Melatonin is a small lipophile molecule, essentially secreted by pineal gland. The synthesis of this hormone shows a circadian pattern with a peak around 2-3 hours am. Many melatonin receptors are found in the body, which explains its multiple functions as biological rhythms resynchronisation, sleep induction, vasoregulation and even immunomodulation. Many experiments realised in this field have permit to discover different interactions between melatonin and the immune system, and especially the link which exists between melatonin and the fight against cancer via the immune system. Phase II studies reported a decrease of thrombocytopenia, an increase of some cytokines rate and an increase of objective responses in cancer patients. In order to confirm these results and to lead further research, we propose to realise a phase II randomised study melatonin versus placebo in metastatic breast cancer patients after two lines of treatment.