This shit is legitimately terrifying. I've had this twice in my life. The second time was better but you're still trapped. I tried asking for help but apparently never said anything. You're completely conscious, but can't even open your eyelids, you can however roll your eyes around. When you finally do complete the REM cycle and wake up, you go from perfect clarity detached from your physical body to normal groggy tired instantly as you slowly regain function. You feel like your willpower is starting to make your limbs move and slowly you can manifest action in your muscles. After a few minutes you can actually move enough to climb out of bed. Personally I can't get back to sleep right away.
I've I controlled a bit better for light and sound in the bedroom it's not happened again. Something to do with autonomic or instinctive responses at the wrong transition time in light sleeping, IDK.