Two theories have traditionally been used to explain why blue eyes occur in Western populations living farther from the equator. First, it might be seen as more attractive to the opposite sex, so it might provide a reproductive advantage.
Second, blue eyes may be a side effect of the same mutation that causes lighter skin color. This mutation evolved because it helps the body make more vitamin D from the sun’s ultra-violet light in parts of the world that receive less radiation, especially during the winter.
But given that blue-eyed people in our study reported lower levels of SAD than their brown-eyed counterparts, this mutation may have occurred as an “anti-SAD” adaptation as a result of the considerable variations in light exposure that our prehistoric ancestors experienced as they migrated to northerly latitudes.
Eye color is, of course, not the only factor here. 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.
If that doesn’t work, “phototherapy”, which involves sitting in front of a light box for an hour daily, could also help. People I have advised to use these methods (whether brown or blue eyed) almost invariably have reported a noticeable improvement. However, people with SAD are advised to consult a GP regardless, especially if their symptoms do not improve, or if the condition becomes difficult to manage.