Sleep paralysis can be scary. I had a few times about a year ago. Psychologists think it's the part of the brain that restricts voluntary muscle
movement during sleep (preventing you breaking your arm when you thrash about running from the Rottweiler/skunk ape/Republican). It basically stays on
a few seconds after you wake up, but it kicks in... there are some diseases where you experience this for the rest of your life...
Then, of course, there's the whole alien abduction thing.