What causes Seasonal Depression?

Is it the lack of sunshine in our days, or the hustle & bustle of the holiday season.

I just don’t seem to be modivated to do anything. I would like nothing more than to just sit around and do nothing.

Everything anyone does makes me angry, and I realize that, but not sure how to snap myself out of it.

Have you had this happen, and what did you do to “cheer yourself up?!