The pineal hormone melatonin is secreted with a marked circadian rhythm. Normally, maximum production occurs during the dark phase of the day and the duration of secretion reflects the duration of the night. The changing profile of secretion as a function of daylength conveys photoperiodic information for the organization of seasonal rhythms in mammals. The role of melatonin in mammalian circadian physiology is less clear. However, exogenous melatonin can phase shift, and in some cases entrain, circadian rhythms in rodents and humans. It can also lower body temperature and induce transient sleepiness. These properties indicate that melatonin can be used therapeutically in circadian rhythm disorder. Successful outcomes have been reported, for example in jet lag and shift work, and with cyclic sleep disorder of some blind subjects. Melatonin receptors of several subtypes are found in the brain, the retina, the pituitary and elsewhere. They are currently under intense investigation. Melatonin agonists and antagonists are under development.