seasonal affective disorder 
noun [mass noun]
Depression associated with late autumn and winter and thought to be caused by a lack of light.For people with a type of depression called seasonal affective disorder, this need for light takes on greater significance....
- For many, the lack of sunlight contributes to seasonal affective disorder, depression that grips most strongly in the dead of winter.
- Many Americans suffer from the form of depression known as seasonal affective disorder, with a strong peak in the fall and winter.