When I'm not working out - I also suffer a peculiar kind of depression mentioned here in the comments. Yet I'm not sure how much of this is due to a release of certain chemicals in the body, or the psychological hit of knowing you're letting yourself go. Either way, I'm back in the gym after 3 weeks out: last week I struggled to get up, today I'm up and coding at 5am. Much like exercise itself, work out what's best for you and realise the importance of training your body as much as we all train our mind.
Edit: spelling correction