Dont people read the readme of mods they download and install
For the millionth time posting this.....this is not part of the stock game...this is part of Beerys Real Uboat Mod(RUB) you must have installed or the SH3 Commander program that you installed.
From the RUB readme file
Fatigue: the new fatigue is based on a combat stress model. It is no longer fatigue. The feature is meant to reward careful crew assignments while removing the burden of micromanagement that the original game placed on the player. In this mod the torpedo compartments and the repair area are the most stress-inducing areas of the boat, so try to keep crewmen out of these areas until they are required to work there. Also, try to limit placing watch crews on duty during bad weather. It's best to submerge when the weather is bad. Always make sure not to spend too much time at under 32x time compression.
Combat stress and crew management: When crewmen become fully stressed they will work at lower capacity. In the first couple of missions (until you train specialists) this can make it difficult to run the boat. In the short-term, place NCOs and officers in compartments which are having trouble. Between patrols, give the necessary specialist badges to NCOs and officers so that they can work the worst-affected compartments more efficiently.
Sleeping areas and fatigue: Fatigue (now combat stress) will no longer heal during time spent in the sleeping areas, but these areas will prevent crewmen from becoming more stressed. Fill these compartments with all non-essential personnel in order to keep them rested for duty.
Fatigue Marker: the fatigue marker's shape and colour has been changed. This is to remind players that high fatigue is not a 'red alert' situation. Crewmen can and will do their job even when highly stressed, but their efficiency will drop considerably. A stressed torpedo crew often takes 24 minutes to load a tube, while a fresh crew takes only 12 minutes.