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Humans are pegged¹ to a 24-hour cycle. We're locked into it not just by day and night—there’s the master timepiece in the brain called the circadian clock. But it doesn't make sense to live by a 24-hour clock in the Arctic, where it's dark or light for months at time. The solution? Lose the daily clock. Which is exactly what reindeer² seem to have done, according to a study in Current Biology.                                           

Reindeer don't sleep eight hours like we do, and there's no obvious 24-hour pattern to their lives. They just chomp³ on tundra⁴, nap a few hours and feast again. But they still need to know when to mate, pack on fat or thicken their coats. So they probably rely on an annual clock instead, set by the hormone⁵ melatonin.

In humans melatonin levels rise at night, in response to darkness and cues from the circadian clock. In reindeer, even if they’re missing a circadian clock, melatonin levels still spike⁶ when it's dark and drop when it's light, making the equinoxes an ideal time to synchronize⁷ their annual clocks.

So ask a reindeer what time of year it is, and it may be able to give you the date. Just don't be offended if it won't give you the time of day.