The watch keeping system on US Navy systems is totally different from the system on merchant ships, anyway in my time. We had three officers who were responsible for the watches. The chief or first officer had the 0400-0800 and 1600-2000 watch. After getting of watch at 0800 hours he was busy for may be half an hour of instructing the boatswain and sailors, who started their work day at that time, about the days work which consisted mainly of rust and paint removal and repainting. We called it therapeutical, perpetual exercises that kept them out of harms way.
The second officer had the 0000-0400 and 1200-1600 watches. He sometimes had extra overtime duties in correcting seacharts for which he was responsible.
And finally the third officer who did the 0800-1200 and 2000-2400 hours watch. He was the youngest and required some extra attention by the captain who was awake during his watch keeping hours. He had to spend some time on administration, only a couple of times per week.
With this watch keeping system there was especially at sea no fatigue involved. The only time that this came in question was on mostly European trips going from one port to the other in short period of times. Apart from sometimes stressful navigation, making fore and aft when mooring and unmooring, there was the always the hectic cargo handling business which required utmost attention.