I have had it a few times in my life. The worst that it was is when I was working a full-time, third shift job and was going to school for biomedical sciences, so I was going straight to my university classes at 8am and would be in class until 3pm. Then I'd sleep, and have to return to work at 11pm. And do homework in there somewhere, of course. My sleep was so fragmented, and I was so tired, that my brain never knew if I was coming or going, and never fully shut down. That's my uber scientific explanation of it.

But essentially, sleep paralysis is your brain not transitioning as smoothly through the stages of sleep as you're supposed to. If you Google "sleep paralysis," there is some pretty good info out there.

I was told to start paying more attention to sleep hygiene and to relaxation in my waking life, including my caffeine intake. That usually solves the problem for me, but I know that isn't the solution for everyone.