Endurance (D20 Modern Feat)
From D&D Wiki
You are capable of amazing feats of stamina.
Benefit: The character gains a +4 bonus on the following checks and saves: hourly Swim checks to avoid becoming fatigued, Constitution checks to continue running and to hold your breath, Constitution checks to avoid damage from starvation or thirst, Fortitude saves to avoid damage from extreme environments, and saves to resist suffocation or drowning. Also, the character may sleep in medium or light armor without becoming fatigued.
Normal: A character without this feat who sleeps in armor is automatically fatigued the following day.