Martin Luther, as recorded in January 1532 by his friend Conrad Cordatus and recently translated and published in Off the Record with Martin Luther [(Hansa-Hewlett, 2009), page 110]:

“I find nothing that promotes work better than angry fervor; for when I wish to compose, write, pray and preach well, I must be angry. It refreshes my entire system, my mind is sharpened, and all unpleasant thoughts and depression fade away.”