To prospectively examine the association between sleep quality before embryo transfer with pregnancy outcomes in a population with infertility. Prospective observational cohort study. Center for Reproductive Medicine, Shandong University. From 7,847 women who enrolled from July 2019 to July 2020, 3,183 were eligible. Information about sleep, including sleep quality, sleep duration, and sleep chronology, were collected before embryo transfer using an integrated questionnaire. Sleep quality is quantified by the Pittsburgh Sleep Quality Index (PSQI) with a cut-point of 5 (PSQI >5 identifying poor sleep vs. PSQI ≤5 identifying good sleep). Average weekly sleep duration was calculated and divided into 5 groups (≤7, 7-8, 8-9, 9-10, and >10 h/d). In defining sleep chronotype, women with a sleep midpoint earlier than 2:30 AM were defined as morningness type, whereas those with a sleep midpoint later than 3:30 AM were defined as eveningness type, and the remainder were defined as an intermediate type. Rate of clinical pregnancy and live birth. Compared with those reporting poor sleep quality, those reporting good sleep quality showed higher clinical pregnancy (69.3% vs. 65.1%) and live birth rates (50.5% vs. 45.7%). After adjusting for confounding factors, women who self-reported good sleep had a higher probability of acquiring clinical pregnancy (RR, 1.07; 95% confidence interval, 1.01-1.13) and of live birth (RR, 1.12; 95% confidence interval, 1.02-1.23). Women with the morningness chronotype had the lowest rates of clinical pregnancy and live birth and had the highest rate of miscarriage. Sleep duration was found to have no significant association with any outcomes. In the stratified analyses, the positive associations of good sleep quality with clinical pregnancy and live birth existed only among women younger than 35 years old or who had undergone fresh embryo transfer. Good sleep quality was positively associated with outcomes in in vitro fertilization embryo transfer (IVF-ET), particularly with clinical pregnancy and live birth. Poor sleep quality may be a risk factor for adverse IVF-ET outcomes for women <35 years old. Treating sleep disorders and providing sleep behavior guidance to patients receiving IVF-ET may improve pregnancy outcomes.