Natural selection has given rise to an exceptional diversity of animals. Here, we explore this diversity as it relates to sleep behavior and neurophysiology. We provide a comprehensive review of sleep in mammals, birds (or avian reptiles), nonavian reptiles, amphibians, bony and cartilaginous fishes, and invertebrates. We also discuss how ecological factors, such as predation risk, have shaped sleep in prey and the growing literature on the influences of light pollution on sleep in humans and wildlife. Lastly, we highlight recent studies on species that can forgo sleep for extended periods of time and, by doing so, challenge notions about the adaptive value of sleep. Whenever possible, we interpret evolutionary patterns in the light of functional hypotheses for sleep. Throughout, we emphasize the unique contribution that comparative sleep research has made to our broader understanding of sleep and sleep functions.