Although African Americans (AAs) are less likely to get breast cancer than European Americans (EAs), they get more aggressive forms. We previously showed that short sleep is associated with higher tumor grade. It is well documented that AAs get less sleep, on average, than EAs. We studied the contribution of short sleep to racial disparities in breast cancer aggressiveness among 809 invasive breast cancer patients who responded to a survey on their lifestyle. Multivariable regressions and mediation analyses were performed to assess the effect of sleep duration on the association of race with tumor grade. AAs reported shorter average sleep (mean [standard deviation] 6.57 [1.47] h) than EAs (mean [standard deviation] 7.11 [1.16] h; P<0.0001) and were almost twice as likely to report less than 6 h of sleep per night (48.0% vs. 25.3%, P<0.0001). AA patients were more likely to have high-grade tumors (52.6% vs. 28.7% in EAs, P=0.0002). In multivariate analysis, race was associated with tumor grade (P<0.0001). On adjustment for sleep duration, the effect of race was reduced by 7.1%, but remained statistically significant (P=0.0006). However, the Sobel test did not indicate statistical significance (z=1.69, P=0.091). In other models accounting for these and additional confounders, we found similar results. Because of the conservative nature of the mediation analysis and smaller sample size, replication of our results in larger studies with more AA patients is warranted.