Reindeer have no internal body clock, according to scientists.
Researchers found that the animals are missing a "circadian clock" that influences processes including the sleep-wake cycle and metabolism.
This enables them to better cope with the extreme Arctic seasons of polar day, when the sun stays up all day, and polar night, when it does not rise.
The team from the universities of Manchester and Tromso report their study in Current Biology journal.
The body clock, or circadian clock, is the internal mechanism that drives hormone release on a rhythmic 24-hour cycle.
Light also influences these hormonal rhythms, but in most mammals, this "circuit" also involves the circadian clock, which can influence the release of hormones without the influence of light.
Anyone who has experienced jet lag is familiar with the effect of the body clock.
But the research team from research institutes in the UK and Norway found that, in Arctic reindeer, this circadian clock was absent.
Professor Andrew Loudon from The University of Manchester took part in the study.
He said that the reindeer may have "abandoned use of the daily clock that drives biological rhythms" in order to survive the extreme conditions in the Arctic.
He and his colleagues studied reindeer living in Northern Norway, 500 km north of the Arctic circle. Here there are 15 weeks of continuous daylight in summer and eight weeks during the winter where the Sun does not appear over the horizon.
They investigated levels of the hormone called melatonin - which is important in the sleep-wake cycle - in the reindeer's blood
They found that there was no natural internal rhythm of melatonin release into the blood - the hormone simply responded to the cycle of light and dark.
Professor Loudon said he believed that evolution had "come up with a means of switching off the cellular clockwork" and that the result was "a lack of internal daily timekeeping in these animals".
He commented: "Such daily clocks may be positively a hindrance in environments where there is no reliable light dark cycle for much of the year.
Organisms use their circadian clocks to correspond with their living environment; but if their environment has a very different cycle, it may be better to follow that rather than use the internal clock.
"This could be the case for a range of animals living at the poles of the Earth or in the depths of the ocean."