The winter blahs are a particularly bad problem when you're living in Chicago. Victims report wanting only to sleep, eat, and kill time on the internet. I've heard it called "hibernating," which is a good way to describe it.
There are only two ways to cure my own winter blahs. One: take a long vacation to someplace less blah-y. Arizona and Colorado come to mind.
Two: exercise. Even though it's only 12 degrees out and already dark by 4:30, get those wooly clothes on and go shuffle around outside until you think your face is about to fall off. Then come back home and have a big hot chocolate.
It worked for me in Ann Arbor, and it's working for me now in Chicago.