I'm not sure to what extent it's related to my bipolar episodes, partially because I haven't experienced it that much since my bipolar diagnosis, and I wasn't paying close attention before that. I think I've noticed an increase in this sort of thing during hypomanic or mixed episodes. I don't know if there's anything neurological going on, but I do experience some other parasomnias as well, such as "exploding head syndrome."

To my understanding sleep paralysis is caused by REM cycles that continue while you start to wake up. During REM sleep the body is normally prevented from moving. With sleep paralysis, you start to wake up but you're still dreaming and your body is "locked" and can't move at all. It can be pretty terrifying and it's thought to be the explanation behind things like alien abduction stories.

I just experienced a brief episode of sleep paralysis again for the first time in a while, and I also just came out of hypomania, so maybe there's a relation there. I know that both parasomnias and manias can be triggered by stress and lack of sleep, so maybe there's only a correlation between the two and not a causal effect. On the other hand, sleep paralysis apparently has some genetic basis, so perhaps some of these genes overlap with the bipolar genes? I don't know but it's interesting to hear about other people's experiences.