The common perception of sleep involves closed eyes and deep unconsciousness, making the resting behaviors of fish seem mysterious. Since most fish lack mobile eyelids, they rest with their eyes perpetually open, leading many to question if they truly sleep. Fish do enter a state analogous to sleep, but their unique physiology has led to a highly adapted form of rest. This resting state is characterized by periods of inactivity and a marked decrease in responsiveness, serving the same restorative function as sleep in terrestrial animals.
Defining Rest and Sleep
Sleep in a biological context is defined by criteria that go beyond the familiar sight of a closed eye. For non-mammalian species, scientists rely on four behavioral indicators to identify a sleep-like state. These include prolonged inactivity, adoption of a specific resting posture or location, regular recurrence following a 24-hour circadian rhythm, and a significantly elevated arousal threshold. The requirement for an increased arousal threshold means the animal is much harder to rouse, indicating a reduced state of consciousness.
Applying the strict mammalian definition of sleep, which relies on complex brain structures like the neocortex and specific brainwave patterns, would exclude fish entirely. Since fish lack these specific structures, behavioral criteria are necessary to establish a functional equivalent to sleep. When fish are deprived of this period of behavioral quiescence, they exhibit a “rebound effect,” resting longer afterward. This confirms the state is biologically necessary and regulated, much like mammalian sleep.
Observable Resting Behaviors
The most common sign of a fish entering its resting phase is a profound reduction in movement and activity. Many diurnal species, which are active during the day, find a secure spot on the bottom substrate or within aquatic vegetation at night. This reduced activity is accompanied by a noticeable metabolic slowdown, including a lower rate of respiration and gill movement.
Some species also display changes in body posture or color as part of their resting routine. While motionless, they may hover in place, wedge themselves into crevices, or even lie on their side. Certain fish can exhibit a dimming of their skin coloration, which functions as camouflage while they are most vulnerable. Nocturnal fish, such as catfish and loaches, display these same behaviors, but their resting period occurs during the daylight hours.
Physiological Differences from Mammalian Sleep
The absence of mobile eyelids is one physiological distinction between fish and mammalian rest. For decades, it was assumed that fish did not experience the deep neurological states of Rapid Eye Movement (REM) and non-REM sleep due to their less complex brain structure. However, recent studies on zebrafish have revealed two distinct brain activity states during rest that are functionally analogous to human sleep phases.
These states have been termed “slow-bursting sleep,” comparable to non-REM sleep and involving synchronized, slow neural activity, and “propagating wave sleep,” similar to REM sleep. During the slow-bursting phase, a fish’s heart rate can decrease significantly, such as from around 200 beats per minute while awake to about 110-120 beats per minute. The propagating wave state is characterized by an even greater reduction in heart rate and full muscle relaxation. Unlike the deep unconsciousness of mammalian sleep, a fish’s resting state is often a lighter form of awareness, allowing for a rapid escape response necessary to avoid predators.
Diverse Resting Strategies in Fish
The vast diversity of the aquatic world has resulted in numerous specialized resting strategies across different fish families. The parrotfish, a common reef dweller, secretes a transparent mucus cocoon around its body before resting in a crevice. This protective envelope is thought to mask the fish’s scent, potentially warding off nocturnal predators and parasites.
Other species have evolved strategies to cope with specific breathing requirements. Sharks that rely on ram ventilation, such as the Great White, must keep water flowing over their gills to breathe and therefore cannot stop moving. These species are thought to achieve rest by slowing their swimming speed significantly or by utilizing ocean currents, rather than entering a period of complete immobility. Meanwhile, some bottom-dwelling fish, like the Pacific sand lance, physically burrow into the sand to create a secure, hidden refuge for their rest period.