I knew exactly what was happening when I had it. What is actualy happening is your body is awake and your mind asleep. When your in REM sleep your body secrets a hormone which paralyses your body, to stop you acting out your dreams in REM. If you wake partly from this state you will find you cant move because of this. Because you are still asleep if you let yourself get carried away it can turn into a nightmare. I paniced a tiny bit, and this caused my mind to go into overload. I felt my mind racing madly, and this was after I struggled a bit. Then I realised, Ok if i struggle again its going to get fricking even worse than it is now. I also felt like something was standing by my bed, but I didnt pay attention to it because I knew I was just in a *****ed up type of sleep. I relaxed and felt my mind slowly slow down, and the adrenalin to stop...then I felt myself wake up a bit more and knew that I could move.