I think this is worth repeating every few days until it is thoroughly internalized. I burn out every once in a while...and when it happens, my productivity is shot...sometimes for weeks. I have to be careful to keep exercising every day, spend a reasonable amount of time watching movies or playing music or games and interacting with other people, or a weird sort of do-nothing depression kicks in. I don't even realize it while I'm working, until it's too late. I can even be enjoying what I'm doing, and having some satisfaction about results, but if I overdo it, something clicks and the next time I run into anything slightly frustrating/boring/repetitive it's all over and the burnout kicks in.