Fish require periods of rest to maintain their health. While they do not “sleep” as humans do, lacking eyelids and not entering a state of unconsciousness, they enter states of reduced activity and responsiveness that serve similar restorative functions.
Defining Fish Sleep
Fish lack a neocortex, the part of the mammalian brain associated with complex thought and deep sleep. Their resting states thus differ from human sleep, which involves distinct brain wave patterns and rapid eye movement (REM) cycles. While most fish do not exhibit REM sleep, some research on zebrafish identifies two sleep stages resembling slow-wave and REM sleep, characterized by specific brain activity, heart rate, and muscle movements. This suggests sleep’s neural signatures may have evolved over 450 million years ago, before vertebrates transitioned from water to land.
Fish enter a “resting state” important for their well-being. This state is marked by a decrease in metabolic rate, reduced responsiveness to external stimuli, and a general lack of purposeful movement. Researchers observe this behavioral sleep, noting fish become less alert and may be handled without immediately reacting. This period of reduced activity allows fish to conserve energy, recover from daily activities, process information, and contribute to disease resistance.
How Fish Rest and Conserve Energy
When fish enter a resting state, observable behaviors and physiological changes occur. They reduce their fin movements and may float motionless or settle at the bottom. Many species seek secluded spots, such as crevices, caves, or dense vegetation, to feel secure and avoid perceived threats.
Physiologically, resting fish exhibit a slowed metabolism. This reduction in metabolic activity is important for energy conservation, allowing them to save energy for future activities and recovery. Their gill movements, responsible for breathing, may also slow, indicating a decreased respiratory rate. Observing these behavioral and physiological cues, such as remaining still for periods, resting at a consistent time, and reduced responsiveness, helps indicate that a fish is in its resting phase.
Variations in Fish Resting Patterns
Fish resting patterns are diverse, reflecting their varied habitats and survival strategies. Many open-water species, like tuna, continuously move to maintain water flow over their gills for oxygen, even while resting. These fish often engage in “unihemispheric sleep,” where one half of their brain rests while the other remains active, allowing them to continue swimming slowly.
Conversely, bottom-dwelling fish, such as cod and flatfish, may enter “torpor,” becoming sluggish and nearly motionless at the bottom with fins tucked close to their bodies. Some fish exhibit more specialized resting behaviors. Parrotfish, for example, secrete a mucus cocoon around themselves at night, forming a protective barrier that may deter parasites and mask their scent from predators.
Other species, like certain wrasses and the Pacific sand lance, burrow into sand or mud for protection and energy conservation. Fish also display diurnal or nocturnal resting patterns; many are active during the day and rest at night, while others, like certain catfish, are nocturnal and rest during daylight in hidden spots. These varied strategies highlight fish adaptability in ensuring adequate rest within their aquatic environments.