You may have heard about this and thought it was an urban legend, but it's true. In order to determine the long term effects of doing nothing, NASA will pay you $5000 to lay in bed for 90 days.
They set you up in conditions similar to what astronauts face in space, with their legs elevated. The extended lack of muscle usage is similar to what astronauts face in space. If you do it, you can't do much physical stuff. You can watch tv, read or use computers.
Read more at the source.