Where does the term “Dogwatch” come from and What does Dogwatch mean?

A dogwatch is a two-hour vigil on shipboard, so arranged as to alternate from day to day the regular four-hour watches of the divisions of a crew.

That is, instead of dividing the twenty-four hours of the day into six periods of four hours each, they are divided into five such periods and two of two hours each, one from four to six and the other from six to eight.

The name derives from the fact that each of such crew has only a short period of rest before being aroused into alertness, comparable to the alertness expected of a watchdog.