Internal clocks are fundamental features of all living things. The circadian rhythm, the 24 hour cycle coincident with a single light and dark (day and night) cycle, is particularly considered fundamental to life on Earth. Internal daily cycles are expressed in the body as daily melatonin cycles. A recent paper asks how it is that reindeer set their internal clock in the arctic where the sun neither rises nor sets.

The paper titled “A Circadian Clock Is Not Required in an Arctic Mammal?” was published March 23, 2010 in Current Biology.

The authors found that regulation of melatonin production in reindeer is fundamentally different from in typical animals. Arctic reindeer lack the molecular clocks that normally drive cellular circadian rhythms throughout the body. Instead, melatonin cycles are set directly from exposure to light and dark. Daily light and dark cycles do not occur in arctic reindeer except for during a few weeks around the spring and autumn equinoxes!