The mystery of sleep has fascinated scientists for decades, yet despite great advances on several fronts, basic questions about sleep remain unanswered. What neural or somatic mechanisms govern the normal alternation between sleep and wakefulness? What genes control the quality and quantity of sleep? Do all animals sleep and if so, do similar biological mechanisms govern sleep in different animal species? And, perhaps most critically, what are the functions of sleep? In this book, leading scientists explore these fascinating questions by presenting their latest findings spanning a) neurological structures and genes underlying sleep expression b) interactions between sleep and the immune system c) the evolution of sleep in invertebrate species and d) the role of sleep in neurogenesis, macro-molecule synthesis and brain plasticity in both developing and adult animals.