Introduction: Melatonin, a hormone that exists in most living organisms, is produced in the pineal gland, retina, alimentary tract and various other tissues of the body and secreted by the pineal gland. It can be said this hormone exists in most living organisms. Production of this hormone is affected by different factors like age, light and other environmental and physiological agents. Considering its vast effects, the aim of this review article is to provide an update of current data available on the production, metabolism and clinical effects of this hormone in humans and its roles in different diseases like obesity, some cancers and diseases affecting brain function. Materials and Methods: This article is the result of reviewing 60 articles, of which 34 are directly referred to. Results: Based on the articles investigated, several environmental factors participate in the regulation and synthesis of this hormone which acts as a protective agent against chronic and degenerative diseases, gastrointestinal diseases, metabolic and behavioral disorders. Additionally, Melatonin intake improves some pathological conditions. Conclusion: Considering the positive effects of melatonin and the fact that no side effects of melatonin supplementation, have been documented it can be said that should, for any reason its synthesis and secretion cease in individuals taking a suitable physician-prescribed dose of this hormone would be beneficial, preventing increase in disease.