I've just had a quick look at the GWX manual orangehat44.
There is a section that deals with fatigue.
Pages 58-64.
Got some useful info.
In fact, as you are new to GWX I would recommend reading the manual anyway.
Not only is it packed with useful stuff, it is also one of the most well written and interesting game manuals I have ever read
EDIT: Also just found this on page 47
The GWX team suggests you set the following SH3 Commander options to get the most enjoyment while running with GWX:
. . .
• Do not use the ―Grey Wolves 24 hour fatigue model