The common belief that fish never truly sleep stems from the observation that they lack eyelids and cannot close their eyes. Unlike mammals, fish do not possess the complex brain structures required for Rapid Eye Movement (REM) sleep cycles. However, science confirms that fish engage in a deep state of rest, often called quiescence, that fulfills the biological criteria of sleep. This physiological process allows for restoration and survival across nearly all species.
The Scientific Definition of Fish Sleep
Sleep in fish is defined by observable behavioral and physiological changes, not by brain wave patterns like those measured in humans. Researchers rely on four main criteria to identify this resting state: a distinct resting posture, a predictable 24-hour rhythm, prolonged inactivity, and a marked increase in the threshold of arousal. A fish in this state will not react as quickly to a sudden stimulus as it would during its active period.
This period of quiescence is accompanied by a significant reduction in the fish’s metabolic rate, which is a key function of sleep across the animal kingdom. Some species decrease their gill and mouth movements, lowering their respiratory rate by as much as half while resting. If fish are experimentally prevented from entering this resting state, they later exhibit a “sleep rebound” effect, catching up on lost rest with longer, more intense bouts of inactivity.
Studies on fish like zebrafish, often used in sleep research, have shown that their brain activity during rest displays synchronized oscillations. This slow-wave pattern is functionally similar to the Non-REM deep sleep found in mammals, suggesting that the restorative purpose of sleep is conserved across vertebrates.
Observable Behaviors of Rest and Quiescence
The visible signs that a fish is resting are highly variable but follow a general pattern of reduced mobility and responsiveness. Many common bony fish cease active swimming and assume a fixed position in the water column, often hovering near the bottom or drifting slightly. This cessation of movement is the most obvious external indicator of rest.
Other species settle directly onto the substrate, resting their bodies against the ocean floor, rocks, or aquatic plants. Many maintain a specific, stereotypic posture, which can include lying on their side or wedging themselves into a crevice for physical support and protection. While their eyes remain open, their overall demeanor changes from alert to relaxed.
This state of deep rest is characterized by significantly decreased reaction time to external threats. Fish that are normally wary and quick to flee can often be approached or gently touched at night without an immediate escape response. This elevated arousal threshold is a necessary trade-off for the biological benefits of restoration.
Specialized Resting Strategies Across Species
The method a fish uses to rest is dictated by its environment and the constant threat of predation, leading to a wide array of specialized strategies. Wrasse and parrotfish, for example, secrete a translucent, protective mucus cocoon around their bodies before settling down for the night. This envelope is thought to mask their scent from nocturnal predators like moray eels, providing a chemical shield during their vulnerable resting period.
Large, pelagic species such as tuna and certain sharks are obligate ram ventilators, meaning they must swim continuously to push oxygen-rich water over their gills. Since these fish cannot stop moving, they are hypothesized to achieve rest through unihemispheric sleep. This allows one half of the brain to rest while the other half remains active enough to maintain swimming. Other species, like some white sharks, may find a current and rest stationary, allowing the water flow to perform the necessary ventilation.
Fish that live in schools often rest in tight groups, relying on the sheer number of individuals to reduce the risk of predation. Conversely, many benthic species, including burrowing fish and damselfish, seek the safety of the substrate. They retreat into holes, rock crevices, or bury themselves in the sand. This physical hiding strategy provides maximum protection when their ability to react to danger is temporarily impaired.