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.