normally im not really much of a skeptic - kinda an eager believer, in fact. but i'm prettttty sure what you are experiencing is Sleep Paralysis...http://en.wikipedia.org/wiki/Sleep_paralysis
It is supposed to account for many of the reports of incubbi/succubi attacks in the past. Alien abductions, too. I've experienced it a shit load of times, and it's fucking scary...
If it doesnt fit that description, and you reallllly are pretty sure it's a demon or something... ummm... i'd suggest confronting it in a lucid dream and kicking the shit out of it.