There have been times when I've been coming down sick without having any real symptoms yet. Even if you can't pinpoint down why you are "fatigued" or not up to par to fly, you shouldn't fly. That is the flight when something will go wrong and your reaction time will suffer for it. No questions asked... if you're fatigued, you're fatigued. We don't even have subtle punishment for fatigue calls at my airline and I am even guilty of flying fatigue before because its hard enough to recognize it when your ability to assess yourself is already impaired.