Hemingway had some excellent advice for where to break off for the day when you’re working on a long piece of writing. Maybe it works for more than just writing. I think Hemingway used the analogy of drilling or mining – of course he did.
Anyway, the advice goes like this: if you’re breaking for the day, don’t break just before you have to write something difficult. Break before you have to write something easy. The easiness of what you have to write will bring you back in the next day and let you get properly on your way.