what u guys are describing, in the American version, are the demons succubus & incumbus. they are demons, of the opposite sex who seek to have "sex" with you, resulting in you having the feeling of something sitting on you as well as you being semi-conscious, trying to wake from the encounter.

in oriental cultures, they have a different name. but the results are the same, you are half awake, can't move, sometimes hear things and have the feeling an evil supernatural force is present w/you.