The idea that fish never stop moving and do not need to rest is a misconception. Like nearly all animals, fish enter a necessary state of reduced consciousness and decreased physical activity to restore energy and process information. This state is often referred to as “rest” rather than “sleep” because it differs significantly from the deep, restorative cycle experienced by mammals. Observing a resting fish can be confusing, as the visual signs are profoundly different from a sleeping cat or dog. A fish’s resting appearance and location vary widely, depending on its specific environment and the constant threat of predators.
The Physiological State of Fish Rest
Fish rest is a period of restoration characterized by prolonged inactivity, decreased movement, and a high threshold for arousal. The fish’s metabolic rate slows down, leading to reduced oxygen consumption and a lower heart rate. Behaviorally, a resting fish remains motionless for extended periods and responds slowly to external stimuli.
This restful state is governed by a circadian rhythm, dictating when a fish is active and when it rests. Most fish are diurnal, resting at night, but some species are nocturnal and rest during the day. This state is homeostatically regulated; rest deprivation leads to a rebound effect where the fish rests for a longer duration.
Fish rest differs from mammalian sleep, as most fish lack the neocortex structure associated with complex sleep stages. However, studies on species like the zebrafish show brain activity patterns similar to deep slow-wave sleep and rapid eye movement (REM) sleep. This suggests the need for a multi-stage rest period may have evolved more than 450 million years ago.
The Visual Signs of Resting
The most striking visual difference between a resting fish and a sleeping mammal is the appearance of the eyes. Since the vast majority of fish species do not possess eyelids, their eyes remain wide open, giving the false impression that they are awake. Because the fish is constantly exposed to its surroundings, a reduction in responsiveness is the primary marker of rest.
When a fish enters its resting period, its posture usually becomes completely still, often settling on the substrate or floating motionlessly in the water column. Non-essential movements, such as fin fluttering, largely cease, with the fish relying on neutral buoyancy to maintain position. In many species, a subtle change in coloration also occurs as a form of camouflage.
Some species, such as various cichlids, temporarily dim their vivid colors or become noticeably paler when the lights go out. This color change helps the resting fish blend into its dark surroundings, making it less visible to nocturnal predators. Full, bright coloration returns rapidly upon waking or when light levels increase.
Diverse Resting Strategies Across Species
The visual appearance of a resting fish depends heavily on its specific survival strategy and environment. Many reef fish seek out sheltered locations to avoid predators, wedging themselves into small crevices, caves, or dense vegetation. Other fish, such as bottom dwellers, settle directly onto soft substrates like sand or mud. Flounder may partially bury themselves for cover, while some reef sharks rest completely still on the ocean floor.
The ability of some sharks and bottom dwellers to rest while stationary is often facilitated by special anatomical features. For instance, a spiracle allows them to pump oxygenated water over their gills without needing to swim.
A unique and highly specialized strategy is seen in the parrotfish, which secretes a transparent, protective mucus cocoon around its entire body before resting. This jelly-like bubble serves two purposes: it masks the fish’s scent from nocturnal predators like moray eels, and it provides a barrier against blood-feeding parasites.
In contrast to these stationary strategies, some large pelagic fish, such as tuna and certain sharks, must engage in “active rest.” They require constant forward motion to force oxygenated water over their gills, a process known as ram ventilation. These species likely achieve rest through unihemispheric sleep, where only one half of the brain is inactive at a time, allowing the fish to continue swimming slowly.