The aim of his study is to review the basic features of biological rhythms and the involvement of the pineal hormone melatonin in the regulation of biological clock, role of melatonin receptors, melatonin is endocrine functions and its use in insomnia therapy. Melatonin is an important component of the internal time-keeping system, its production is regulated by the lightening conditions, light suppresses melatonin synthesis, and darkness enhances it. It regulates physiological processes including the sleep wake cycle, pubertal development and seasonal adaptation. Melatonin possesses also antioxidant activity, modifi es immunological processes and the stress response. Many of its proposed therapeutic or preventive indications are based on these properties. The multiplicity of actions and variety of biological effects of melatonin suggest the potential for a range of clinical and wellness-enhancing uses. Special emphasis, is given to the effectiveness of melatonin prolonged release supplementation therapy in the treatment of the biological rhythm-related disorders. Synthetic melatonin supplementation has been used for chronobiotic therapy and assists with tapering or cessation of benzodiazepine depending. Exogenous melatonin reduces such disturbances related to sleep as adverse effects induced by beta-blockers which depress melatonin secretion. Melatonin administration does not cause toleration and serious adverse effects. Preclinical and clinical data indicate that melatonin can be helpful in many disorders. Synthetic melatonin supplements have been used for a variety of medical conditions, most notably for disorders related to sleep but a survey of the medical data should be done to determine its effi cacy and side effects in a number of conditions, the main attention focused on: sleep disturbances, seasonal affective disorder, neuroendocrine disorder and cancer therapy. This review summarizes the physiology of melatoninergic system and discusses the potential therapeutic uses of melatonin.