"Men Fear Most What They Cannot See." sleep paralysis "Ghost Intruders" and faceless "Shadow-People"-The role of the right hemisphere and economizing nature of vision - PubMed
pubmed.ncbi.nlm.nih.gov
Sleep paralysis is a curious condition where the paralyzed person may hallucinate terrifying ghosts. These hypnogogic and hypnopompic visions are common worldwide. They often entail seeing and sensing shadow beings; although hallucinating full-fledged figures (e.g., cat-like creatures and witches) a …

I’ve always found sleep paralysis fascinating, having had it a few times myself it’s much less scary if you know what’s going on. I think there’s lots of good research out there on the mechanisms of the paralysis, but would love to see more work done on some of the even more bizarre features, like the tendency to hallucinate a shadowy figure in the room with you.