Biological rhythms enable organisms to measure time and to synchronize their endogenous behavior and physiology with the time constraints of their environment. Since the inhabitants of the littoral zone of marine environments are exposed to complex temporal and environmental changes, biological rhythms play an important role in their lives.Timekeeping is especially important for those with short lives like octopuses. Circadian rhythms, which are rhythms of about a day, are one of the most prominent of biological rhythms. They are ubiquitous through all phyla of the animal kingdom, light being one of the most important “Zeitgebers” for almost all of them. To examine the function of light as a synchronizer for their activity, four octopuses were held in a potentially entrained state with a 24-hour L:D(light/dark) cycle. After this phase the animals were placed in constant conditions to document free running activity with circadian components. The results of this study clarify why publications up to now show conflicting results about circadian aspects of Octopus vulgaris activity. While light was used inefficiently as a Zeitgeber for activity the results of our study proved the presence of an endogenous circadian rhythm in O. vulgaris. These generalistic and opportunistic animals may use several different time cues to synchronize their activity and behavior with the time constraints of their environment.