To evaluate the impact of postoperative exogenous nocturnal melatonin supplementation on the early regulation of the sleep-wake cycle and its clinical impact (subjective improvement of the quality of sleep and reduction of cardiopulmonary events) during the first postoperative week following anesthesia and surgery in older breast cancer patients receiving a unilateral segmental mastectomy with or without intraoperative lymph node mapping, sentinel node biopsy and axillary node dissection.