Sleeping in space, having closed eyes in zero gravity, is a very strange experience. Usually taking a day or two to get accusmed to, is related to the extension of the spine. But when you close your eyes, many astronauts experience "those never ending fall scenario that makes up many nightmares." The astronauts in space never took long periods of time to sleep. In order to sleep in zero gravity, they must be strapped into place, touching a surface. Most people lose a large percentage of their bone mass in orbit which is similar to the bone loss issue with people lying horizontally.