Why can some birds sleep with only half of their brain? What really happens during REM sleep? Why do our sleep patterns change across a lifetime? Sleep is one of the most important but least understood aspects of our life, health and longevity. Until very recently, science had no answer to the question of why we sleep, or what good it served, or why we suffer such devastating health consequences when it is absent. Compared to the other basic drives in life - eating, drinking, and reproducing - the purpose of sleep remained elusive. In this book, neuroscientist and sleep expert Matthew Walker charts twenty years of cutting-edge research, looking at creatures from across the animal kingdom to find the answers that will transform our appreciation of sleep and reverse our neglect of it.