Today (20th March) is the Spring equinox for 2025 – when the hours of daylight are the same as the hours of darkness – and Spring has officially started. As well as the daffodils and crocuses, you may also be noticing some changes in the behaviour of animals around you. Perhaps you’ve been hearing the sound of great tits, blackcaps, willow warblers and chiffchaffs, blackbirds, songthrushes. If you live in the countryside, you might even be able to spot badger cubs making their first explorations of the world outside their sett, hares engaging in mating rituals and frog spawn appearing in ponds.
So what about us humans? Are we also influenced by the rhythms of Planet Earth? As we make our annual rotation around the sun, our own behaviour is inevitably influenced by the changing seasons – but our mental processes become impacted too – and in some quite fundamental ways.
Perhaps the best known of the seasonal changes in our brain function is mood. Our emotions generally take a dip in the winter months. For some of us, that can lead to Seasonal Affective Disorder (SAD) – a type of depression that comes and goes with the seasons. But, based on an analysis of social media, this appears just one extreme of a broader trend, with an analysis of 800 million tweets in the United Kingdom finding a peak in sadness in winter. A poll suggests almost 1 in 5 of us suffer low moods more in the Autumn and Winter.
Spring brings changes in meteorological conditions (sunlight, temperature, and rainfall) which are associated with many changes in neurochemical processes involving cortisol, μ-opioid reception, serotonin, dopamine and testosterone – all of which are known to change how our brain operates. For example, increased sunlight effectively produces higher levels of serotonin in the brain which helps explain why we feel generally happier when we leave winter behind and also why we tend to be more helpful towards others on sunny days. Some researchers have even found evidence for a biannual seasonal change in our “binding” moral values (loyalty, authority, and purity), most strongly able to endorse them in spring and autumn and least keen on doing so in summer and winter.
Perhaps more unexpectedly, however, the seasons may also affect some very basic brain function. A sample of volunteers in Belgium were asked to undertake, at various points in the year, a vigilance task that required them to sustain their attention over time. The researchers found brain regions related to alertness were least activated in mid-December and most activated in mid-June. The participants in this study did not show any difference in cognitive tests, but a larger study of dementia patients in the Netherlands found their cognition to be better in the summer. There are still too few studies in this area to be sure exactly of what’s going on, but it certainly seems possible that the seasons are influencing our brain function in complex and fundamental ways.