Seasons and the psyche

If the gloomy days and long nights of December are getting you down, you're not alone. Seasonal affective disorder (SAD), sometimes known as the 'winter blues', affects about two million people in the UK alone.

However, SAD isn't the only example of a seasonal impact on our psychological functioning. In fact, it's the proverbial tip of the iceberg, write Ian Hohm at the University of British Columbia and colleagues in a new paper in Perspectives on Psychological Science. As the team reports, recent work has documented seasonal impacts on everything from generosity to colour preferences. 

Some of these impacts are linked to changes in temperature and light, as well as cycles of infectious disease. But there are seasonal cultural influences, too. These relate to the set timing of Christmas, for example, as well as other national holidays. Though we might not typically think of set cultural events as being seasonal influences on our psychology, the team argues that they need to be considered, too. Whatever the precise nature of the seasonal trigger, 'seasons appear to be a fundamental source of variability in how people think, feel and behave,' they write. 

Their review reveals that these are the seven aspects of psychological functioning with the strongest evidence of seasonal influences:

Mood

Psychologists have known about SAD for decades. People suffering from SAD eat more, sleep more, feel less motivated, and are less productive. They also find it harder to learn and remember. Some researchers have likened SAD to animal hibernation and argued that the core symptoms can be viewed as energy-conserving. Other studies have suggested that low mood during winter is common, and that people with SAD represent the extreme end of suffering. 

A study of UK-based Twitter/X users did find a peak in sadness in winter, while an analysis of 765 million streams by people in 51 different countries from the music service Spotify, published in Nature Human Behaviour, found a link between day length and the 'intensity' of streamed songs. Longer days were associated with spikes in downloads of energetic songs, while in places with shorter days, more relaxed, less invigorating songs were more popular. The team, led by Fabon Dzogang, thinks this reflected variations in mood, from more positive in sunnier, warmer times and places, to more subdued in darker, cooler regions.

The precise mechanisms underpinning SAD are still debated. But research has found higher levels of serotonin in the brain in summer and lower levels in winter, which could help to explain the impacts on mood. A team in Finland (led by the nominatively determined Lihua Sun) also recently reported finding more opioid receptors in the brains of people who had been scanned in summer versus winter. Since endogenous opioids affect sociability and mood, this could also explain some of the seasonal mood effects. 

Seasonal variations in levels of infectious disease may also be involved in causing symptoms of depression. The immune system's inflammatory response to infection leads to 'sickness behaviour', including social withdrawal. 

Physical harm

The hypothesis that heat makes people more irritable, and so increases the chances of violence has been supported by a number of studies. For example, research on the rates of violent crime in Philadelphia, US, found a 9 per cent hike on hotter than average days. Other research has found higher rates of domestic violence and prisoner misbehaviour during the summer.

Some psychologists are concerned about how rising temperatures as a result of global climate change might increase aggression and violent conflict. According to one recent analysis of data from the US, by Andreas Miles-Novelo and Craig Anderson, a 1°C rise in average daily temperatures could lead to 25,000 more serious assaults in the country each year.

Others are exploring potential links between seasonal changes and suicide. Some of this research suggests that men might be affected more — in England, male, but not female, suicide rates peak between April and June. Exactly what might underpin this is not clear, but some research has linked spring allergies to suicide risk.

Sexual behaviour

There is evidence of seasonal variation in human sexual activity. In the US, Hohn and his colleagues note that condom sales, the timing of first sexual intercourse, and Google searches for pornography and prostitution all show two clear seasonal peaks: one at Christmas, the other during the early summer. Both of these peaks coincide with holiday periods, especially for college students. This, then, seems to be an example of a seasonal impact that's due to culture, rather than biology.

Recent research led by Laura Symul at Stanford, still in preprint, has found peaks in birth rates linked to increased sexual activity during holiday periods. However, the biggest birth rate peak was linked to seasonal changes in fertility, which seems to peak between the autumn equinox and winter solstice in the northern hemisphere and shortly following the winter solstice in the southern hemisphere.

Diet and exercise

It's perhaps no surprise to those who can't resist an extra treat or two when the weather draws in, but we consume more calories in autumn and winter than during other seasons. During winter, we are also less physically active and gain the most weight.

Some research implicates biological mechanisms. Other animals bulk up during autumn in preparation for food scarcity during winter, and it's thought that we may feel this urge, too. The temperature may even have a direct impact on feelings of fullness. A study of young Japanese women found that they felt fuller after eating outdoors on a warm day, compared with a cooler day. If warmer temperatures make us feel full after fewer calories, this could certainly help to explain why we eat more – and gain more weight – in winter. 

There are also seasonal cultural influences on levels of exercise and diet. One 2022 Twitter/X study led by Scott Griffiths found that dieting-related tweets peaked in the spring, presumably in anticipation of feeling more on display in fewer clothes in the summer.

Prosocial behaviour

Hohm and his colleagues highlight the 'Christmas effect' on prosocial behaviour. For example, one analysis found that 29 per cent of charitable giving in the US happens during December. Another study, which focused on a restaurant in New York State, found that wait staff received the biggest tips during the Christmas holiday season.

Within other cultures and religions, there are different peaks in charitable giving, of course. For Muslims, Zakat ul-Fitr is a charitable donation of food, or a money equivalent, that must be given during Ramadan.

For some people, Christmas intensifies feelings of loneliness or anxiety, however. Managing the holiday season during a cost-of-living crisis brings its own challenges.

Colour preferences

Various studies support the idea of seasonal variations in the colours that we find most appealing. For example, a study in the US found that people living in the north-east of the country prefer yellow and greenish-yellow colours in the autumn – when, as Hohm and his colleagues note, these colours may be readily associated with autumn leaves – and less in the winter, when these colours might instead be associated with symptoms of sickness. However, the participants' preferences for colours that we never associate with leaves showed no such seasonal change. Though these results are only suggestive, they support an 'ecological valence theory' of colour preferences, which holds that our liking for a colour depends on the degree to which we like things that we associate with those colours.

Cognitive performance

Research in Belgium, published in PNAS, on healthy young people, found that their ability to maintain sustained attention was at its maximum around the winter solstice and at its minimum around the summer solstice. The same study also found that performance on a working memory task was highest in the autumn and lowest in the spring.

In the US, Joshua Goodman at Harvard University and colleagues have found that in years with hotter school days, secondary school children get poorer test results. (School air conditioning could almost entirely eliminate this problem, the team adds.)

There is also evidence of seasonal impacts on the cognitive performance of older people. A recent study of older adults in the US, Canada and France found that those diagnosed with Alzheimer's disease had more severe symptoms in winter and early spring. However, even participants without dementia did significantly better on tests of thinking and concentration in the late summer and early autumn versus the winter and early spring. This was not related to variations in their mood or levels of physical exercise, but it was correlated with seasonal variations in levels of amyloid beta (a protein involved in Alzheimer's disease) in their cerebrospinal fluid.

A fundamental feature of human psychology

The wealth of evidence for seasonal variations in human psychology has a few major implications, argue Hohm and his colleagues. Firstly, for academic research: psychologists should take note of when experimental data are collected. This would allow them to control, if necessary, for the season when analysing their data and would also generate a much larger databank to mine for further insights into the psychological impacts of seasonal changes.

The second major implication is that psychological fluctuations relating to the seasons deserve far greater recognition and research attention. 'Rather than remaining an overlooked source of noise in the database of the psychological sciences, seasonal variation can instead be systematically documented as a key source of signal – a fundamental feature of human psychology,' the team concludes.