Final answer:
Seasonal affective disorder (SAD) is a mood disorder characterized by depressive symptoms that occur primarily during the winter months.
Step-by-step explanation:
Seasonal affective disorder (SAD), informally known as winter depression, is a mood disorder characterized by depressive symptoms that occur primarily during the winter months. SAD is most commonly experienced in regions with long winter nights. It is estimated to affect up to 10% of the population during the fall and winter.