人類按照24小時一循環的作息規律來生活，是因為大腦中有晝夜節律鐘的協調。

研究發現，馴鹿等北極動物，在北極持續數周白天或黑夜里生活時，是沒有晝夜節律鐘的。

How Its Internal Clock Is Read, Knows Reindeer

Humans are pegged 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 reindeer seem to have done, according to a study in Current Biology. [See Weiqun Lu et al, http://bit.ly/btOYTL]

Reindeer don't sleep eight hours like we do, and there's no obvious 24-hour pattern to their lives. They just chomp on tundra, 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 hormone 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 spike when it's dark and drop when it's light, making the equinoxes an ideal time to synchronize 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.

—Christopher Itagliata