June 4th, 1938. Nathaniel Kleitman makes his way down the 71 steps which lead to the mouth of Mammoth Cave. Despite the midsummer heat, he and his research student carry heavy overcoats, galoshes and several pairs of woollen trousers. They are about to embark on one of the most intrepid and bizarre experiments in medical history, one which will change our understanding of sleep forever. Then, together, they enter the darkness. Award-winning writer Kenneth Miller weaves science and history to tell the story of the outsider scientists who took sleep science from the fringes to a mainstream obsession. Reliving the spectacular experiments, technological innovation and single-minded commitment of these early pioneers, Miller provides a tantalising glimpse into the most mysterious third of our lives.