Seasonal Affective Disorder (SAD)v

The American Psychiatric Association estimates that as many as 10 percent to 20 percent of Americans may experience a mild form of seasonal-affective disorder (SAD), a mood disorder that is often attributed to a lack of light and is common during the colder months of the year. The disorder is characterized by depression, exhaustion and a lack of interest in people and regular activities, and can interfere with a person’s outlook on life and ability to function properly.

Here's an interesting article about SAD from CNN: Less Sun, More Depression With SAD.