AbstractThe reasons why we sleep have been debated endlessly and the arguments have tended to focus upon one key biological function that applies to the human species. Such explanations cannot account for the ubiquity of sleep across all animal life. This Opinion Paper explores the logic which enables all animal sleep to be defined as follows: Sleep is a period of physical inactivity during which time an individual of a particular species avoids movement within an environment to which it is poorly adapted, whilst using this time to undertake a range of essential biological activities that allow optimum performance during the period of activity.