I’ve been writing and revising several papers this summer, and my experiences match the heart of this passage from H. L. Mencken.
Every man who writes, or paints, or composes knows by hard experience that there are days when his ideas flow freely and clearly and days when they are damned up damnably. On his good days, for some reason quite incomprehensible to him, all the processes and operations of his mind take on an amazing ease and slickness. Almost without conscious effort he solves technical problems that have badgered him for weeks. He is full of novel expedients, extraordinary efficiencies, strange cunnings. He has a feeling that he has suddenly and unaccountably broken through a wall, dispersed a fog, got himself out of the dark. So he does a double or triple stint of the best work that he is capable of—maybe of far better work than he has ever been capable of before—and goes to bed impatient for the morrow. And on the morrow he discovers to his consternation that he has become almost idiotic, and quite incapable of any work at all.
-H. L. Mencken, from “The Divine Afflatus”, reprinted in A Mencken Chrestomathy: His Own Selection of His Choicest Writing.