Life evolved under the permanent influence of environmental cycles, the most prominent being the daily light/dark cycle, caused by the earth’s rotation about its axis. As a consequence almost all organisms have developed biological clocks that allow them to anticipate cyclic changes in the environment and thus to adjust their behavior and physiology accordingly. A biological clock has also been identified in the copepod Calanus finmarchicus, where it is thought to underpin diel and seasonal rhythms in behavior and physiology. C. finmarchicus plays a central role in sustaining the food webs of the North Atlantic and Subarctic regions, however, climate change induced latitudinal range shifts have introduced C. finmarchicus into the Arctic region, where it experiences extreme light conditions, with almost constant light throughout the Summer months and constant darkness in Winter. Therefore, this thesis centers on the question whether the C. finmarchichus clock stays functional throughout the High Arctic Summer, when diel fluctuations in light reach a minimum. Net based 24 h samplings have been conducted at two stations along a latitudinal gradient from the southern Barents Sea (74.5 °N, 30 °E) to the Nansen Basin (82.56 °N, 30.85 °E) north of Svalbard, within 9 days of the Summer Solstice 2018. Further, temporal expression patterns of clock genes have been analyzed and the behavioral activity of individual C. finmarchicus has been assessed in onboard laboratory experiments. Results from gene expression analysis show significant rhythmic oscillations in a number of core clock genes in wild caught C. finmarchicus, suggesting a functional and synchronized endogenous clock during periods of minimal fluctuations in light intensity. Further, a period shortening could be observed in several clock genes at the northern station associated with lower diel oscillations in light properties. Results from behavioral experiments indicate overall low rhythmic behavioral activity during Summer in the High Arctic. The findings from this study are further discussed in the context of seasonal timing, concluding that the circadian clock likely stays functional throughout the whole active phase at high latitudes, including periods of Midnight Sun. This may further point out the importance of the circadian clock as a tool to track the progression of the season and help to time seasonal events, which is of fundamental importance for C. finmarchicus to survive in the extreme conditions of the Arctic.