I mean the definition of death is when the light fades right? Everything else is just an approximation. Would be wild if true.

It would be wild because it would be wrong. We acknowledge someone who has suffered brain death to be truly dead, even if their body can be kept alive (and presumably shining!) for weeks and weeks through modern medicine. If this light is a side effect of biochemical processes then presumably someone who is decapitated will also continue to shine for minutes.

The fading of the light would be sufficient but not necessary for someone to be considered dead, so it would make a poor definition.

Sleeping probably also fades light.