I've heard of the brain still being alive around 5 minutes after death, before the cells start to really die.

In that moment of relative time before the brain dies, like Coelho mentioned, the possibility that our spiritual or dream-like state of mind during that time could seem to last forever and actually be the afterlife. That's a pretty cool common theory, but it makes me wonder about the people who die that have their heads smashed to bits and don't get to experience the slow process of what happens chemically and physically in the brain.