I'm a pretty happy guy so I don't think that makes you immune to getting it. I only get it if I'm travelling, never at my home in Australia. And it not necessarily demons.
For me its always a skinny homeless looking man with long hair and a dark wet raincoat standing next to my bed. He never looks at me and stares towards something out of my field of view and I can't move or turn my head.
Weird thing is when it happens I'm always in my bed upside down. Head where my feet go and feet where my head go and I have no idea how I turn around, and why it only happens when I get sleep paralysis.