Seasonal affective disorder (SAD), a depressive tendency in some individuals associated with winter's lack of light and dreary weather, can turn many's holiday season into a crippling period of severe depression.
But a recent study claims to have found one possible factor which increases people's chances of suffering from the disorder: their eye color.
The new study, published in the Open Access Journal of Behavioural Science & Psychology, observed "175 undergraduate and graduate students from South Wales and Cyprus, with an average age of 24."
The results indicated that people with lighter eyes (light brown or blue) showed less variability when it came to any season's "mood, weight, appetite, sleep, and social activity."
Basically, the mood of people with lighter eyes didn't change as much from season to season.
One of the study's authors, Dr. Lance Workman, offered a theory why this might be the case on The Conversation:
"The reason that eye colour may make some people more susceptible to depression or mood changes might be because of the amount of light an individual's eyes can process."
It all comes down to melatonin, a hormone all human beings produce to help us sleep. Scientists believe having too much melatonin can make individuals feel lethargic or depressed.
Melatonin also plays a role in how we absorb and process light through our eyes.
People with lighter eyes are more sensitive to light and, thus, need less melatonin to process the fading light during fall and winter. Having less of the hormone also happens to give light-eyed individuals better chances of dodging SAD as the winter months drag on.
Previous studies have also noted that people with darker eyes are more likely to become depressed, and some scientists have gone so far as to suggest that light eye color mutations like blue, which are most prominent far away from the equator, "may have occurred as an 'anti-SAD' adaptation" in populations who experience less light during the year.
Workman was also sure to note that eye color is far from the only determining factor when it comes to SAD, and that there are ways to combat the disorder if you find yourself suffering from wintry doldrums:
"Fortunately for those with SAD, simply going outside for a regular walk, especially at times when it's sunny, will help improve their mood."