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The vertebrate pineal organ rhythmically synthesizes and secretes melatonin during nighttime and forms an essential component of the photoneuroendocrine system which allows humans and animals to measure and keep the time. Regulation of the melatonin biosynthesis depends on signals from photoreceptors perceiving and transmitting environmental light stimuli and endogenous oscillators generating a circadian rhythm which is independent from any environmental time cue (zeitgeber). In non-mammalian species the photoreceptors responsible for regulating melatonin biosynthesis reside within the pineal organ itself. In several nonmammalian species (e.g., lamprey, zebra fish, house sparrow, chicken) the pineal organ is also capable of generating circadian rhythms and thus serves all key functions of the photoneuroendocrine system: photoreception, endogenous rhythm generation, and production of neurohormones. These may even be accomplished by a single “photoneuroendocrine” cell.