Biological functions at many levels of the organism show time dependent oscillations so called biological rhythms. The rhythmic variations are organised by exogenous and endogenous pacemakers involving melatonin. Biological rhythms, are affected by some factors such as aging, diseases, shift-work and jet-tag and, as a consequence of this situaition, synchronization of the organism deteriorates. It is thought that melatonin may resynchronize the internal rhytms. On account of its synchronising properties, melatonin is accepted as a chronobiotic that adjusts the timing of internal biological rhythms. Aging process causes chronobiological alterations including changes in circadian rhythms and/or rhythm characteristics in the organism. With the help of the broad range of biological functions of melatonin like chronobiotic, hypnotic, antioxidant, immunmodulatuar, oncostatic and hypotensive activities, melatonin may have beneficial effects on age-associated, diseases such as sleep disturbances, degenerative diseases, rhythm changes in endocrin system, deterioration of immunity, hypertension and cancer.