Bright lights influence not only our internal clocks but also our production of serotonin, and in turn this causes some people to suffer from seasonal affective disorder — a gloomy winter mood cured, strangely enough, by sitting in front of a dazzling lamp. The prevalance of this disorder grows with increasing distance from the equator, University of Liege researchers propose how our brains work might also be season-dependent. Although we perform mental tasks equally well throughout the year, certain tasks “cost” us more during different seasons, their new study reveals.

Our lives on earth are shaped by circadian rhythms; daily we wake and sleep in response to the sun’s passage across the sky. Yet we also are influenced by the seasons, the researchers explain, with evidence of some physiological functions, including blood pressure, and the expression levels of certain genes related to the time of the year. According to the authors, several behaviors peak based on season: Conception (winter/spring peak); death (winter peak); and violent suicide (spring/summer peak). While behavior and moods are the most frequent focus of investigations, a few rare studies from the past suggest that “cognitive brain functions show annual variations in healthy individuals,” the authors note.

Led by Dr. Gilles Vandewalle, a post-doctoral researcher, the team of Belgian scientists investigated this possibility with a unique experiment.

Not even a Cell Phone

To study the effects of season on brain function, a total of 28 healthy volunteers spent 4.5 days in a lab, living beyond the reach of daylight and without access to the external world. As these intrepid volunteers dutifully performed tasks related to attention and memory, the researchers used MRI technology to observe activity patterns between brain networks. Throughout the year, volunteers arrived at the lab for their 4.5 day “vacations.”

At year’s end, the team gathered and analyzed the data. What did they discover? Though the performance of participants remained stable throughout the year, brain activity varied depending on the seasons. In other words, more (or less) of an involved brain region would be recruited to complete a given task during a specific time of year.

For the sustained attention task, for example, the volunteers’ maximum brain activity occurred around summer solstice, the minimum around winter solstices. For the working memory task, the researchers observed maximum brain engagement around autumn, the minimum around spring equinox.

“One might postulate that more basic cognitive processes, such as attention, are more tightly related to basic environmental changes (e.g., day length), whereas higher cognitive processes are related to more complex cues, such as, for instance, social interactions,” wrote Vandewalle and his colleagues. Though we may see ourselves as separate and distinct, we are inextricably bound to this planet, whirling through empty space, and forever influenced by its seasonal moods.

Source: Meyer C, Muto V, Jaspar M, et al. Seasonality in human cognitive brain responses. PNAS. 2016.