In temperate latitudes, the right timing is crucial for almost all living things: Plants sprout with the advent of spring, bees know the best times to visit flowers, people get tired in the evening and wake up again in the morning. The constant change between light and dark is the rhythm to which all living beings must adapt if they want to survive and reproduce. Circadian clocks regulate the timekeeping mechanism in each organism and adjust it to changes.
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Source: Phys.org