Director Nobuaki Komoto revealed more about the fatigue system in Final Fantasy XIV, shedding some light on a feature that many players didn't fully understand.

To sum it up, you'll earn normal experience for eight hours, and after another seven hours you'll stop gaining experience completely. Only after a week the XP earning rate will be normal again.

The reason appointed for this by Komoto is to balance the game and not give an advantage to those players with more free time in their hands. Something that probably won't be well received by all players.