This is my first winter bike commuting, and I have to say that it's totally changed my life. In high school, I ran almost every day, and was perfectly fine, emotionally. Once I hit college and started to be more sedentary (especially in the winter) I started getting bouts of depression that the doctors labeled as SAD. This year, I've had no depression, I'm not as pessimistic, and even my students can see a change.
Heck, they can see a change from day to day, if I bike one day and not the next. They'll even tell me to go for a bike ride so I'll be in a better mood! lol.