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

Our behaviour and decisions can vary with the changing of the seasons. The trick is to learn to make the most of this natural variation.

The northern hemisphere may have ed its shortest day, but the winter is far from over. After the sparkle of the holiday celebrations, January can feel like the darkest month for many.

It's not just our emotions that are affected. A recent scientific paper, published in the journal Perspectives on Psychological Science, explores the many ways that the seasons can influence our brains – from our sexual appetites to our intelligence and our social activity.

While it is well-accepted that creatures like Canada geese or black bears adapt their behaviour to the time of year, these subtle variations in human psychology have been far less discussed – but they may be essential for understanding our decision-making and wellbeing.

Here are some of the most notable findings to have caught researchers' attention:

Mood

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 people 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, analysed 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, s tended to employ fewer positive words.

There are many potential explanations for the winter blues and Sad. One popular theory is that the reduced light levels interfere with the body's biological clock – known as its "circadian rhythm" – which can in turn impair the healthy regulation of neurotransmitters involved in emotional processing. This has inspired light therapy, which uses special lamps that mimic the Sun to recalibrate the body clock, although a 2019 Cochrane Systematic Review concluded that the evidence for its effectiveness as a preventative treatment was limited.

Recent research by the health psychologist Kari Leibowitz suggests that our mindsets may also play a role. Working with Joar Vittersø at the University of Tromsø, Leibowitz questioned participants from various regions of Norway about their attitudes to winter. For example, they were asked to rate their agreement with statements such as "winter is an especially beautiful time of year"; "I love the cosiness of winter months"; and "I like the soft light we have during the winter months". Leibowitz and Vittersø found that people who agreed strongly with these statements tended to cope much better with the cold and dark, with higher levels of life satisfaction and more positive emotions in the winter months.

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A changed mindset certainly cannot be a cure-all for people with severe Sad, but Leibowitz suggests that many of us might beat the blues by learning to notice and embrace the positive features of winter, such as the natural beauty that it brings. 

We know, after all, that mindsets can play a role in many other phenomena. People with anxiety disorders, for instance, are more likely to engage in "catastrophising" and other forms of thinking that focus on the most negative and frightening elements of a scenario. Cognitive behavioural therapy can help people to take a more balanced view of the situation, leading to an overall improvement in their mental health. There is some evidence this talking therapy can also be useful against Sad. Similar strategies could help lift our spirits in the year's darkest months.

Memory and concentration

If you have ever noticed your mental acuity waning with the daylight, you may not be alone. Sanne Mooldijk at Erasmus University Medical Centre in Rotterdam, the Netherlands, and her colleagues, recently analysed data from a large longitudinal study of more than 10,000 participants aged 45 and over. They found that those assessed in winter showed slightly worse performance on measures of learning, memory and concentration, compared to those assessed in the summer.

We do not yet know the causes of this seasonal variation. The poor cognitive performance may be a consequence of people's generally depressed feelings – we find it harder to think straight when we feel blue. Another possibility is that it reflects a winter-time deficit in vitamin D, which is thought to aid brain health. Vitamin D is produced by the body when our skin is exposed to sunlight, and while we can also absorb the nutrient from certain foods, it is very difficult to get enough vitamin D from our diet alone. The short winter days in higher latitudes in particular, combined with less clement weather, means many people can struggle to get enough vitamin D.

Probing this underlying mechanism could help scientists to understand why there is a rise in dementia diagnoses over the same period. The slight dip in mental acuity over winter may amplify existing symptoms of cognitive decline, leading more people to meet the clinical criteria for dementia.

Sociality and sexuality

We may consider our descriptions of "warm" and "cold" behaviour to be merely metaphorical, but emerging evidence suggests that these words may reflect ancient associations between ambient temperature and social connection.

According to the theory of "social thermoregulation", we evolved to look towards others as sources of physical warmth and comfort. In this way, we're similar to emperor penguins and many other creatures that naturally huddle together to share their body heat.

If this theory is true, then lower temperatures should prompt us to seek greater social connection. To test this prediction, a team led by Hans IJzerman at the Université Grenoble Alpes, , asked participants to hold either hot or cold drinks while completing various questionnaires probing the contents of their thoughts. He found that those with cold drinks were considerably more likely to think of close loved ones – people who would fulfil their need for social connection – than those holding the hot drinks. This was provided that the participants did indeed have stable and ive relationships in their lives – a fact that could not be taken for granted for some participants.

Further evidence comes from our film-viewing habits. Data from online movie rentals reveals that people are more likely to pick romance films over other genres when the temperature drops – a finding that has been replicated in multiple laboratory experiments. A heart-warming movie apparently fulfils our evolved desire for emotional warmth and affection prompted by the chill outside.

Our sexual activity follows a more complex cycle. According to one study from Villanova University in Pennsylvania and Rutgers University in New Jersey, both in the US, Google s are more likely to seek pornography in the deep mid-winter and early summer. They are also more likely to hunt for dating websites. Once again, many factors are likely involved, but it seems reasonable to speculate that the wintertime peak may, in part, reflect our desire for greater human . Whatever the explanation, this variation has real-world health consequences, with further studies revealing a rise and fall in sexually-transmitted infections over the same periods.

With further research, scientists may identify many more cycles in human behaviour that result from the complex interplay of biology and culture. And with this greater knowledge, we can better for these changes in our own behaviour. By cultivating a more positive mindset, forgiving ourselves for the odd moments of forgetfulness, and making more concrete plans to fulfil our increased social needs, we can all take steps to beat the winter blues and make the most of the new year.

* David Robson is a science writer and author of The Expectation Effect: How Your Mindset Can Transform Your Life, published by Canongate (UK) and Henry Holt (USA). He is @d_a_robson on Twitter.

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