The only reason they see these figures, I believe, is because of their fearful emotions. They were sleeping, and all of a sudden they are breathing shallow, their heartbeat is rapid, and they can't move! They have no clue what's going on, so naturally they feel scared. This fear literally manifests itself as the Old Hag hallucination. I think that just by knowing what sleep paralysis is, and the benefits it brings, people won't fear it anymore.