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The mood disorder seasonal affective disorder (SAD) is a type of winter depression, in which people who have normal mental health throughout the year exhibit depressive symptoms during winter.
In Germany alone, around 800,000 people are affected by this winter depression.
Although it is difficult to say who exactly becomes affected by SAD, researchers have recently discovered that eye colour could play a role in making people prone to the disorder.
A British researcher made the discovery that eye colour could affect people's proneness to winter depression.
The study surveyed nearly 200 people on their moods, amount of sleep, and social activities in both summer and winter.
The result from this survey shows that those with light or blue eyes tend to be less affected by SAD, whereas people with brown or dark eyes are more prone to winter depression.
This reinforces discoveries from earlier studies, which suggest that people with darker eyes tend to be more depressive.
According to the researcher in charge of this study, this tendency could be due to how your eyes process light.
Darker eyes are less sensitive to light, which in turn affects the moods of those with darker eyes in light of the amount of melatonin that eyes release.
Lighter eyes release less melatonin, which means that those with darker eyes release more, and in turn experience more mood swings.