I have found that the fatigue accompanied with times of furious writing often has anger associated with it. My drive moves me past a certain point, and I suppose I am really more angry with myself than the project. The need to nurture ourselves as writers involves stimulating other areas of the brain. If we have been too long into the left brain–words, words, words, then we need to get back to the intuitive part of our right brain (which is “big picture”). Activities such as exercise, art, dancing, sex, nature, or even spending time with our pets, can give the left brain time to renew.
For the actual block, I feel that it is an extreme form of fatigue. An analogy might be adrenal exhaustion versus adrenal fatigue. We are only human. Why should we try to be more?