When Slaves of Sleep was first published (Unknown magazine, July 1939), John W. Campbell, Jr., sent Ron Hubbard a letter that shines a revealing light on the crucial role the author was playing in the fortunes of both Unknown and Astounding Science Fiction. Noting at the outset that he had mailed Mr. Hubbard the largest check in Unknown’s history, and the second largest in the combined history of Unknown and Astounding Science Fiction, Campbell went on to enjoin the author to “please start now on your next Arabian Nights yarn. What’ll it be about? I’d like to get it in about four weeks.”
There would be a sequel, of course—a celebrated one—but ten years, a world war and the tumultuous dawn of the Atomic Age would intervene before it appeared as The Masters of Sleep in the October 1950 issue of Fantastic Adventures.