I don't think I need to tell you, Brethren, that ships don't run themselves. Being the original "don't drink and drive" vehicle they had to be watched constantly (and soberly) during the age of wooden ships and iron men. And watching the ship gave us the term "watch" and being on watch meant that the ship was at no time left to her own devises.
Watches, originally adopted of course by the great navies, were - and still are - common to all ships. Even the most undisciplined freebooter afloat had a watch system of some kind. There's no pay in running aground or wrecking on a reef regardless of your chosen profession. The general watch system was simple and easy to follow, and it ensured that no man had the same watch two days at a time. This was helpful given that sleep could only be got in a maximum of four hour increments for the average, healthy sailor. Here's how it worked.
A ships company was divided in half, each half being a "watch" (on very large men-of-war and ships of the line which could carry upwards of 800 men, the company might be divided into thirds). They were named for the sides of the ship with one being the larboard (now port) watch and the other being the starboard watch. A man identified with his watch and there was often a friendly rivalry between them. Each group stepped in to mind the ship alternately during the periods - also called watches - that divided the day. They were as follows:
Middle, or Graveyard, watch: midnight to 4:00 AM
Morning watch: 4:00 to 8:00 AM
Forenoon watch: 8:00 AM to noon
Afternoon watch: noon to 4:00 PM
First dog watch: 4:00 to 6:00 PM
Second dog watch: 6:00 to 8:00 PM
First watch: 8:00 PM to midnight
As you can see, the "dog watches" were each two hours long and they were the mechanism by which no man had to serve the same watches two days in a row. Obviously it was a grueling schedule but doubtless once the daily routine set in no one thought much about it.
Virtually everyone aboard ship was subject to the watches which were kept on track by careful timekeeping (with a half hour glass) and a system of bells (which is another topic all together). While the Captain was entitled to take watches as he saw fit, and non-service officers like surgeons and chaplains did not participate in the system, every other healthy soul aboard - even boys as young as seven - were subject to their watch.
Makes you appreciate an eight hour day and a good night's sleep a little more, doesn't it mate? Me too!