I always come out of winter 7 lbs heavier. I think it's partly boredom, with not being able to get outside and do stuff in the garden or on the allotment, which also results in lack of exercise. I also hate the winter months with a vengeance, and all I really want to do is curl up in a ball and sleep through them.
Having said this - for the past couple of mornings, some time between 05:30 and 6:00 am, I have heard the beginnings of the dawn chorus, so spring isn't too far away. HOORAYYYY11111