Seasonal Affective Disorder is more than just a case of the blues. It hits us most often in winter months when there is less light and is a real problem in northern countries where winter days are very short. You can ease the symptoms by eating foods with tryptophan, like turkey, legumes, and nuts, and by exposing yourself to more sunlight. Take a B vitamin and get outside for starters. This blog has tips that can help.
http://floliving.com/blog/blog-2/