With dwindling daylight hours and ongoing battles against colds, winter can be pretty gloomy for everybody. But for many thousands of people across the world, the change of season can bring a clinically recognized form of depression called Seasonal Affective Disorder (SAD).

All kinds of circumstance can affect a person's experience of SAD, from genetic influences to lifestyle choices, but it's also debated whether eye color can have a “direct effect” on the condition too.

Writing for The Conversation, Professor Lance Workman cites a 2002 study that asked 175 students from two universities in Wales and Cyprus to answer a seasonal pattern assessment questionnaire to determine whether they had SAD and if so, how severe it was. It found that people with SAD who had brown or dark eyes were significantly more depressed than those with blue eyes. 

So, could blue eyes actually keep the winter blues away? With this study alone, it could be argued that this is a case of correlation not equalling causation; the darker eyes are not directly making the individuals' SAD any harsher, it could be a mere coincidence or a less direct relationship.

However, a 2018 study by Professor Workman suggests that the levels of pigmentation in the iris could actually have a causal link with the severity of SAD, even suggesting that blue eyes evolved to "provide resilience" to SAD.

One of the leading theories says that SAD is strongly related to the reduction of sunlight (UVB light) we receive in winter months when the days are short and the nights are long. It could also be argued that the decrease in light reaching the retinal cells in the eye results in a reduction of melatonin, an important hormone that regulates wakefulness.   

“Eyes with lower pigment (blue or grey eyes) are more sensitive to light,” writes Workman. “This means they don’t need to absorb as much light as brown or dark eyes before this information reaches the retinal cells. As such, people with lighter eyes release less melatonin during the fall and winter.

“This mechanism might provide light-eyed people with some resilience to seasonal affective disorder (though a smaller proportion may still experience SAD).”

Alternatively, it is possible that blue eyes are a bit of a red herring. Lighter eyes, generally, are associated with paler skin, a mutation that evolved among inhabitants of colder climates and high altitudes to maximize vitamin D conversion in low levels of UV light. The skin's adaptation to lower light levels could, therefore, also play a role.

In reality, though, it's safer to assume that SAD is caused by a cocktail of many components, from deep-rooted genetic factors to fiddly cultural differences.

“Eye colour is, of course, not the only factor here," concludes Workman. "People who spend too long indoors are also more susceptible to both winter blues and full-blown SAD. Fortunately for those with SAD, simply going outside for a regular walk, especially at times when it’s sunny, will help improve their mood.”