I think most of us here know of effects of general anesthetics from own experiences

Tibetan Book of the dead you bring as an example speaks directly that from the first pages that the awareness never fades, the afterlife challenges and the cycle of rebirth.

Ever been on a DMT trip so grand that you can't remember a nanosecond of it? That's because brain hardware doesn't have capacity to memorize certain high-vibrational dimensions (both negative and positive).