Seasonal Affective Disorder

A type of depression that occurs during the same season every year, typically in the fall or winter but some people may experience season-linked symptoms in the summer. The major symptom of seasonal affective disorder is a sad, despairing mood that is present most days, lasts more than two weeks, and impairs a person’s performance at work, at school, or in social relationships. The other signs and symptoms of seasonal affective disorder mirror those of depression such as loss of interest, sleep problems, fatigue, and irritability. 

Further readings and resources
CAMH: SAD overview
APA: SAD treatments
NIMH: Risk factors