An overview of that most vital, most underrated and most elusive of human activities, sleep. Paul Martin looks at the purposes of sleep, drawing on neuroscience and classic literature to do so. We spend one third of our lives asleep, but know hardly anything about it, and can remember so little of it as we come out of it. Why? Does sleeping keep us sane? Are dreams the place we go to resolve our problems, emasculate our fears and rehearse our hopes? Why are we paralysed when we dream? Why did sleep evolve? And is anybody getting enough sleep?