What is sleep? Why do we dream? And how does it change as we get older? Sleep is a fundamental biological process, yet scientists are still uncovering exactly what happens when we close our eyes. Good-quality sleep is essential for almost every aspect of human health - from brain function and immunity to physical repair and mental wellbeing. Not getting enough can affect everything from mood to susceptibility to illness. This book traces our understanding of sleep, from how our ancestors slept to modern habits, and offers practical tips for getting a better night's sleep.