I wholeheartedly agree that we need a break - and regularly; when focusing so hard on one thing, other perspectives and just our energy can go 'out of balance' or 'out of whack' and leave you feeling like you are working so hard yet getting nothing done!
Have you heard of the 'Pomodoro Technique?' It's basically a time-management technique that also helps to keep your focus fresh and alert. You time yourself for small to medium amounts of time and also time breaks within your timeline; it places importance, as it should! on the value of taking a short and regular break:
I like to think about the power of this simple technique and combine its concepts with some writing prompts when I have trouble getting something written.
For example, themed lists make a wonderful prompt to get your mind flowing And the words to come forward easier. Using a starter phrase to keep the list focused, such as 'I never..' 'S/he would always..' Or like Joe Brainard's book, 'I remember..' These are phrases you can use to just get writing. You begin to make a list and each sentence or line begins with this same phrase. Your job is to finish it with a new idea each time. You can write this list in timed segments; 3min, 10min, 25min etc. To have some more stimulus you can even grab a few snapshots or photos of any kind, you can even get them from online, and write the timed lists in response to the pictures.
They don't necessarily have to have anything to do with what you are ultimately trying to write about. It's just to get your mind going, into 'writing mode.' It's like any other voice. You have to sometimes warm-up before doing the real thing :] This and other prompts I have on hand help me all the time.
Hope this is helpful!