Port and starboard watches in the Age of Sail

Why did Age of Sail ships use two watches rather than three?

Two crew divisions suited the organisation and work rhythm, with several daily watch periods and the dog watches rotating their hours.

Limited timekeeping facilities and the uneven rhythm of work aboard sailing ships help explain the preference for two crew divisions. A three-watch system would require more precise timing and organisation in this explanation. Port and starboard watches did not each work one continuous twelve-hour shift. The day contained five four-hour watches and two two-hour dog watches, with first watch running from 20:00 to midnight. Alternating through that odd number changed which division took the first watch on successive nights. The arrangement left no uninterrupted sleep period longer than four hours.