Caribou living in the Arctic have switched off the internal biological clock most mammals use to distinguish day from night, biologists have found.

Researchers in the U.K. and Norway say the lack of an internal clock is the caribou's adaptation to the "land of the midnight sun," where the sun doesn't set for months at a time during the summer and doesn't rise during the winter.

"Our findings imply that evolution has come up with a means of switching off the cellular clockwork," Andrew Loudon of the University of Manchester said in a statement.

"Such daily clocks may be positively a hindrance in environments where there is no reliable light-dark cycle for much of the year."

The findings appear Thursday in the journal Current Biology.

The researchers examined the expression of clock genes in the skin cells of caribou, also called reindeer, but suspect that similar results would be seen in other animals that live in the Arctic.

In many animals, the daily release of hormones, including melatonin, is mediated by the nervous system and light sensors in the eye. This daily, cyclical regulation of hormone levels, known as circadian rhythm, influences the animal's behaviour, especially sleep.

Even if the cycle of light and dark isn't present, such as if the animal is in a cave or exposed to certain lab conditions, hormone levels are kept consistent by means of an internal clock.

"In reindeer, it is this clock element that seems to be missing," Loudon said.

Arctic animals' clocks work on seasonal cycle

Scientists in Manchester and the University of Tromso in Norway found that two known clock genes, called Bmal1 and Per2, are not regulated in the same rhythmic fashion as in other animals.

"We suspect that they have the full range of normal clock genes, but these are regulated in a different way in reindeer," Loudon said.

Hormone levels in caribou instead rise and fall in response to light and dark without an internal regulating clock, the researchers found.

Melatonin levels in the animals are low during daylight hours and spike when it turns dark.

Caribou don't appear to require a daily cycle of melatonin to regulate sleep, as many animals, including humans, do.

"Reindeer sleep in a series of naps, often associated with rumination," Loudon wrote in an e-mail. "Since rumination cycles occur many times a day, the animal accumulates sleep but in many episodes."

The findings were surprising, but it is likely other Arctic animals also lack an internal clock, the researchers wrote.

It's possible the daily cyclical regulation of hormones present in other animals morphed into a year-long seasonal cycle in Arctic animals.

"Synchronization of seasonal cycles in mammals is a prominent feature of physiological adaptation in northern temperate and Arctic species," the researchers wrote.

"It is attractive to speculate that in reindeer, informative melatonin signals associated with equinoxes directly entrain a 'circannual clock' that, at least in reindeer, may not involve circadian mechanisms."