If you want to live a freaking nightmare... Try having sleep paralysis. It usually happens when you're having a lucid dream or just after you wake up or something... Your body can't move no matter how much you try and you can actually see horrifying hallucinations and there's a huge feeling of dread... Little is known about it but it's real, it happened to me once. 

 I didn't see any hallucinations (I didn't open my eyes, I was lucid dreaming and wanted to go back to sleep... grr.) but I couldn't move at all, it was like my muscles wouldn't respond and I felt trapped and hopeless... >.<