The study evaluate the effect of melatonin in improving quality of life and reducing post operative pain and chemotherapy-induced toxicity in breast cancer patients. This is a multi-center, randomized, double-blind, placebo controlled trial conducted in stage II or III breast cancer patients. Mixed-block randomization, stratified by center and treatment scheme is used to divide eligible patients into two groups: melatonin 20 mg or matched placebo. The patients are required to take the studied drugs at night (after 21.00 pm) on the first night prior to surgery and continue for 24 months. Standard treatment is surgery followed by chemotherapy according to each center's standard protocol. Study endpoints are QOL (FACT-B), pain (VAS 0-10), adverse event frequency, sleep quality (VAS 0-10), recurrence rate and progression-free survival.

Inclusion Criteria:

• histologically proven stage II or III breast cancer
• Eastern Cooperative Oncology Group (ECOG) performance status (PS) ≤ 2
• platelet count ≥100,000 cells/mm3
• white blood cell count ≥ 3,000 cell/mm3
• hemoglobin ≥ 10 g/dL
• serum creatinine ≤ 1.5 mg/dL
• bilirubin ≤ 2 mg/dL
• AST ≤ 2.5 times upper limit of normal (ULN)
• New York Heart Association grade ≤ 2
• written consent

Exclusion Criteria:

• received prior chemotherapy or biotherapy, radiotherapy or surgery within 1 month preceding randomization,
• had more than one type of cancer or brain metastasis
• moderate neuropathy (CTCAE grade ≥ 2)
• active infection
• uncontrolled complications (i.e. blood glucose > 200 mg/dL, uncontrolled hypertension, unstable angina, history of congestive heart failure or history of myocardial infarction within one year).