Source: Player's Handbook
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 nonlethal damage (see Swim), Constitution checks made to continue running (see Run), Constitution checks made to avoid nonlethal damage from a forced march (see Forced March), Constitution checks made to hold your breath (see Swim), Constitution checks made to avoid non-lethal damage from starvation or thirst (see Starvation), Fortitude saves made to avoid nonlethal damage from hot or cold environments (see Environmental Dangers), and Fortitude saves made to resist damage from suffocation (see Suffocation). Also, you may sleep in light or medium armor without becoming fatigued.
Special: A ranger automatically gains Endurance as a bonus feat at 3rd level. He need not select it.
Normal: A character without this feat who sleeps in medium or heavier armor is automatically fatigued the next day.