Some days I can't focus, other times I can't fall asleep as I'm restless over projects. I've been known to go to bed, get up after an hour of tossing and turning, and work through the night finishing a project. And not because I have a deadline - I do this for fun. That project might be something I didn't touch at all the day before.
I just go with it, an hour of a project, an hour doing something else - they say moving around every hour is good to do. But if I'm on a roll, I can sit at my machine forever!