diurnal ones including humans. many animals use the variation in duration of melatonin production each day as a seasonal clock. in animals including humans, the profile of melatonin synthesis and secretion is affected by the variable duration of night in summer as compared to winter. the change in duration of secretion thus serves as a biological signal for the organization of daylength-dependent (photoperiodic) seasonal functions such as reproduction, behavior, coat growth, and camouflage coloring in seasonal animals. in seasonal breeders that do not have long gestation periods and that mate during longer daylight hours, the melatonin signal controls the seasonal variation in their sexual physiology, and similar physiological effects can be induced by exogenous melatonin in animals including mynah birds and hamsters. melatonin can suppress libido by inhibiting secretion of luteinizing hormone and follicle-stimulating hormone from the anterior pituitary gland, especially in mammals that have a breeding season when daylight hours are long. the reproduction of long-day breeders is repressed by melatonin and the reproduction of short-day breeders is stimulated by melatonin. during the night, melatonin regulates leptin, lowering its levels. cetaceans have lost all the genes for melatonin synthesis as well as those for melatonin receptors. this is thought to be related to their unihemispheric sleep pattern (one brain hemisphere at a time). similar trends have been found in sirenians. plants until its identification in plants in 1987, melatonin was for decades thought to be primarily an animal neurohormone. when melatonin was identified in coffee extracts in the 1970s, it was believed to be a byproduct of the extraction process. subsequently, however, melatonin has been found in all plants that have been investigated. it is present in all the different parts of plants, including leaves, stems, roots, fruits, and seeds, in varying proportions. melatonin concentrations differ not only among plant species, but also between varieties of the same species depending on the agronomic growing conditions, varying from picograms to several micrograms per gram. notably high melatonin concentrations have been measured in popular beverages such as coffee, tea, wine, and beer, and crops including corn, rice, wheat, barley, and oats. in some common foods and beverages, including coffee and walnuts, the concentration of melatonin has been estimated or measured to be sufficiently high to raise the blood level of melatonin above daytime baseline values. although a role for melatonin as a plant hormone has not been clearly established, its involvement in processes such as growth and photosynthesis is well established. only limited evidence of endogenous circadian rhythms in melatonin levels has been demonstrated in some plant species and no membrane-bound receptors analogous to those known in animals have been described. rather, melatonin performs important roles in plants as a growth regulator, as well as environmental stress protector. it is synthesized in plants when they are exposed to both biological stresses, for example, fungal infection, and nonbiological stresses such as extremes of temperature, toxins, increased soil salinity, drought, etc. herbicide-induced oxidative stress has been experimentally mitigated in vivo in a high-melatonin transgenic rice. fungal disease resistance is another role. added melatonin increases resistance in malus prunifolia against diplocarpon mali. also acts as a growth inhibitor on fungal pathogens including alternaria, botrytis, and fusarium spp. decreases the speed of infection. as a seed treatment, protects lupinus albus from fungi. dramatically slows pseudomonas syringae tomato dc3000 infecting arabidopsis thaliana and infecting nicotiana benthamiana. fungi melatonin has been observed to reduce stress tolerance in phytophthora infestans in plant-pathogen systems. danish pharmaceutical company novo nordisk have used genetically-modified yeast (saccharomyces cerevisiae) to produce melatonin. bacteria melatonin is produced by α-proteobacteria and photosynthetic cyanobacteria. there is no report of its occurrence in archaea which indicates that melanin originated in bacteria most likely to prevent the first cells from the damaging effects of oxygen in the promitive earth's atmosphere. novo nordisk have used genetically-modified escherichia coli to produce melatonin. food products naturally-occurring melatonin has been reported in foods including tart cherries to about 0.17–13.46 ng/g, bananas, plums, grapes, rice, cereals, herbs, olive oil, wine, and beer. the consumption of milk and sour cherries may improve sleep quality. when birds ingest melatonin-rich plant feed, such as rice, the melatonin binds to melatonin receptors in their brains. when humans consume foods rich in melatonin, such as banana, pineapple, and orange, the blood levels of melatonin increase significantly.