Everyone has sleep paralysis every time they dream. It’s a mechanism that stops you acting out your dreams. What happens occasionally is that you come out of the dream state enough to become aware of being paralysed. You’re not awake, so your unconscious mind is grappling with the horror of paralysis.

My own experiences were nightmares where I was being threatened by an unseen figure, but couldn’t move to escape. I had a lot of them, some really horrible. Then I read an article with the above explanation, and I haven’t had one since. It was like once my unconscious knew what was going on, it stopped freaking out.