It would seem that information on sleep paralysis is not as readibly available as it should be (present company excluded), and I have actually met people who don't believe that this can happen. I experience sleep paralysis regularly myself, both in wonderous and terrifying ways.

But an interesting fact is that many stories of people who allegedly have been abducted by aliens actually are surprisingly similar to descriptions of experiences with sleep paralysis. I will here relate, in short terms, an example of an abductee's experience.
Judge for yourself if it is similar, or if I am just paranoid:

"I woke up in my bedroom. The room was completely still, and there was an odd 'crispness' to the air. All I could hear was a strange screeching noise that seemed to occupy everything.
I tried to move, but my entire body was completely paralysed. Suddenly, two figures appeared in the room, one beside me, and one at the foot of my bed. Both appeared to be looking down on me. Their blank faces and featureless eyes gave nothing away, but I could feel them staring all the same.
I felt myself floating from the bed, first upwards, and then backwards through my bedroom window. Here I was enveloped in a piercing white light..."

I can only speak for myself, of course, but to me that sounds suspiciously like a sleep paralysis with hallucinations. I find this particularly interesting since one of the of the major arguments for the validity of alien abductions is that most of the stories are so alike.

"How can so many people imagine the same thing?"

Just a thought.