Scientists have confirmed that animals that live where the sun don't (often) shine have evolved not to use their body clocks.
Most scientists accept that the majority of life studied on Earth, 
whether moulds, mice or men, uses a body clock to synchronise its activity to the environment. In humans that means we go to sleep when it gets dark and wake up when the sun rises. For nocturnal animals, on the other hand, the reverse is true.
Consequently, the body clocks of most organisms have a roughly 24h cycle. But what about animals that live in high latitudes, like reindeer, where for 6 months of the year the Sun never sets and for the rest of the year it's dark?
To find out, Manchester University researcher Andrew Loudon, together with colleagues in Norway, collected blood samples from reindeer that they exposed to short 2.5 hour cycles of light and dark. The samples were tested for melatonin, a mammalian sleep-hormone secreted by the brain's pineal gland, usually in response to signals fromthe adjacent body clock.
But in the reindeer, the team found, melatonin secretion was switching on in the dark and immediately turning off again in the light, without any apparent regard to what the animals' body clocks were doing.
To find out why, the team added to cultured reindeer cells glowing DNA markers linked to two of the genes known to power the body clock. These showed no apparent clock activity, unlike in a mouse where the genes turned on and off sequentially, like clockwork.
This shows that, under selective pressure applied by the extreme environment in which they live, reindeer have evolved not to use their body clocks.
To do so, point out the researchers, would be disadvantageous; it's better to doze only when you feel tired, otherwise you might miss out on a feeding opportunity...
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