Yesterday, I slept late, got in to work with more rest, and fixed an even dozen bugs, including one that left me feeling smug about how clever I was. And then when I started feeling tired, I went home, feeling content.
And then today I discovered a well-developed feature that will make this project much better. This is going to be awesome.
It's not all roses and song, but it's not all thorns and drudgery either. I do have to pay careful attention to only work as hard as I can sustain, but it's not out of the question to do so.