Meaning
A late-night/early-morning work shift.

Origin
I am loath to do it, as there may just be someone who will take the following passage as literal truth, but here's a reprint of the last (and quite possibly the least) paragraph of the collection of invented and untrue twaddle that has been circulating on the Internet for some time, under the name of 'Life in the 1500s':

England is old and small and they started running out of places to bury people. So they would dig up coffins and would take the bones to a "bone-house" and reuse the grave. When reopening these coffins, one out of 25 coffins were found to have scratch marks on the inside and they realized they had been burying people alive. So they thought they would tie a string on the wrist of the corpse, lead it through the coffin and up through the ground and tie it to a bell. Someone would have to sit out in the graveyard all night (the "graveyard shift") to listen for the bell; thus, someone could be "saved by the bell" or was considered a "dead ringer."

From: http://www.phrases.org.uk