Seasonal affective disorder: How the dark days of January shape your mood, intelligence and sex drive

Our behavior and decision-making processes can fluctuate with the changing of the seasons. The key is to learn how to optimize this natural variation. The northern hemisphere may have passed its shortest day, but winter is far from over. After the festive glow of the holiday celebrations, January can feel like the gloomiest month for many. It's not just our emotions that are impacted. A recent scientific paper, published in the journal Perspectives on Psychological Science, delves into the numerous ways that the seasons can influence our brains – from our sexual appetites to our cognitive abilities and our social activity.

While it is widely accepted that creatures like Canada geese or black bears adapt their behavior to the time of year, these subtle variations in human psychology have been far less discussed – but they may be crucial for understanding our decision-making and wellbeing. Here are some of the most notable findings that have piqued researchers' interest: The existence of wintertime depression, known as 'Seasonal Affective Disorder' (Sad), is now well-accepted. The symptoms include a persistent sadness or anxiety lasting for at least two weeks; a sense of hopelessness and worthlessness; decreased energy; overeating; and oversleeping.

Many individuals may experience a subdued mood without meeting all the criteria for a clinical diagnosis of Sad. This general melancholia is known informally as the 'winter blues', and research suggests that it is widely shared. In the early 2010s, researchers from Cornell University in Ithaca, New York, analyzed the content of 509 million tweets from 84 countries. They found a correlation between the change in daylight hours and the emotional content of the posts: as the days shortened, users tended to employ fewer positive words.