Characters who do not get a full night’s sleep may suffer the effects of fatigue. If a PC does not get at least 6 hours of sleep, she must make a DC 15 Fortitude save or be fatigued and take a -1 penalty on all other checks and saving throws against sleep effects. A second night without sleep requires another DC 15 Fortitude save. A failed save results in the character becoming exhausted and the penalties increasing to -2. A third failed save on the next night increases the penalties to -3.
Section 15 here.