Most animals and plants follow an internal clock timed to the rising and setting of the sun--the so-called circadian rhythm. But above the arctic circle, the sun never rises in winter and never sets in summer. For the reindeer living there, it might be tempting to sleep away half the year and remain awake throughout the long summer days. Instead, Norwegian researchers have determined that the creatures set their own schedule, grazing and resting in cycles of less than 24 hours for most of the year.

Biologist Karl-Arne Stokken of the University of Troms in Norway and his colleagues found that reindeer on the polar island of Svalbard ignore the sun--or lack of it--completely, except for a few weeks in spring and fall. During the long darkness of winter Rangifer tarandus platyrhynchus prefers to alternate a few hours of grazing with a few hours of resting. And although the periods of grazing grow longer during the long summer brightness, they remain hours short of being a continuous day-night cycle.

The scientists also studied another subspecies of reindeer in northern Norway, eight degrees of longitude south of the Svalbard population. Rangifer tarandus tarandus, they observed, follow a 24-hour cycle when it is available in autumn and spring, and even curtail their activity in winter to match the short hours of light, supplemented with some after-dark snacking. But in summer, these reindeer also follow a looser clock, grazing for long periods, followed by short periods of rest not matched to any cycle in the sunlight.

The reindeer are not the only polar animals observed to have their own rhythm. Svalbard ptarmigans--small white Arctic grouses--also follow their own clock. The researchers therefore speculate that most Arctic animals seasonally adjust their biological rhythms. No word on whether Santa and his elves do the same.