Actually, I don't think that things like sleep paralysis, deja vu, hallucinations etc are paranormal or supernatural. They have very valid scientific explanations and theories for these states of mind.
During sleep paralysis, a person will feel awake but without the ability to move or speak. Other symptoms also include feeling suffocated, hearing noises, and seeing things around them. And that leads people to believe in experiencing a supernatural phenomena, when in reality, it is merely a mistransmission of brain signals. When we sleep, the brain sends signals that inhibit any muscle contraction. If we come back into consciousness before our body has received the signal to activate muscle contractions, we experience that state of 'paralysis'.
During the night, our body alternates between non-REM and REM cycles. 80 minutes of NREM slepe and 10 minutes of REM sleep make up one 90-minute sleep cycle. We go through this cycle repeatedly all night. During our non-REM cycle, they body has the ability to move, allowing us to toss and turn in our sleep. This is also the part of the cycle where sleepwalking and talking in one's sleep occurs. The REM (rapid-eye-movement) part of the cycle is where dream, and we can watch a dream occur in our sleep. But in this part of the cycle, the brain sends signals inhibiting any muscle contractions, and preventing us from mobility perhaps so we don't actually act out the dream. If something goes wrong and those neurotransmitters that inhibit muscle contractions are continuously released by the brain even when the person is awake, the person experiences sleep paralysis.
During sleep paralysis, auditory and visual hallucinations can also occur. I am not very informative on this particular cause, but I think it has something to do with the fact that there is way too much activity occurring in the temporal and occipital (auditory and visual) lobes of the brain that causes us to see/hear things when we are awake, when in fact, that activity should be reserved for dreams.
I haven't really studied the exact mechanics of deja vu, but I do know that there are a lot of reliable scientific theories out there explaining the phenomena. Hallucinations and delusions occur when the mind is unable to distinguish between the conscious and the subconscious. We usually make consciousness the 'awake' state, and the subconscious the sleeping mind. Reality then becomes more commonly associated with the conscious mind. However, in a mind that is undergoing hallucinations, the transition between the conscious and subconscious do not occur smoothly, and the thoughts of subconscious continue even in a state of consciousness, leading to people believing that this is in fact reality.
I just made a light-hearted topic into a scientific one, sorry! For kicks, I'm sure you guys have all heard stories of spirits taking over bodies ... people acting weird and unnatural, completely unlike themselves, and then coming out of these episodes without any memory of what has occurred. My dad also tells us a story of a girl that lived in his neighborhood when he was young who would be taken over by a spirit, she would get angry, and behave completely unlike herself. In fact, her voice would change into a man's, somehow indicating a male spirit that had entered her body. As a child, I would get scared of these stories, but now I just roll my eyes. I'm sure my dad is not lying about this episode, but there are scientific explanations to explain these things. In fact, did you know, that is Dissociative Identity Disorder, where people display multiple identities without being conscious aware of these identities, sometimes also experience a change in their voices, which clearly explains this phenomena of spirits taking over.
I certainly did not intend to write this much. Apologies to those who bothered reading it. ;-)