I've never been able to work night and day on project. I recommend a gap of 21 or 45 hours between writing sessions, between 12 noon one day and 9 the next morning, or between noon and nine a.m. two days from now. Writing, then, should be continual but not continuous, for those who observe the differentiation between those two adjectives. You need to write frequently, but with fresh eyes on your project at the beginning of every writing session.
I have to stop myself from writing even though I know I could produce more on a particular day, because I know from experience that I will be better on a fresh day. I try not to do much on the project between writing sessions, except to think about planning in the general sense (how many more days do I need?) or check a few books I need out of the library. Sometimes, a brilliant simplification will occur to me when I'm not writing or even thinking about my project.
A brilliant simplification means a way of eliminating unnecessary elements that are getting in your way. Sometimes being away from a project a week will help, because when you return to it you will ask, "Why am i even talking about this irrelevant and tangential issue?"