The Science of Sleep Paralysis

This clip discusses sleep paralysis from a scientific standpoint. It explains why and how it occurs, and why it is often associated with the idea that someone is being abducted, or that strange, shadowy figures are in the room with you.

Why is it biologically necessary for us to be paralyzed while in REM sleep? What are hypnopompic hallucinations and how are the sensations they produce explained by a carryover from NREM sleep?