BACKGROUND: Hormone therapy is commonly used to treat breast cancer but can cause mood disorders and sleep disturbances, negatively impacting patients' well-being. OBJECTIVES: This trial aimed to evaluate the effects of melatonin on sleep problems and mood changes in breast cancer patients undergoing hormone therapy. METHODS: The study was conducted at Omid Hospital in Isfahan, Iran, using a randomized, double-blinded, placebo-controlled design. Participants were assessed using the Hospital Anxiety and Depression Scale (HADS) and were randomly assigned to receive either 6 mg of melatonin or a placebo daily for 4 weeks. Sleep quality, depression levels, and mood states were measured using the Pittsburgh Sleep Quality Index (PSQI), the Center for Epidemiological Studies-Depression Scale (CES-D), and the Profile of Mood States (POMS) Questionnaires at the beginning and end of the 4-week follow-ups. RESULTS: Sixty participants (34 in the melatonin group and 26 in the placebo group) completed the study. Melatonin administration significantly improved sleep quality, latency, duration, and reduced the use of sleep-promoting medication, according to the PSQI scores. However, there were no significant improvements in depression severity or mood disorders, as assessed by the CES-D and POMS questionnaires, in either group following the 4-week melatonin supplementation period. CONCLUSIONS: Melatonin supplementation effectively alleviated sleep disturbances caused by hormone therapy in breast cancer patients. However, the study did not find substantial evidence supporting the use of melatonin for improving mood disorders or depression in this specific context.