I was thinking depression too. It happens to my husband at this time every year (also in PA).
Its called Seasonal Affective Disorder triggered by a lack of sunlight like you'd get over the summer. It can range from just the winter blues to all out depression. Treatment options include going for more walks, especially in the morning, and using full spectrum lights to simulate the outside. We just outfitted our whole house in full spectrum lights so hopefully that will help him.
Have you ever seen the commercial with the bouncing ball who is sad, and then he takes some drug and then he is happy? Every year DH sees that ad and says "I wish I could be the happy bouncing ball," but of course he refuses to see a doctor.
If you are like him and don't like to get help, I'd recommend the walks and lights.