Climate warming is predicted to lengthen the growing season, particularly at high latitudes, which provides increased foraging opportunities, although biological interactions can also be disrupted due to intra- and inter-specific variability in the response to climate forcing. Using patterns of body temperature in free-living arctic ground squirrels we can precisely determine the timing of key seasonal events including hibernation, mating and parturition, and immergence and emergence from the hibernacula. Long-term data collected from two arctic ground squirrel populations living 20 km apart that differ in timing and duration of snow-cover indicate that individuals can adjust their seasonal timing. Additionally, it has been hypothesised that a seasonal absence of circadian rhythms is common to all vertebrate residents of polar regions. While free-living arctic ground squirrels do not express circadian rhythms during the heterothermic and pre-emergent euthermic intervals of hibernation, they display entrained daily rhythms of body temperature throughout their active season which includes six weeks of constant sun. In winter, ground squirrels are arrhythmic and regulate core body temperatures to within +/-0.2 ^oC for up to 18 days during steady-state torpor. In spring, after use of torpor ends, male but not female ground squirrels, resume euthermic levels of T_b in their dark burrows but remain arrhythmic for up to 27 days. However, once activity on the surface begins, both sexes exhibit robust 24-h cycles of body temperature. We suggest that persistence of daily rhythms through the polar summer enables ground squirrels to minimise thermoregulatory costs.