We know that taking a short break during a long writing session is helpful, but research shows a surprising reason. Apparently, focusing exclusively on one task for a long time can actually make it disappear from your mind.
One of the authors of the study, Alejandro Lleras, compared it to the way we tend to stop noticing anything that is constant in our environment. For instance, you may put a poster on your wall. The first few times you see it, you notice the colors and perhaps it cheers you up. After you’ve seen it every day for a while, your eyes still see it but it stops registering. As reported on the Psyblog site, Professor Lleras says:
“Constant stimulation is registered by our brains as unimportant, to the point that the brain erases it from our awareness…If sustained attention to a sensation makes that sensation vanish from our awareness, sustained attention to a thought should also lead to that thought’s disappearance from our mind!”
He tested this with two groups. One focused on one task for 50 minutes without interruption, the other switched tasks from time to time. The performance of the first group went down, the performance of the other group stayed comparatively high.
The brain responds to changes, so even if you feel you are too busy to take a total break, at least switch to doing another task for a few minutes and then get back to the main one. Since research also suggests that sitting for too long is bad for you, an easy way to do this is simply to get up, walk around the room, perhaps drink a glass of water, and then get back to work. During this brief break be sure to think about something other than just the project you’re working on, or it may not function as intended.
However, in terms of writing, if you find yourself in a flow state, with the words coming to you easily, I'd advise that you keep going.