A cen­tu­ry ago, sleep was con­sid­ered a state of noth­ing­ness — a prim­i­tive habit that we could learn to over­come. Then, a Jew­ish immi­grant sci­en­tist spent a month in the depths of a Ken­tucky cave. In the 1920s, Nathaniel Kleit­man found­ed the world’s first sleep lab at the Uni­ver­si­ty of Chica­go, where he sub­ject­ed research par­tic­i­pants (includ­ing him­self) to a dizzy­ing array of tests and tor­tures. But the turn­ing point came in 1938 when his head­line-grab­bing cave exper­i­ment awak­ened the pub­lic to the unknown — and vital — world of sleep. Kleit­man went on to men­tor the tal­ent­ed but trou­bled Eugene Aserin­sky, whose dis­cov­ery of REM sleep revealed the aston­ish­ing activ­i­ty of the dream­ing brain, and William Dement, a jazz-bass-play­ing rev­o­lu­tion­ary who became known as the father of sleep med­i­cine. Dement, in turn, men­tored the bril­liant out­sider Mary Carskadon, who uncov­ered an epi­dem­ic of sleep depri­va­tion among teenagers and launched a glob­al move­ment to fight it. Weav­ing sci­ence and his­to­ry, award-win­ning jour­nal­ist Ken­neth Miller shows how these researchers changed our lives — and why it matters.