a 30-minute light treatment can be compared to a one-hour delay in sunrise and sunset, and how large potential effects could be in the general population.

In fact, only once scientists were able to directly compare time periods with perennial summertime and wintertime in northern Russia. Here, in a sample of 10 to 24 year-olds, researchers found a two to three percent increase in the rate of symptoms related to winter depression. However, only at first sight, this seems a definite argument against summertime. After all, we need to ask: Can results from regions in extreme latitudes simply be transferred to regions in, for instance, central Europe, in particular, when the effects became larger the further up north people lived? And are the reported effects practically relevant or merely a statistical effect? Answering these questions is difficult. It requires a combination of careful interpretation, experience, and methodological or statistical knowledge — and even when these are most expertly applied, scientists’ conclusions may vary and even be conflicting.

2. It’s complex: The relationship between solar and body clock

Light is the most important zeitgeber for the internal biological clock. Photo by Pablo Heimplatz on Unsplash.

Some opponents of summertime argue that it ‘messes with the body clock’. Why? Because light is the primary zeitgeber, which entrains the internal biological clock to the earth’s 24-hour light-dark cycle every single day. Under summertime, ‘solar time’, that is, the sun’s motion, is shifted by one hour relative to ‘social time’, our daily schedules. And this may cause problems, some argue.

But is the relationship between biological clock and sunlight really invariant and deterministic? One study from Europe claims that it is, although the researchers’ conclusion presupposes a number of assumptions rendering it debatable. Indeed, a 2017, a modelling study suggested that the case is more complex. Here, the researchers investigated the extent to which varying amounts of artificial light in the evening modulate the relationship between body and ‘solar’ clocks. They found that light intensities of only 100 lux, comparable to staircase lighting in buildings, greatly reduced the influence of solar time on sleep-wake rhythms. Additionally, a recent study among UK students showed that sleep-wake timing depends on when and how much light we are exposed to rather than at what time the apex of the sun’s motion is reached. In sum, while natural sunlight is important beyond doubt, the question of how meaningful the effects of a shift in solar time by one hour will be in a modern light environment, can currently not definitely be answered.

3. It’s unresolved: The magnitude of negative health consequences

Picture by Yerson Retamal on Pixabay.

‘Cancer’ is a buzzword triggering attention and fear in many people. But could perennial summertime really increase cancer rates and should we therefore be worried? In this context, two studies investigated cancer incidence rates in relation to the position within a time zone in Russia/China and the US. This is a very elegant approach as the western end of a time zone resembles summertime whereas the eastern end resembles wintertime. First, both studies concluded that the risk for cancer increases when moving westwards. However, the researchers from the US admit that the effects are very small — especially compared to other well-known risk factors. Furthermore, time zones in the Chinese/Russian study were two to four times the width of a ‘standard time zone’, therefore representing rather extreme cases. Last, data from the European Cancer Registry do not seem to back these findings — at least, nothing along these lines has been published and the data do not suggest an obvious effect. Thus, while we certainly need to be cautious, I recommend to carefully evaluate the scientific evidence before drawing simplified conclusions that may alarm the public.

All things considered, perennial wintertime seems the safer choice. However, the potential consequences of perennial summertime and their magnitude currently cannot be predicted.

Beyond this though, the debate exemplifies how challenging adequate communication of scientific evidence can be. Even in controversial debates, scientists need to inform in an unbiased way and not ‘sell’ an emotionalized story. As experts, it is their task to tell the whole story and this includes informing about still lacking evidence. After all, scientists should not underestimate nonexperts’ ability to understand scientific considerations and flag concerns for instance about the quality of evidence. Only this way, insecurities and doubt in science can translate into public trust.

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Christine Blume
sci five | University of Basel

Senior sleep scientist at the Centre for Chronobiology of the University of Basel, Switzerland.