For many people, the shorter days and long, dark nights of fall and winter can lead to a mild or severe depression known as seasonal depression, or seasonal affective disorder (SAD). Here are some resources to further explain, and provide help with, this condition:
Seasonal Affective Disorder (Mayo Clinic)
Seasonal Affective Disorder (PubMed Health)
Winter blues : everything you need to know to beat seasonal affective disorder
Rosenthal, Norman E.