University of Manchester Professor Andrew Loudon and biologist Karl-Arne Stokkan of Norway's University of Tromso said in the northern arctic, the sun doesn't set for part of the year and at other times it never appears.
"Our findings imply that evolution has come up with a means of switching off the cellular clockwork," Loudon said. "Such daily clocks may be positively a hindrance in environments where there is no reliable light-dark cycle for much of the year."
In most mammals, Loudon said, the process involves an internal clock that drives hormone levels, in particular melatonin, in a rhythmic 24-hour fashion, even when there is no light-dark cycle.
But, Loudon said, northern Arctic reindeer show no natural internal rhythm of melatonin secretion. Instead, hormone levels rise and fall in direct response to light and dark. Such hormone concentrations spike almost as soon as the light goes out, only to dive again when the light again appears.
Although the researchers do not yet know how the hormone concentrations affect the reindeer, they said: "It is attractive to speculate that in reindeer informative melatonin signals … directly entrain a 'circannual clock' that, at least in reindeer, may not involve circadian mechanisms."
Loudon said he suspects similar patterns will be uncovered in other arctic animals.
The study appeared in the March 11 online edition of the journal Current Biology.