Melatonin is a ubiquitous natural neurotransmitter-like compound produced primarily by the pineal gland. This agent is involved in numerous aspects of the biological and physiologic regulation of body functions. The role of endogenous melatonin in circadian rhythm disturbances and sleep disorders is well established. Some studies have shown that melatonin may also be effective in breast cancer, fibrocystic breast diseases, and colon cancer. Melatonin has been shown to modify immunity, the stress response, and certain aspects of the aging process; some studies have demonstrated improvements in sleep disturbances and “sundowning” in patients with Alzheimer’s disease. The antioxidant role of melatonin may be of potential use for conditions in which oxidative stress is involved in the pathophysiologic processes. The multiplicity of actions and variety of biological effects of melatonin suggest the potential for a range of clinical and wellness-enhancing uses. This review summarizes the physiology of melatonin and discusses the potential therapeutic uses of melatonin.
Melatonin is a widely occurring neurotransmitter-like compound derived primarily from the pineal gland. It is also produced in a number of other areas, for example the gastrointestinal tract.[1–3] Once labeled as a master hormone, it has been found to be involved in numerous aspects of biological and physiologic regulation.

http://www.pubmedcentral.nih.gov/articlerender.fcgi?artid=1395802
Neuroendocrinol Lett 2002 Apr;23 Suppl 1:14-6
Karasek M, Reiter RJ.
Laboratory of Electron Microscopy, Chair of Pathomorphology, Medical University of Lodz, 92-216 Lodz, Czechoslowacka 8/10, Poland.