This is a link to a slideshow illustrating the physical and mental issues from being in space for long periods. Among the most interesting facts, astronauts commonly have issues with gravity when returning. They get so used to the lack of gravity that upon their return, they commonly drop things only to forget that objects don’t just float there as they do in the space station. Sleep is also hard due to the lack of protection from cosmic rays. They travel through your head, causing zaps and bright flashes while trying to sleep.