Dog watch, in naval terminology, is a period of work duty between 1600hrs and 2000hrs. The period is usually split into two to allow the crew to have meals and exercise.
There are various stories about the origins of the term. It is said to be a direct translation from either German or Dutch, originally referring to the night watch on ships — a time when only dogs would be found awake. The name is also said to have been derived from Sirius, the "Dog Star", as it was claimed that Sirius was the first star that can be seen at night.