Melatonin: Both master clock output and internal time-giver in the circadian clocks network

► New vista on the circadian clocks network in mammals. ► Melatonin: both a master clock output and an internal time-giver. ► Nervous and endocrine pathways involved in the distribution of circadian signals. ► Role of endogenous melatonin in the circadian system of mammals. ► Chronobiotic effects of exogenous melatonin.