Melatonin is a hormone that is found in all forms of life on earth. In humans, it is produced by the pineal gland, a gland about the size of a pea, that is located in the center of the brain. The secretion of melatonin occurs during the night as a response to darkness, peaks in the middle of the night, and gradually falls during the second half of the night. The synthesis and release of melatonin is inhibited by light. For this reason melatonin has been called "the hormone of darkness."