Animals living permanently at high latitudes must cope with long periods of low temperatures and severely reduced food availability. This has resulted, in long-lived vertebrates, in the development of characteristic annual cycles in physiological functions such as the growth and moulting of fur or plumage, food intake, fat deposition and reproduction. These cycles are typically initiated in advance of, and as preparation for, predictable seasonal changes in the environment. In general, the timing of annual cycles is believed to be based on the mechanisms which regulate the timing of daily rhythms in physiological and behavioural function and which are ubiquitous in living organisms. The timing of daily rhythms generally involves the interplay between endogenous, self-sustained (circadian) rhythms of genetic origin, known as biological clocks, and ambient time cues (zeitgeber) of which daylength is the most important. However, this timing is compromised in high latitude residents by extreme photic conditions where permanent daylight in summer and permanent darkness in winter obliterate the principal zeitgeber. A solution has been to dispense with normal clock function/circadian mechanisms. Thus, reindeer/caribou, which are well adapted to high latitudes, possess a weak or non-functioning circadian clock. Instead of being governed by an endogenous timer entrained by the environment, this species responds directly and passively to the prevailing light/dark cycle. Consequently, the animals are largely active across the 24 h day, having shed virtually all traces of underlying, circadian rhythmicity. This may reflect a general adaptation among resident Arctic species to polar photic conditions and may represent a fundamental departure from corresponding regulation of seasonal and daily rhythms in species inhabiting lower latitudes.