I've had pretty severe sleep phase disorder and got it when I was really spent, about once a month.

For me at least it didn't really look like a demon, it's more of a feeling. That and exploding head syndrome is pretty awful. Didn't get that too often.

Nowadays I'm on low dosage anti psychotics and I sleep when I want. I'm still not entirely caught up on missed sleep but my exploding and paralytic friends leave me alone, fortunately.