One theory of mine is that when we have SP we are breifly visiting hell , purgatory or some kind of weird afterlife different alltogether from religious ideas of where our souls end up .
The "presence" could be the gaurdian of this afterlife , an omnipotent being whose duty is to frighten and confuse the souls that enter his/her/its domain .
When we die our souls supposedly go somewhere , maybe the SP limbo is a place where they accidentally end up (lost souls?) , and the SP experience of living people is a flaw in the atomic structure of the universe that allows the souls of the living to travel there upon sleeping (the confusion in the universal mechanics arising from the fact that the brain activity during sleep is reminiscent of death) . But the souls cannot stay there , much as the Evil Presence would like us to , because they have a kind of ball and chain link to their living bodies and are pulled back eventually .
Note this is just a theory , I have given it a lot of thought , and I have others . The reason being , although I am willing to accept rational explanations for anything , the most rational explanation isn't always the most likely or satisfactory conclusion . I feel the scientific theory of SP has a certain something lacking , it's too simple and it leaves many questions unanswered .