The demon thing is very common, it's known as 'sleep paralysis'
Basicly it's hallucinations that occur when a person is about to fall asleep or just wake up, and then realise that they can't yell or move at all.
Often people say they felt a presence of a demon or something evil with them the demon has been reported to attack and the victim can then barely breath.
It happens because a person will awake from REM sleep but there body will still be asleep (hence the paralysis) and they cannot move, also images and such will still be occuring from the REM sleep, so basicly it's like dreaming awake... I can only guess that people belive demons are near them because there paralysized body induces a sense of fear that is very extreme hence they assume that some sort of demon is attacking them.
I think thats what it is, I remember ready about it but thats about all I can remember.
Iv'e had this happen to me before and it scared the crap outta me