Is This A Plausible Theory?

Tell me if you think this is a plausible theory. It's a scientifically proven fact that the brain stays active something like 20 minutes after all vital signs shut down. Is it possible to say this is a time when the minds activity is primarily on a subconscious level? In this dream-like subconscious state, a person may experience what they believe to be an afterlife of some sorts, be it heaven or whatever he or she believes in. Is it possible that people who have had near-death experiences and saw dead relatives or Jesus were experiencing a similar phenomenon?