While fish do not exhibit the same sleep patterns as land animals, they certainly experience periods of rest. This aquatic rest involves distinct behavioral and physiological changes.
Defining Sleep in Fish
Sleep in fish is characterized by a notable reduction in activity and responsiveness to external stimuli. During these periods, a fish’s metabolic rate and breathing often slow down considerably. They may adopt specific resting postures, such as hovering motionless in the water column, settling on the substrate, or finding a secure spot within their habitat, like crevices or plants. Most fish do not possess eyelids, so their eyes remain open during these resting states, allowing them to maintain vigilance against threats.
Scientists often refer to this state as “rest” rather than “sleep” due to the absence of the typical brain wave patterns found in mammalian sleep, which are associated with the neocortex that fish lack. Despite these differences, fish do follow regular sleep-wake cycles, often influenced by light and darkness, similar to circadian rhythms observed in other animals. Some species are diurnal, resting at night, while nocturnal fish are active at night and rest during the day.
Observing Fish Sleep
Sleep-like states in fish are identified by specific behavioral and physiological changes, primarily prolonged inactivity. During these periods, fish exhibit a reduced responsiveness to stimuli, meaning they take longer to react to external cues such as sudden movements, light changes, or gentle nudges. Some fish, like the Spanish hogfish, can even be gently handled without immediately waking.
Studying fish sleep involves monitoring these behavioral cues and, in some cases, brain activity. Researchers have used advanced techniques, such as fluorescent polysomnography in translucent zebrafish larvae, to observe neural patterns. These studies have revealed that zebrafish can exhibit two distinct sleep stages, one similar to slow-wave sleep and another resembling rapid eye movement (REM) sleep, though without actual eye movements. If sleep-deprived, fish like zebrafish experience a “sleep rebound,” resting for longer periods to compensate for the lost rest, which further supports the existence of a sleep-like state.
Do All Fish Sleep?
Most fish species engage in some form of rest, though variations and exceptions exist. Many pelagic fish, such as tuna and certain shark species, must continuously move to breathe. These “obligate ram ventilators” rely on swimming forward with their mouths open to force water over their gills for oxygen. For these fish, stopping would mean suffocation, so they achieve rest through unique adaptations like unihemispheric sleep, where one half of their brain rests while the other remains active, allowing them to continue swimming slowly. Some sharks, like the draughtsboard shark, can also rest on the seafloor and still breathe using a method called buccal pumping or spiracles.
Blind cavefish, like the Mexican cavefish (Astyanax mexicanus), are another example. These fish live in environments of perpetual darkness and have significantly reduced sleep durations, often sleeping for as little as two hours a day, compared to their surface-dwelling relatives who sleep for around eight hours. This adaptation is linked to their environment, where scarce food and lack of visual input make prolonged sleep less advantageous. The diversity in resting behaviors across different species highlights how fish have evolved varied strategies to balance rest with the demands of their aquatic lives.