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Purpose: Short and long sleep duration and poor sleep quality are risk factors for weight gain and cancer mortality. The purpose of this study is to investigate the relationship between sleep and weight change among postmenopausal breast cancer survivors. Methods: Women participating in the Women’s Health Initiative who were diagnosed with incident breast cancer between year 1 and year 3 were included. Self-reported sleep duration was categorized as ≤5 hours (short), 6 hours, 7-8 hours (optimal), and ≥9 hours (long). Self-reported sleep quality was categorized as poor, average, and above average. Post-diagnosis weight change was the difference of weight closest to, but preceding diagnosis, and year 3 weight. We used linear regression to evaluate sleep duration and sleep quality associations with post-diagnosis weight change adjusted for potential confounders. Results: Among 1,156 participants, 63% were weight stable after diagnosis; average weight gain post cancer diagnosis was 3.2 kg. Six percent of women reported sleeping ≤5 hours, 26% reported 6 hours, 64% reported 7-8 hours, and 4% reported ≥9 hours. There were no differences in adjusted estimates of weight change among participants with short duration (0.37kg; 95%CI -0.88, 1.63), or long duration (-0.56kg; 95% CI -2.03, 0.90) compared to optimal duration, nor was there a difference among poor quality (-0.51kg; 95% CI -1.42, 0.41) compared to above average quality. Conclusion: Among postmenopausal breast cancer survivors, sleep duration and quality were not associated with weight change after breast cancer diagnosis. Future studies should consider capturing change in adiposity and to expand beyond self-reported sleep.