This study was performed to investigate the effect of sleep quality of new mothers on their breastfeeding self-efficacy. The data of 52 mothers were analysed in the study. Data were collected by the midwife at the first, sixth and 12th weeks after the birth. In the analysis, the repeated measures ANOVA, independent samples t-test, chi-square test and correlation analysis were used. Sleep quality was poor in 38.5%, 17.3% and 15.4% of the participants in the first, the second and the third follow-ups respectively. Breastfeeding self-efficacy levels of the mothers whose sleep quality was good improved. In promoting breastfeeding, the sleep factor should be considered, and it should be included in social support and postpartum care services.