Does cognitive function change with the seasons? This question had never been properly investigated. In an Article published by the American journal PNAS, researchers at the University of Liege have been able to show that some brain function, at least, changes in keeping with the seasons. For tasks requiring attention, brain activity is maximal in June, around the time of the summer solstice, and is minimal in December near the time of the winter solstice; for executive tasks (short-term memory), brain activity is maximal around the time of the autumn equinox (September) and is minimal around the time of the spring equinox (March).

We know that humour has a seasonal dimension in humans. It goes without saying that most of us are happier and more cheerful in summer. Conversely, around 3% of individuals suffer from a recurrent depression syndrome, seasonal depression, which begins around October and ends around March. Likewise, in our latitudes, 15% to 20% of the population experience a loss of moral, upset sleep, an increase in fatigue and irritability… ;in short, a slight non-clinical depression syndrome: the “winter blues”. “Moreover, some studies seem to show that serotonin, whose importance in regulating humour is known, and dopamine, also involved at this level, but also in the reward circuit and addiction as well as in the regulation of cognitive function, change with the seasons”, explains Gilles Vandewalle, a research associate in the FNRS (the Belgian National Funds for Scientific Research) in the GIGA-CRC-In Vivo Imaging (1) unit of the University of Liege (ULg).

In addition to the regulation of humour, the seasonal aspect of the expression of some functions in the organism, such as immunity, blood pressure or the level of cholesterol, is also present in humans while there is a peak in the number of deaths in winter, violent suicides in spring and procreation in winter and spring. On the other hand, the impact of the seasons has been well established for various functions in animals. For example, reproduction, the search for food, or, for some species, general metabolism culminating in the phenomenon of hibernation.

Loss of bearings

But what happens at a cognitive level? Not only do we have no data on animals in this regard, but, in addition, the  studies on humans are very rare and focussed almost exclusively on the P300 event related to attention and cognition. These electroencephalographic studies yielded contradictory results with regard to the influence of the seasons on certain cognitive functions.

Furthermore, research on animals suggest that the suprachiasmatic nucleus, a structure situated in the anterior hypothalamus and containing around 10,000 cells, is not only the biological clock which regulates the circadian rhythms (a time period of around 24 hours), but it is also one of the structures which controls seasonal rhythms. This points to the fact that, in humans, the suprachiasmatic nucleus, which is known to play a key role in variations in circadian rhythms of cognitive functions, could be involved in possible yearly variations in these functions.

In summary, apart from all the speculations and strongly-held views, the study of possible seasonal aspect in cognitive functions in humans appears to be a blank canvas. This situation stems principally from the fact that measuring pure seasonal rhythms in the human brain is an extremely arduous process due to the necessity to control a high number of factors likely to affect brain function,: exposure to light, sleep-waking rhythm, external temperature, food intake, physical activity, social interactions…

(1) Formerly, the Cyclotron Research Centre (CRC).