You are capable of amazing feats of stamina.
Benefit: You gain a +4 bonus on the following checks and saves: Swim checks made to resist non-lethal damage, Constitution checks made to continue running, Constitution checks made to avoid non-lethal damage from a forced march, Constitution checks made to hold your breath, Constitution checks made to avoid non-lethal damage from starvation or thirst, Fortitude saves made to avoid non-lethal damage from hot or cold environments, and Fortitude saves made to resist damage from suffocation. Also, you may sleep in light or medium armour without becoming fatigued.
Normal: A character without this feat who sleeps in medium or heavier armour is automatically fatigued the next day.
Special: A Ranger automatically gains Endurance as a bonus feat at 3rd level. He need not select it.
Source: Player’s Handbook (Page 93)