Personality, time of day, and day of the week were assessed as predictors of state fatigue. After completing an in-lab questionnaire, 172 participants (N = 172) reported their state subjective fatigue three times a day for 8 days. Trait neuroticism, conscientiousness, positive affect, and negative affect were predictive of aggregate state subjective fatigue at different points in the day and over the course of the study. Results indicated mean differences in subjective fatigue at different points in the day and week. Personality traits displayed incremental validity over time and day in predicting subjective fatigue states. Multilevel analyses demonstrated that personality traits have an impact on both between individual and within individual sources of state fatigue variance. The relative contribution of personality traits to state subjective fatigue is discussed.