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Since the origin of life, organisms have been profoundly influenced by the cyclic lighting conditions of their environment, which result in daily (diurnal) and seasonal rhythms. Plants and animals respond to rhythmic changes in the environmental lighting conditions via reactive and - even more interestingly - via anticipatory mechanisms. The anticipation of environmental light pulses requires an internal program providing a kind of “memory” about the length of the dark and light phases. To fully adapt body functions to the temporal organization of the environment this internal program needs (a) to be adjusted to environmental conditions via afferent connections and (b) to convey its signals to the body via efferent pathways. In vertebrates, including humans, these functions are carried out by a specific circuit of the brain, the photoneuroendocrine system.