Some people become depressed, lack energy, and crave carbohydrates in the winter months. If you experience these symptoms in the winter and they seem to go away as spring and summer arrive, you could have Seasonal Affective Disorder (SAD).
Fewer daylight hours means less sunlight to your body, which can trigger a biochemical response and affect moods.
Taking walks during daylight hours can help. Try to get more natural light into your home and office by sitting near windows without drapes. If symptoms don't go away, consider buying a special fluorescent light that is used to treat SAD.
(Taken from NYSCA)