There is a Character Trait named Relentless that gives you some Constitution bonuses. As a drawback, it states that if something makes you Fatigued, you become Exhausted instead. The Tireless feat states that you're immune to Exhaustion, and something that if you get Exhausted you're Fatigued instead. That makes anyone immune to Fatigue Effects... or... it makes you mind explode (paradox). xD