A great thing with it is that you can just basically force yourself to do it, and still reap the rewards.
I find it's much easier to procrastinate when I don't feel like working. But if I just push myself to go out for a walk or lift some weights, once I get going I'll usually see it through and feel much better afterwards. It doesn't even have to be much, just a few rounds of pushups and squats (without weights) is enough to get the blood flowing and get my spirits back up. There's something about feeling your body working that's very mentally relaxing.
Bonus points for this having an effect on my food cravings, which I've seen have an effect on mood. I think there was a related thread on HN recently, but I can't seem to find it.