Follow-Up:
Sleep paralysis is pretty common. The brain has to shut off voluntary muscle control while in REM sleep, otherwise the body would make all of the movements that you make in your dream. The semi-dream / semi-waking paralysis feeling probably occurs because the brain shuts down your voluntary muscle control a tiny bit early (in other words, you are about to enter REM sleep, and the brain says "ok, time to shut down body control"). But you haven't quite fully entered the sleep mode yet. This has happened to me many, many times. For some reason, as is the case with most of you, it is a scary feeling. But I really think the scariness of it is just because you are caught between states. Your body has shut down for the sleep period, but you aren't sleeping yet. I still try to yell out to my wife sometimes, to no avail. But I have also been able to relax, accept what is happening, and experience an incredible influx of energy. Try it if you can. Just try to relax - you will feel a tingling energy building up. Tell yourself that this is ok, and let it keep happening. The result is indescribable, but for certain it is a good thing. Also - this in-between state is a great sign to jump off into a lucid dream.