n all organisms, biological rhythms arise from the interaction between endogenous clocks and a rhythmic environment. The timing of sunrise and sunset synchronize internal timing processes and inform the organism not only of the time of day, but also of the time of year. For animals living in polar regions biological timekeeping represents a challenge, given that the sun does not rise during the winter, nor set during the summer months. Due to the difficulty of assessing real time food intake in field studies, activity has so far been used as a proxy for feeding. Long term timing mechanisms such as seasonal rut, reproduction, appetite, antler growth and moult rely on circadian timing in order to sense changes in daylength indicating a change in season. In order to investigate this dilemma we analysed both, activity and food intake as parameters of rhythmic output. The aims of this study were to 1. Analyse the effect of photoperiod on the distribution of feeding and activity across season. 2. Analyse ultradian and/or circadian patterns of activity. 3. Use of simultaneous food intake and activity measurements to assess the validity of previous assumptions about activity as a proxy for food intake. 4. Assess age differences in activity and feeding between reindeer calves and yearlings Locomotor activity and food intake in semi-domesticated Norwegian reindeer (Rangifer tarandus tarandus, n=8) were repeatedly measured in Tromsø (69° 39′ N,18° 57′ O, Norway) between November 2017 and August 2018. Half the animals were around a year of age, and half of them were only 6 months old at the start of the project. For the first time food intake and activity were measured simultaneously, demonstrating that activity is not a good proxy for food intake. All animals displayed higher appetite and weight increase in the months prior to winter, preparing for the extreme cold and lack of food ahead. Overall activity and food intake were suppressed by darkness, a likely adaptation against nocturnal predators. Ultradian rhythms of activity prevailed in all animals both during day and night, throughout the year. Overall activity was lower in winter than summer, presumably as a strategy to conserve energy when resources are limited. Despite the dominance of the ultradian activity pattern during the times of the midnight sun or constant darkness, periodograms revealed an underlying 24-hour rhythm indicating entrainment. At those times of the year with a rhythmic light dark signal, activity and feeding concentrated in the light part of the day with the biggest food intake occurring around civil twilight. We could demonstrate that timing of feeding is co-regulated by photoperiod and endogenous ultradian feed requirements. Contrary to Svalbard reindeer, mainland Norwegian reindeer synchronize feeding and activity bouts within the herd.