Many years ago, when wood was in short supply it was a common practice to dig up and reuse coffins.
Quite often the grave diggers would see scratch marks on the underside of the coffin lid, left there by someone who was buried but still alive!
This happened so often, in fact, that all newly buried people had a length of string tied to their finger, leading all the way aboveground where it was tied to a bell.
When someone buried alive would move their hand trying to desperately claw their way out, the bell would ring aboveground.
Those people were called "Dead Ringers".
The men hired to listen for the bells during the night were known to be working the "graveyard shift".