Sleep paralysis is a condition which occurs up to as many as 4 out of every 10 people according to some statistics. Sleep paralysis happens when one is about to fall asleep or while waking up during the transitional states between wakefulness and sleep. In the course of sleep paralysis one will experience temporary paralysis that makes it unable to move or speak. The paralysis may last usually only few minutes. In addition the individual might also perceive terrifying hallucinations that are made worse with the inability to move or speak.
Some things are just psychological, but there are interactions with petas (ghosts), yakhas, etc. where one can experience sleep paralysis, getting high pressure/ force applied to chest, body parts feeling like you're suffocating, a cold chill going down your spine preventing you from sleeping, or just outright possession of your body/mind for periods of time by other beings (much less common).
Most people have no experience with any of the above, but if you've spent enough time in serious forest monasteries or living alone in the wilderness, you and/or people around you are going to have those kinds of interactions. Read Ajahn Mun biography for many examples.
I myself was on a retreat near the himalayas with monks and nuns, and at least 10 of us were disturbed by yakkha(s). I didn't have sleep paralysis in that ocassion, and I'm not sure if the other monks and nuns did either. But we experienced the typical ghost/yakkha things of cold chills, getting pressed with great force into the bed, weird or terrifying visions and dreams.
The more interesting question IMO, is how do you prevent these kinds of problems from happening?
What I've found, is that the stronger your internal energy gets, not just your biological immune system gets stronger, but your psychic immune system against mental stress, ghost and yakkha interacitons also strengthens greatly.
The way to build up internal energy, is a proper combination of meditation and physical exercise, healthy eating, every day.
P.S. It's important to keep in mind not everything we have not personally experienced can be casually dismissed as hallucination.
Encouraging people to be kind to new (and old users) alike is important to establishing a good culture on Dhamma (and any) forums.
The reality you see is created by your mind. It's like a robot vacuum that builds a map of the room based on what it encounters. If you stand in its way it will think there's a wall there. It doesn't really know, it figures it out from what it picks up with its sensors.
Similarly, our reality is based on what our mind picks up. If you focus on negative things, you see hell. If you focus on competition you see the world of jealous demons. But the real world is infinite and includes all of these words, we just don't see all of them at the same time.
The way we maintain our focus on our favorite world, is by thinking about it all the time, talking about it, watching movies about it, and so on - feeding our mind with thoughts about a particular world. A world is a story the mind keeps telling itself.
But when we are falling asleep, we stop feeding our mind with our favorite stories, and for a brief moment of time it stops reproducing our favorite world, and picks up other stuff. And that's when you see other "layers of reality", for a lack of better word.
It's not bad per se, and not good, just a glimpse outside of our regular world. It's not real, but then our typical world is not strictly real either. It's not completely fake, it's just the robot vacuum of our mind trying to make sense of things beyond our normal focus, things it doesn't understand.
'Sleep paralysis' sounds like the manifestation of 'mental formations'. 'Mental formations' are emotions, images, volition & thoughts.
'Sleep paralysis' sounds like the mind is in a sleep dream state (the emotional creation of mental images & stories) however a part of the mind believes it is awake & attempts to exercise volition to escape from the bad dream created by the emotion of fear.
Buddhism explains there are five aggregates, namely, the physical body (rupa), feeling (vedana), perception (sanna), mental formations (sankhara) & consciousness (vinnana). Buddhism says all experience is a manifestation of these five aggregates.