One must be pitiless about this matter of "mood." In a sense, the writing will create the mood. If art is, as I believe it to be, a genuinely transcendental function—a means by which we rise out of limited, parochial states of mind—then it should not matter very much what states of mind or emotion we are in. Generally I've found this to be true: I have forced myself to begin writing when I've been utterly exhausted, when I've felt my soul as thin as a playing card, when nothing has seemed worth enduring for another five minutes ... and somehow the activity of writing changes everything. Or appears to do so.
From "Joyce Carol Oates" in George Plimpton, ed., Women Writers at Work: The Paris Review Interviews (1989).