Fish enter a restful state marked by minimal movement and decreased responsiveness. During this time, they maintain their balance and continue slow, gentle movements, allowing them to conserve energy while remaining able to respond quickly to threats. Although this state resembles sleep, it differs in several ways from the deep, sustained sleep experienced by mammals. One key difference lies in the physical signs of rest. Unlike mammals, fish do not have eyelids and therefore cannot close their eyes while resting. Instead, they remain partially alert, able to sense changes in their environment even as their activity slows.

Different species of fish exhibit varying sleep behaviors. While most fish maintain a slow movement during rest, some species, such as certain catfish, loaches, and eels, are nocturnal and rest during the day in protected areas like holes or thick vegetation. Other fish, like some species of sharks, have adapted to rest while still moving, as they need to keep water flowing over their gills to breathe. These adaptations highlight the diversity of sleep behaviors among fish species.

The environment plays a crucial role in how fish sleep. Fish that live in environments with high predation risk may have developed the ability to remain more alert during rest. Additionally, some fish may choose specific locations to rest, such as the bottom of a body of water, where they can blend in with their surroundings and avoid detection by predators. This behavior demonstrates the importance of environmental factors in shaping the sleep patterns of fish.