On average, people spend one third of their lives asleep. But what is sleep? In the sixth century BC, it was thought that people fell asleep because all the blood had drained from their heads. Scientists have come a long way since then, but many mysteries remain. For example, what causes narcolepsy? And how can someone get out of bed, leave home, walk to a nearby construction site, and climb a 130-foot-tall crane, all without waking up? Introduction. Afterword. Annotated list for further reading. Index. Black-and-white illustrations.