[...]—Well, I too have been so low in health again recently that, since I finished the rough draft for the first act of Siegfried ten days ago, I have literally been unable to write down another note without being chased away from work by the most agonising headaches. So every morning I sit down, stare at the paper, and am lucky in the end if I can manage Walter Scott. It is simply that I have overexerted myself again; but how am I to recover my strength?? I got on well with The Rhinegold, but I had great trouble even with The Valkyrie and now I am like a pianoforte badly out of tune (as regards my nervous system)—and I have got to hammer out Siegfried upon it. A pretty state of things! This last, I imagine, will break the strings ultimately and then all will be over. Well, we cannot alter matters—but really it is a dog's life!—[...]
Source: Letters of Richard Wagner, Vol. 1, selected and ed. Wilhelm Altmann, trans. M. M. Bozman (J. M. Dent & Sons, 1927), No. 327