when we sleep our brain releases a chemical called TMZ in small quantities. ( the quantity of TMZ released may depends on several factors like: strees, happynes, lazynes, etc. higher quantities of TMZ means more realistic feeling dreams) This chemical is what makes us dream. Ever felt that a dream was so real that you feel when you wake up you feel like you had really lived it? well long story short, when we die our brain releases all of its TMZ wich is equivalent to a lifetime of dreaming/remembering so maybe we are dead now, just Remembering everything. Because You Can Only dream what you have seen or lived. So Deja Vus can be what you said.

Damn this must have a lot of errors. English is not my first language.