Mummelmann on 27 April 2008
One of my closest friends experiences this from time to time, it is apparently very uncommon and seems to occur in less than 5 % of the population.
Some refer to it as "Paralyptic Nerve Resonance" (freely translated from Norwegian mind you) and it is considered a sleeping disorder as far as I know (at least here in Norway).
Its very creepy and unnerving and is believed to be the culprit in the majority of "alien abuction" cases and similar events.
Some suffer greatly at the hands of this disorder and carry trauma from it throughout most of their lives, describing it as "a total state of helplessness".
Perhaps you should see your GP Okey? There are methods of dealing with it through medication or hypnosis as I understand it! 
