Sleep is present in all animals studied, suggesting an early evolutionary origin. This state of relative disconnection from the external world is vital for multiple physiological and behavioral processes. Sleep is regulated by two processes: homeostatic pressure, which ensures daily balance, and the circadian rhythm, which aligns sleep with the day–night cycle. Although sleep has been studied in many models, including invertebrates, it has never been investigated in intertidal animals, which experience a third pressure guiding their activity/rest rhythms: the circatidal rhythm, corresponding to the rise and fall of the tides. One example is the crab Neohelice granulata, which inhabits mudflats. In this project, we analyzed their activity/rest cycle by filming crabs in the laboratory for several days under natural light and constant water levels. They showed a crepuscular activity pattern, with two peaks near dusk and dawn respectively, and a prolonged midday/early afternoon rest phase. During immobility bouts, especially in this prolonged rest phase, crabs often adopted a characteristic posture: flattened cephalothorax, retracted claws, and a preference for arena corners. We also observed changes in the organization (but not the total duration) of resting periods between previously isolated and socialized crabs, consistent with sleep recovery following social interaction.