Sleep is one of the most important aspects of our life, health and longevity and yet it is increasingly neglected in twenty-first-century society, with devastating consequences: every major disease in the developed world - Alzheimer's, cancer, obesity, diabetes - has very strong causal links to deficient sleep. Until very recently, science had no answer to the question of why we sleep, or what good it served, or why its absence is so damaging to our health. Compared to the other basic drives in life - eating, drinking, and reproducing - the purpose of sleep remained elusive. Now, in this book, the first of its kind written by a scientific expert, Professor Matthew Walker explores twenty years of cutting-edge research to solve the mystery of why sleep matters. Looking at creatures from across the animal kingdom as well as major human studies, Why We Sleep delves in to everything from what really happens during REM sleep to how caffeine and alcohol affect sleep and why our sleep patterns change across a lifetime, transforming our appreciation of the extraordinary phenomenon that safeguards our existence.