Oldflame's post got me thinking about it. I've never asked other people about how common it is, but it's pretty scary.
Basically your body is asleep and paralyzed, but your mind is awake. Sometimes you get a feeling of dread, othertimes of euphoria. Sometimes you hallucinate, and there are people or horrible creatures in the bedroom with you, and you can have conversations with them. I almost always get a loud buzzing sensation in my ears/neck that occurs right before my mind "wakes" up with me paralyzed.
From Wikipedia: Many perceptions associated with sleep paralysis (visceral buzzing, loud sounds, excited mental state, [ 22 ] presences, and the paralysis itself) also constitute a common phase in the early progression of episodes referred to as out of body experiences. [ 23 ] Mental focus varies between the two conditions; paralysis sufferers tend to fixate on reestablishing operation of the body, whereas subjects of out-of-body episodes are more occupied by perceived non-equivalence with the body.
I am one of the ones that fixated on reestablishing control of my body, even when I had terrible visions of supernatural creatures, and early on I found out even though I couldn't move myself, I could control my breathing. So I started breathing as fast I could, and within a couple of minutes, my body woke up from the increased oxygen.
Anybody else ever suffer from this?