I suffer occasionally from night terrors and sleep paralysis. It started when I was young and has recurred with decreasing regularity (thank god) ever since. When it happens I awake in the night paralysed with a real sense of a disturbing presence in the room. I have had the sensation of being squashed and even pulled out of bed and across the floor. It used to frighten me a lot and now it just frightens me a bit. Many cultures believe that night paralysis is an indication of a demonic visitation, in this country it is a condition attributed to stress (ironically few things in life are as stressful as a demonic visitation). Of course it is possible to die during sleep and I once read that Nightmare On Elm Street was based around the condition. Terribly comforting.