Fish do sleep, but their resting habits differ from humans. Most fish lack eyelids, so their eyes remain open even when resting. Scientists refer to this state as “rest” rather than “sleep” in the human sense, as the brain wave patterns defining human sleep are not typically found in most fish.
Defining Fish “Sleep”
Fish “sleep” is a physiological state of reduced activity and responsiveness to external stimuli. This state allows fish to conserve energy and undergo periods of recovery. During this time, their metabolic rates often slow, and breathing may become less frequent. This resting period differs from unconsciousness or death, as fish can still be aroused, though less reactive than when fully awake.
Researchers have found that fish exhibit behaviors consistent with a regular sleep-wake cycle, known as a circadian rhythm. While they lack a neocortex, the brain part associated with complex sleep stages in mammals, studies on zebrafish suggest they may experience sleep-like brain activity, including rudimentary forms resembling slow-wave and REM sleep, without rapid eye movement. This fundamental need for rest and recovery is deeply conserved across diverse animal species.
Observable Resting Behaviors
When fish are resting, they display several observable behaviors. Many species become largely motionless, floating, settling on the substrate, or tucking into sheltered areas like plants or rock crevices. Their fin movements may decrease significantly, and they might appear less alert. Some fish can even be gently touched or handled without immediately reacting.
Fish may show subtle changes in body coloration, appearing slightly duller or muted during resting periods. Their respiration rate also tends to slow down. These behaviors conserve energy and reduce vulnerability to predators by minimizing movement and maintaining a less conspicuous presence. Observing these consistent patterns helps identify when a fish is resting.
Diversity in Fish Sleep Patterns
Fish exhibit a wide array of resting patterns, influenced by their environment, predators, and feeding habits. Many fish are diurnal, active during the day and resting at night. Conversely, nocturnal species, such as some catfish and eels, are active at night and rest in hidden spots during daylight hours. These varied schedules optimize survival and foraging opportunities.
Some fish have developed unique adaptations for resting. The parrotfish, for instance, secretes a mucous cocoon around its body before resting, offering protection from predators or parasites. Certain sharks and rays, requiring constant water flow over their gills to breathe, engage in unihemispheric sleep, allowing one half of their brain to rest while the other remains active. Other species, like the Pacific sand lance, burrow into sand or mud for protection and energy conservation.