My great grandmother, who was 83 or 84 at the time, had had a brain tumor (which was removed once it got to between golf ball and baseball size) and was pretty far along into Alzheimer's as she got got close to her death.

For the last several months, she wasn't able to feed or clothe herself, and she was basically immobile.

The day she died, she got up, went to her room, made her bed (unthinkable in the state she was in), put on her "Sunday's best" and laid down peacefully in the middle of her bed and passed.

Her daughter (my grandmother), was floored. She just said she must have just known it was her time, and that she had a few minutes of lucidity before dying.

Whether there's any truth to that I'm not qualified to answer, but if you had seen the state she was in for quite some time prior to that day, her actions would certainly have been surprising to say the least.

Terminal lucidity as well as activity may happen when the brain/body realizes it is really losing it, and mounting any and all last reserves of energy, instead of conserving energy for a long healing process. So it's consistent with compromised physiology, when activation can overcome whatever is otherwise stupefying or immobilizing.

Something similar happens in some recoveries which then fade. People will come out of a coma briefly and seem fine, and then go back, never to return.

Very difficult to study.

Also this is quite distinct from what you might have meant: for elderly who have accepted their death, they may experience peace with anything in their experience.