way back when, people used to drink alcohol from lead goblets ...this lead to led poisoning and resulted in people passing out for days, those people would often be mistaken for dead, so to help prevent false readings, they held a "WAKE" where the body would lie out for a few days to see if the poor guy would wake. If they did not, the funeral went on as normal and they were buried. At this time sometimes bodies would be dug up and scratch marks would be found on casket roofs revealing people that had drank the alcohol+lead concoction and been buried alive. So to help prevent this, after a person was buried, they attached a bell to a string from tombstone to casket, so if some poor guy was in fact, buried alive, they could pull the string and alert the caretaker. This was called a "DEAD RINGER." The caretaker that was watching for DEAD RINGERs was working the first ever "GRAVEYARD SHIFT." ...funny how phrases are created :)