Antarctic krill is a species with fundamental importance for the Southern Ocean ecosystem. Their large biomass and synchronized movements, like diel vertical migration (DVM), significantly impact ecosystem structure and the biological carbon pump. Despite decades of research, the mechanistic basis of DVM remains unclear. Circadian clocks help organisms anticipate daily environmental changes, optimizing adaptation. In this study, we used a recently developed activity monitor to record swimming activity of individual, wild-caught krill under various light conditions and across different seasons. Our data demonstrate how the krill circadian clock, in combination with light, drives a distinct bimodal pattern of swimming activity, which could facilitate ecologically important behavioral patterns, such as DVM. Rapid damping and flexible synchronization of krill activity indicate that the krill clock is adapted to a life at high latitudes and seasonal activity recordings suggest a clock-based mechanism for the timing of seasonal processes. Our findings advance our understanding of biological timing and high-latitude adaptation in this key species. The Southern Ocean is home to whales, seals, seabirds and other iconic wildlife. All of these animals depend either directly, or indirectly, on a small marine prey species known as Antarctic krill, which thrives in the harsh conditions of the Southern Ocean. At night, large swarms of krill move towards the water surface to feed on plankton before returning to the depths during the day to avoid whales, fish and other predators. This synchronized movement influences the structure of the ecosystem in a number of ways by transporting carbon and influencing predator-prey interactions. Researchers have observed the movements of krill swarms for many decades, but the processes controlling this swimming behavior remained unknown, in part, due to a lack of tools that can track the movements of individual krill. Do the krill simply respond to light and other external cues, or do they also have internal biological clocks that can maintain the observed rhythms even without such cues? In 2024, researchers developed a new monitor known as AMAZE, which can record the swimming activity of individual krill in tanks of seawater. Hüppe et al. – including many of the researchers involved in the 2024 work – have now used this technique to trace the movement of individual wild-caught krill under different light conditions and seasons. Hüppe et al. captured krill from the Southern Ocean on a commercial fishing boat and transferred them into a tank on the vessel for experiments. Observations revealed that the krill were most active at night, matching their natural patterns of migration in the wild. These patterns of nighttime activity adjusted to the changing length of the night over the seasons. Furthermore, the krill maintained a daily rhythm of activity even when they were kept in constant darkness for several days. These findings suggest that an internal biological clock, in combination with light cues, regulates the swimming patterns of krill and helps them adapt to daily and seasonal changes in their environment. Understanding these internal rhythms will be key to assessing how well krill may cope with rapid changes in their environment due to climate change, which are particularly pronounced in polar regions.