The Earth is in constant movement around the Sun – this has created environments with changing light throughout the day and year. Through evolution animals have adapted to this change in light and circadian and circannual rhythms have evolved to enhance fitness and survival. Animals living in the Arctic region have a different relationship to light changes as for several months a year the Sun is either above or below the horizon all day creating only a minimum amplitude of light change throughout the day. Consequently, circadian organization may be less important is these animals. However, there are limited research on animals in the Arctic concerning circadian rhythms. This thesis took a closer look at the Norwegian reindeer (Rangifer tarandus). The reindeer suprachiasmatic nucleus (SCN) was characterized with the use of DIG in-situ hybridization and found that arginine vasopressin (AVP) and vasoactive intestinal polypeptide (VIP) are expressed in the SCN, similar to what is seen in sheep. To measure the output physiology – locomotor activity, feeding behavior, rumen temperature, heart rate and cortisol – a forced desynchrony protocol was developed. The reindeer had a weak 24-hour diurnal locomotor activity pattern when a light-dark cycle is present. In constant light this rhythm breaks down and this study did not provide any evidence of a strong circadian rhythm in locomotor activity, feeding behavior or rumen temperature. The most dominant rhythm appears to be ultradian. The limitations of the set-up and devices, and the ultradian rhythmicity in reindeer, resulted in no signs of uncoupling of locomotor activity and feeding behavior from rumen temperature. The three parameters, locomotor activity, feeding behavior and rumen temperature were correlated and this correlation was most apparent at an ultradian level, and the relationship between the parameters did not change throughout the protocol. The reindeer have strong social organization and becomes stressed by being indoors for a longer period of time. As reindeer are ruminants living in the Arctic, both the environment and feeding demands can have contributed to a less important circadian organization.