Time for my usual disclaimer that I am not a doctor or an expert in anything and it would be advised to avoid following any advice I give.
I have not officially been diagnosed with seasonal affective disorder(SAD), but last year as winter grew near and the days became shorter, I started to exhibit all the classic symptoms. So like any good internet citizen I self diagnosed. I went out and bought a "happy" light. They are supposed to help with SAD. I can't tell you if it is effective. I being me, never read the instructions and used it wrong all last winter.
This winter I plan on using it properly until January. Why January? The symptoms tend to start dying off in January. Not just because the days start getting longer. Here in Illinois it gets really cold in January, that means the air is really clear and cloudless most of the time. The days during January are brilliantly bright. This reminds me of a funny story a friend told me. It was January and he had been working with a Nigerian immigrant. They sat near the window at a local restaurant. The Nigerian looked out into the brilliant sunshine wistfully and sighed. He turned to my friend and proclaimed, "It's like God is lying to me."