Again, viability does not define human life. Viability refers to chance of survival.
And humans with a 0% chance of survival due to lack of higher brain function are both medically and legally dead.
The start of human life begins at the formation of a zygote.
A zygote is no more a human life than a skin cell.
Do we consider living to be sentient or conscious? If that were the case, that means we are are no longer alive while sleeping?
You don't need to be conscious, but you need to have the capacity for consciousness. Adults without the capacity for consciousness are declared dead. Fetuses without the capacity for consciousness are not yet a life.