I've had sleep paralysis most my life, especially when I was younger. If anyone knows what it is, you wake up some time in your sleep cycle, you wake up but your body is paralyzed and is usually accompanied by a strange sensation of not being able to breath or floating. Your eyes can be wide open and looking at your surroundings but at the same time you're experiencing audio and visual hallucinations, often times the "presence" of someone else close by is imminent.
When I was younger, about 6-10 years old I would get this all the time, and see ghosts, demon like figures holding me down, even aliens. It scared the shit out of me, my parents thought I was insane. I still get it to this day, but instead of the hallucinations being ghosts and demons, it's usually someone has broken in to my house. And I usually only get the audio hallucinations now.
A sleep paralysis episode only lasts about 30 seconds max, but it's very frightening as the hallucinations seem very coherent and lucid which causes people to confuse their episodes with reality. That and the fact that you wake up in a paralyzed state, unable to move any part of your body but your eyes. But after I learned what it was it didn't phase me as much. I can make the paralyzing short lived now by focusing on moving my right finger and then getting the feeling to come back to other parts of my body sooner. I'm used to it now and am able to chill out and stop the hallucinations as well, why? Because sleep paralysis is purely psychological. It's a disorder where your brain will wake up when your body is still paralyzed and in it's sleep/dream mode. In reality it's like a harmless type of seizure.
The theme of my SP episodes changed as I got older from ghosts and demons visiting me to more realistic things like someone breaking in my house. Proving that the themes of my episodes were based purely on my perception, my brain was merely trying to make sense and put together an explanation of what was going on.
Sleep paralysis is actually fairly common, look it up. Anyways, point being people perceive Ghosts/Demons/Spirits for many different reasons. Often times when people experience a loss of someone or something they will see/experience "psychological ghosts", i.e. feeling the "presence" of that person or thing that was lost, flitting images of the person or object in their peripheral vision etc.
Example... My dog died a couple years ago after having him for 13 years, anytime I visited parent's house I would experience these "psychological ghosts". Why? Psychologically, that environment triggers memories of him and his presence... my mind associates that environment with my dog, so I subconsciously anticipate seeing my dog there. It can happen with anything, even objects that are associated with certain environments.
Anyways, I believe in ghosts, demons and spirit in a metaphorical sense. "Ghosts" being memories or the influence a deceased person had, "demons" being evil self-destructive habits/tendencies or mental disorders, "spirit" being your emotional state or well-being. I think it's obvious that these words are also an old way of explaining what was once unexplainable.
I often study various different religions and strive to learn from an objective point of view. For instance when Siddartha (Buddha) refers to "Mara the tempter" in the Dhammapada, I don't believe he is literally referring to an evil entity or demon whom tempts humans to do evil, but rather it's a reference to that primitive selfish thinking in the brain which can be destructive (lust, greed etc.). It's a "you say tah-mae-toe I say tah-mah-toe" kind of thing, using different names and words to explain the same thing... This can be applied to all different types of deities, entities, demons, gods and figures in various religions.... but some people have sadly gotten carried away (and delusional) with the literal sense of these names and figures.