Fish do not have eyelids, so their eyes remain open during rest. Despite this, they enter a state of reduced activity recognized as sleep, distinct from human sleep. This essential biological process allows for recovery.
Understanding Fish Sleep and Stillness
Most fish exhibit a period of stillness considered a form of sleep. They may float motionless, rest on the bottom, or wedge themselves into a secure spot. Goldfish and betta fish, for instance, become less active, hovering or settling near the tank bottom. Even when still, fish remain aware and can react quickly to threats.
During this rest, their metabolic rate slows, and breathing patterns become less frequent. While fish lack a mammalian neocortex, they exhibit sleep-like brain activity. Studies on zebrafish, for example, show neural patterns analogous to slow-wave sleep and even a stage resembling REM sleep, without eye movements. This indicates a restorative function similar to sleep in other animals, distinct from unconsciousness.
How Different Fish Sleep
Sleep patterns vary significantly across fish species, reflecting their diverse environments and survival strategies. While many fish become still, some species, like certain sharks and tuna, must maintain continuous movement to breathe. These “ram ventilators” move water over their gills by swimming, and some may achieve a form of rest through unihemispheric sleep, where one half of their brain rests while the other remains active. This allows them to stay vigilant and continue swimming at a slower pace.
Other fish use hiding as a sleep strategy. Parrotfish, for instance, secrete a transparent mucus cocoon for protection from parasites and predators. Some wrasse and Pacific sand lances burrow into sand or mud for safety. Additionally, some fish display “pajama coloration” during sleep, where their vibrant colors fade or shift to paler hues for camouflage.
Why Fish Need Sleep
Sleep in fish, like in other animals, serves several biological purposes. A primary function is energy conservation, allowing their bodies to recover. During this resting phase, fish reduce their metabolic rate, conserving energy otherwise spent on swimming and foraging. This energy saving supports physiological maintenance.
Rest periods also contribute to brain function and may aid memory consolidation, though this is less understood than in mammals. Adequate rest strengthens their immune system, making them more resilient to disease. Sleep supports fish survival, enabling them to be more alert and efficient when awake.