Melatonin signals night time by its rhythmic profile of secretion. Its measurement provides the best indicator of internal circadian clock timing. Treatment with melatonin induces sleepiness and lowers body temperature during 'biological day' (i.e. when its endogenous secretion is minimal) and it acts as a 'chronobiotic' to change the timing of the circadian clock. It has been used successfully to treat disorders of biological rhythms in humans.