Why are women more fatigued than men? The roles of stress, sleep, and repetitive negative thinking.

Fatigue, a state of prolonged tiredness that cannot be alleviated through rest, is twice as likely to affect women than men. To account for women's higher rates of fatigue, we examined three factors which have both exhibited consistent sex differences in the literature and have been linked to fatigue: stress, sleep, and repetitive negative thinking (RNT). In the first study, 193 undergraduate students reported their levels of fatigue, stress, sleep quality and RNT over the past month. Mediation analysis showed that the effect of sex on fatigue was partially mediated through stress, sleep, and the impact of stress on sleep, but not RNT. In the second study, 205 undergraduate students underwent the same procedures as in Study 1 to test the whether the findings from the first study were replicable in an independent sample. These students were then sent seven follow-ups across 2 weeks to measure their momentary fatigue, daily stress, and previous night's sleep quality. In the second mediation analysis, sleep did not mediate the impact of sex on fatigue; however, all other pathways found in the first study were replicated. With regard to the prospective measurements, women reported greater fatigue, even when accounting for sleep and stress, and poorer sleep and higher stress predicted fatigue both at population and individual levels. Taken together, these findings suggest that sex differences in fatigue could be in part driven by sex differences in stress.