The pineal hormone melatonin is a remarkable molecule, with a conserved time-keeping function across species. It is extensively used as a self-administered remedy for sleep disturbance in countries where it is freely available, and to some extent when it is available on prescription, as in the UK. In some circumstances, notably free-running sleep disorder of the blind, it is the treatment of choice. It is also the marker rhythm of choice for the determination of circadian phase and period. This review outlines the current state of knowledge within a physiological perspective with emphasis on human biological rhythms.