People with brown eyes are more likely to have seasonal affective disorder (SAD) than people with blue eyes.

This is among the findings presented by Professor Lance Workman, Nathalie Akcay and Michaell Reeves of the University of South Wales to the annual conference of the British Psychological Society in Nottingham on May 3rd 2018.

The findings are from a study of one hundred and seventy five students from the University of South Wales and the Girne American University in North Cyprus. The University of South Wales reports the students completed a questionnaire that measures the extent to which an individual’s mood varies with the seasons. The results showed that there was no difference in mood between students in Wales and Cyprus, but that the mood of students with brown eyes varied significantly more than the mood of students with blue eyes.

In a second test, the students were surveyed to see how the two hemispheres of the brain respond when recognising faces with different emotional expressions. The researchers found people with seasonal affective disorder were more likely to favour the left visual field when studying the faces and thus process them with the right side of their brains.

Lance Workman said “This tendency to use the left visual field and right side of the brain for identifying facial expressions is present in the general population, whether they suffer from SAD or not. But people who suffer with more conventional forms of depression generally lose this right hemisphere advantage. In the case of SAD we found this left visual field advantage was actually increased. This suggests SAD has different causes than, say, bipolar depression. We know that light entering the brain causes a decrease in levels of melatonin. As blue eyes allow more light into the brain, it may be that this leads to a greater reduction in melatonin during the day and this is why people with lighter eyes are less prone to SAD.”

In a second study being presented at the conference, Lance Workman and Dr Sandie Taylor reported the results of a survey of two thousand and thirty one adults conducted in collaboration with YouGov.

The results suggest 8% of the sample appeared to have full blown seasonal affective disorder (equivalent to five million people in the UK) and a further 21% had a less serious form of the condition. Combining these two groups, almost a third of the population appears to experience a noticeable change in mood with the seasons. This figure of 8% is twice the level that has previously been estimated for full blown seasonal affective disorder, but it is in line with findings from Greenland and Alaska. The survey also showed that women were 40% more likely to report seasonal affective disorder than men. This was also true of less acute seasonal affective disorder. There is also evidence that seasonal affective disorder is most acute for women during their reproductive years.

Lance Workman suggests the finding of a higher level of seasonal affective disorder can be explained by three factors:

• People are spending more time indoors, and those who take exercise tend to go to the gym, so they are experiencing less sunlight than they did ten years ago
• In the UK winters have been more overcast in recent years
• It is possible that previous estimates with smaller samples were less accurate

He suggests seasonal affective disorder may be an exaggeration of a pattern of responses that occurs primarily in women to conserve energy during their reproductive years. This may be a strategy to help promote the survival of both the mother and child. He added that the symptoms of seasonal affective disorder are accompanied by cravings for carbohydrates. Putting on a bit of weight may have helped people in the past through the winter months.