Melatonin is secreted only during night, irrespective of the habitat of an organism and the site of its synthesis and secretion, and hence known as "darkness hormone". Elevated melatonin levels reflect the nighttime. In vertebrates, the main site of melatonin production is the pineal gland. Species in which melatonin is also secreted from sources other than the pineal, as in some birds, relative contributions of different melatonin producing tissues to the blood melatonin level can vary from species to species. Melatonin acts through its receptors, which are members of the G protein-coupled (GPCR) superfamily. Three melatonin receptors subtypes MT1 (mella), MT2 (mellb), and MT3 (mellc) have been identified in different brain areas and other body organs of vertebrates. Melatonin synthesis and secretion are circadianly rhythmic. Changes and differences in specific features of melatonin signal can vary among species, and under a variety of natural environmental conditions. Two major physiological roles of melatonin are established in vertebrates. First, melatonin is involved in the circadian system regulated behavioural and physiological functions. Second, it is critical for the photoperiodic system. Besides, melatonin has been implicated in various ways both directly and indirectly to human health, including jet lag, sleep, immune system and cancer.