In the Arctic, bright summers and dark winters are a fact of life and can lead humans to rely on clocks and routines to tell them when to eat or sleep, but how do animals function under these conditions? Circadian rhythms refer to the "internal body clock" that regulates the approximately 24-hour cycle of biological processes in animals and plants. Rhythms in body temperature, brain wave activity, hormone production, and other biological activities are linked to this 24-hour cycle. The Earth's light-dark cycle provides the strongest influence on circadian rhythms and is thought to be the primary driver for the emergence and evolution of internal clocks. In the Polar Regions, however, photoperiod exhibits extreme annual variation because of near 24 hour sunlight in the summer and 24 hour darkness in the winter. In the absence of a well-defined light-dark cycle, some arctic residents lose their daily organization of behavior and physiology, and it is thought that the molecular clockwork that drives circadian rhythms may be weak or absent in arctic vertebrates.