Save by the bell

I often wonder about how saying came to be my mother had a book about how certain word came to beeing it also had the origns of a couple of sayings as well.

People who do shift work will know the graveyard shift but it has spooky origins and links with save by the bell. In Europe graveyard were getting kinda crowded so they had to exhume people to make room but some of them had signs that people were buried alived which i can imagine is kinda bad so to solve this problem when a person was buried thay had a bell to ring so they could be dug up but not many people tend to visit graveyard at night so it a perosn job to stay there at night and listen for the bell indicating a person was buried they were working the "graveyard shift" and lucky person who was buried alive was "saved by the bell"