 The most widely recognized chronobiotic is the hormone melatonin, secreted at night in both diurnal and nocturnal species.The concept of chronobiotics arose from the characterization of the pineal gland.In 1917, Carey Pratt McCord and Floyd Pierpont Allen at Johns Hopkins University demonstrated that tadpoles hatched in water that contained crushed pineal gland were much lighter in color than tadpoles hatched in normal water.