The primary function of melatonin in mammals is to convey information about the changing length of the night in the course of the year. This information is used by photoperiodic species to ensure the correct timing of seasonally variable functions such as reproduction, coat growth, and probably the duration and organization of sleep. Melatonin appears not to be essential for circadian organization but reinforces functions associated with darkness. In diurnal humans this of course includes sleep and lowered body temperature. It may act as an adjunct to light for the maintenance of synchrony with the solar day. Exogenous melatonin can both advance and delay the timing of sleep and other circadian functions and appears to stabilize sleep to a 24 h period taken daily at an appropriate time in free running conditions. However, there is as yet little evidence that it can consistently synchronize free running strongly endogenous variables such as core temperature. Its effects on sleep in free run are complex, depend on circadian time of administration, and can in part be interpreted on a photoperiodic basis.