This is a randomized, double-blind clinical trial, parallel, placebo-controlled trial. Breast cancer women treated surgically and with indication for adjuvant chemotherapy, aged between 18 and 75 years will be included. Patients will be randomized to receive 20mg of melatonin or placebo at bedtime. Treatment will initiate seven days before the start of chemotherapy to three days after the first cycle, totaling ten days. The sleep-wake rhythm will be assessed by actigraphy, which use will precede three days to start treatment. We will also evaluate the sleep-wake cycle by sleep diary. Other parameters will be measured as depressive symptoms, pain threshold pressure, effect on platelets and white series, the pain response to conditioned stimuli and quality of life. Serum markers of neuroplasticity will be measured as the Derivate Nerve Factor Brain BDNF and S100 β. The icronucleus effect will quantified by cell material from the oral mucosa. We expect that the use of melatonin, as a competitor to chemotherapy and synchronizer, improves tolerance to adjuvant treatment of breast cancer chemotherapy adverse effects.