This book discusses the evolution of sleep and its possible function in the higher invertebrates and vertebrates, including humans. It describes the current concept of sleep and its functions, based on research on the mammalian system. To date, electrophysiological recordings of the brain waves, muscle activity, and eye movements are the only tools available for characterizing the sleep architecture in the majority of animals. In mammals and birds, only two distinct types of sleep are found - non-rapid eye movement (NREM) and rapid eye movement (REM) sleep. Since the discovery of REM sleep, studies have been performed to understand the purpose of the two distinct sleep states in higher vertebrates (birds and mammals), and how REM sleep was evolved. The book summarizes the role of both REM and NREM sleep in the proper functioning of the brain and body. It covers various aspects of the role of sleep in important physiological processes, including memory consolidation, induction of synaptic plasticity, energy restoration, enhancing immune response, and maturation of neuronal circuitries during early life. Lastly, the book reviews the effects of chronic/acute sleep deprivation on memory consolidation, obesity, and the immune system in animal models and humans.