Many mammalian species, including humans, spend a substantial fraction of their life sleeping. Sleep deprivation in rats ultimately leads to death, indicating the essential role of sleep. Exactly why sleep is so essential, however, remains largely unknown. From an evolutionary point of view, almost all animal species that have been investigated exhibit sleep or sleep-like states, suggesting that sleep may benefit survival. In certain mammalian and avian species, sleep can be further divided into at least two stages, rapid eye movement (REM) sleep and non-REM sleep. In addition to a widely conserved role for sleep, these individual sleep stages may have roles unique to these animals. The recent use of state-of-the-art techniques, including optogenetics and chemogenetics, has greatly broadened our understanding of the neural mechanisms of sleep regulation, allowing us to address the function of sleep. Studies focusing on non-mammalian animals species have also provided novel insights into the evolution of sleep. This review provides a comprehensive overview regarding the current knowledge of the function and evolution of sleep.