Health

As you probably know, your mind has a sort of switch that flips off activity in your body when you sleep.  This state of paralysis is what keeps you from actually running in your bed when you are running in your dreams. 

It isn't a complete paralysis - people will still twitch and mutter, and some may even kick around and cause their bed partners a great deal of consternation.  But it works well enough for what it needs.

This paralysis is associated with REM sleep, and is called "REM atonia."  It is regulated by a hormone, which usually wears off after REM sleep passes.  However, sometimes this switch gets stuck for a little while after you wake up.  If you don't know what's happening, this can be a terrifying experience.