What are the origins of the term dog watch (ie a late night work shift)?

Dog watch is the name given to the 1600-1800 and the 1800-2000 watches aboard ship. The 1600-2000 4-hour watch was originally split to prevent men from always having to stand the same watches daily. As a result, sailors dodge the same daily routine, hence they are dodging the watch or standing the dodge watch. In its corrupted form, dodge became dog and procedure is referred to as “dogging the watch” or standing the “dog watch.”