Who thought it was a sensible idea not to associate lack of sleep with Stamina? Being hungry/thirsty (apparently, they're interchangeable) would be better off affecting health and/or magicka. All I can assume is that as soon as they halted their years of calling the green bar "Fatigue" (which I usually modded to be named "Stamina" anyway), Bethesda promptly forgot that it had ever been fatigue in the first place.