The Winter Solstice has passed, which means the days are now getting longer. The increase right now is still minimal, so the long dark will be around for a while.
Many people suffer from "S.A.D." (a.k.a. "seasonal affective disorder", "winter blues", "cabin fever") resulting from the lack of daylight. It's a common problem in the north during winter, because we don't get much sunlight.
You can read up on SAD on the net. Wiki has a lot of stuff about causes, symptoms and what you can to do prevent / treat it.
Basically, you want to get outdoors as much as possible, especially when the sun is shining (which, at this time of year, only gives you a few short hours, and that is on the clear, colder days). Being in the light is important.
It also helps to keep your home well lit too; especially the room you are in. An extra light or two can really help relieve depression, give you more energy and avoid the effects of SAD.