And it isn't just going over 40 hours. You can feel burned out if it starts to feel like you're working all the time even if it is just from an odd schedule. I was getting this way a few weeks ago, so I stopped working evenings for a few days and it helped immensely. When I start to feel burned out, I step back and do something completely different for awhile. Except at work, I don't touch a programming book, stay off forums, don't even look at programming on the computer and don't even read news about it. After a few days, if I find myself missing it I go back and I'm glad to be back.