Almost all life shows a 24-hour pattern of activity and rest, as we live on a planet that revolves once every 24 hours, causing profound changes in light, temperature, and food availability. Species are adapted to a particular temporal niche just as they are to a physical niche. Activity at the wrong time often means death. We spend approximately 36 per cent of our lives asleep, which suggests this aspect of our 24-hour behaviour provides us with something of huge value. ‘Sleep: The most obvious 24-hour rhythm’ considers two questions: Why has almost all life evolved a 24-hour circadian pattern of activity and rest? And what are the important processes that occur in the body during sleep?