General Info:
In Europe, back in the day, they had such a difficult time determining when people were actually dead that they would often accidentally bury them alive (in a comatose state). They didn't realize their error until they started exhuming bodies in order to move them to different burial grounds, and during the process they noticed many of the inside coffin lids had claw marks. They soon realized that the "dead" were actually being buried alive and waking up in the coffin underground and trying to claw their way out (really fucked up way to die). Consequently, they began to places bells next to each grave, and run a string underground and inside the coffin. So, if the "dead" should awake, they could tug on the string and summon help. The watchmen who listened for bells ringing during the night were said to be working the "GRAVEYARD SHIFT." And, that's why, today, we describe people working at night as working a graveyard shift. (Just dropping knowledge where I can)