In general, we operated on a 24-hour day on the submarine, even in the absence of external reasons to do so. We stood watch for four hours, and were off for eight hours, around the clock, while at sea. In between we did all our administrative work, and we lived our lives. The 72 enlisted men all maintained this schedule, whether we were surfaced or submerged.
The officers kept this regular schedule while submerged. However, while surfaced, we only stood watch as often as necessary. This led to some "days" of unusual lengths. Usually there were five of us who were qualified to stand watch as Officer of the Deck, Surfaced. That led to 20-hour days, with four hours on watch and 16 hours off watch. Sometimes there were only four of us available, so we lived 16-hour days, with four hours on watch and 12 hours off watch. Only once did we have six of us, so that we could live a 24-hour day on the surface.
Once, however, we had an extraordinary experience, in which there were seven of us available to stand the watch on the bridge. We had surfaced in the Norwegian Sea, and we ran on the surface down through the North Sea and the English Channel, to the Canary Islands, thence westward to Charleston, South Carolina. This non-stop trip required almost three weeks to complete. For this length of time, we lived a 28-hour day.
Based on this experience, I would say that a 28-hour day is best. When we had the 16-hour days, I always tried to make them into 32-hour days. I could do it, but it was hard work. A 32- hour day is not as good as a 24-hour day. However, the 28-hour day was clearly superior to the 24-hour day, in my opinion. A couple of experiments that have been conducted in French caves seem to validate my personal observations.
Sometimes I have wondered how our rhythms developed a 28-hour cycle. Surely the length of the day hasn't changed that much that quickly.