`:I believe that in practice, if the traditional system was used, sailors on watch but not actively working were allowed to nap on deck. As you suggest, the replacement watch has to be present and ready before the start of their watch, so they never get the nominal full time off of the schedule. I do not have a reference, but I remember reading that the US Navy conducted controlled experiments where volunteers tried a 4 on 4 off schedule and found they could not keep it up for more than a few days. I know this is true from practical experience though, having tried it once. Naive people (as we were) occasionally try this plan, but I think it is very dangerous, because ``loss of effectiveness`` at sea can easily get someone killed. Eric Newby described watch standing on the Moshulu on his 1938 trip to Australia. On this ship there were normally 3 working watch positions, that had to be awake, the runner, the lookout and the helmsman. These jobs were rotated among watch standers hourly, and it seems the rest of the watch (6 sailors) was often able to nap, called by the runner when they were needed. There were two officers on each watch, so one, I assume, could nap when not needed. In addition, they followed a ``Swedish`` watch schedule of 4,4,5,6,5, so each watch had 6 hours off every other day. I have followed this schedule a number of times and found it tolerable, as long as I made sure I slept during that (almost) 6 hours off. None of this is a problem when there are 3 watches, which is a much better plan.        `