You don't realize what a strain it is on the nerves to write or think-of-writing all day long, and to sleep full of nervous dreams, and to wake up not knowing who one is: this all stems from anxiety about finishing the book, about time 'growing short', etc., and the perpetual strain of invention.
Windblown World: The Journals of Jack Kerouac, 1947-1954 (ed. Viking Press, 2004)