A.  Conventional wisdom: REM atonia is  a normal function of the body.
The muscles   that move the  body  are "turned  off" during REM sleep,
which prevents   you from acting   out  dreamed actions   in  reality.
Non-REM  sleep paralysis after  waking up  ("old hag") is  caused by a
failure   to   re-activate the  muscles   immediately.   Normally this
condition lasts only a few   seconds, but sometimes it   can go for  a
minute, which causes  a very scary  feeling.  You are damn sure you're
awake  now  but you  can't move. This  is  extremely unpleasant but at
least not dangerous.