That used to happen to me a lot when I was a 12-14 yrs old. scared the crap out of me! Usually it would happen when I was just sleeping on the couch.

I think its from being disturbed during REM?

First thing the body does when you sleep is shut down unnecessary muscle movement. Like your arms and legs. The heart, brain, and respiratory system still works. So when you get startled or awaken suddenly it takes time for the signal to travel to your brain and let the muscles wake up.

I read that it happens often when you are stressed. So take time to relax before sleeping.