I have been in real situations that legitimately threatened my life and safety, and sleep paralysis consistently gave me comparable levels of stress and fear in each episode.
Imagine having a bad nightmare, so bad that you wake up from it. But when you wake up, your eyes are open and you cannot move. You see things that your mind logically knows cannot actually be there, but they are real for you. The REM dream state continues, overlapping reality.
In one case I woke up one my side to a large, dimly lit figure on the side of the bed, staring at me with red eyes. It just stared at me, while I physically felt something at the bottom of the bed dragging my body off. I couldn't move or even shift my head.
I was aware that I was awake, that this wasn't a dream anymore, but that this sort of paranormal situation should't be happening. I couldn't move, couldn't struggle - then the thing staring at me unhinged its jaws wider than should be humanly possible and screamed at me at the top of its lungs. It sounded like a human mixed with the shrill cry of a velociraptor from Jurassic Park.
I just remember trying to yell but not making noise, and feeling like I was drowning. I knew I was awake but the supernatural sense of wrongness was almost more terrifying than the hallucination.
Then, after what felt like minutes of struggling to breath, and the figure almost eating me, it all vanished, and I shook my body so violently I threw myself off the bed (seriously). I think I actually ran to the lights, turned them on and sat shuddering on my bed in the blanket for a while.
It's no joke. I used to regularly have these, and I sympathize with anyone who has had them. They're primarily caused by stress and anxiety (and have a high incidence in people with PTSD). It's not like a nightmare where you can convince yourself it wasn't real, because the things I hallucinated were as real to me as the lamp and nightstand.
Obviously I should say I didn't consume anything the night before. It's almost debilitating, and caused me insomnia for quite a while.
It was hard to describe to people - they are not the same things as night terrors. I would talk to people about it and they would say it was a bad nightmare. But it felt so real, it was like that scene from the Matrix with Neo and the bug.
Afterwards, details were always fully lucid and clear, and never became fuzzy. I eventually realized that the level of obsession I would get into when trying to get people to understand that it wasn't just a nightmare mirrored the hysterical reactions you would see from characters in Nightmare on Elm Street or the Exorcist. I never really believed in things like alien abduction, but I can fully understand why repeated exposure to sleep paralysis would cause someone to seriously question reality.