Do fish sleep? |

The simple answer to this question is, yes, fish do sleep. But, some fish do not sleep, and fish sleep is very different between different fish species and very different from the sleep other animals experience. These differences are so great that the NOAA says, in a very brief summation of the topic, that "fish do not sleep," at least not "in the same way that land mammals sleep." 

Much recent research has been done (is being done) on fish and sleep in hopes that it may lend insight into human sleep disorders. One example of this is the inquiry into the relationship between insomniac behavior in Zebrafish and insomnia and narcolepsy in humans. The Zebrafish that don't have a functioning brain receptor for hypocretin, due to a genetic mutation, have abnormal sleep patterns--abnormal as compared to non-mutational Zebrafish--sleeping significantly less often and less long (about 30 and 50 percent). Since hypocretin has been linked in research studies to human narcolepsy, and since hypocretin is linked to insomniac behavior in Zebrafish, fish sleep research has an important relationship to human sleep disorders.

Sleep in fish takes on several different aspects depending upon the specific species of fish being discussed. Some popular and noteworthy examples of these wide differences between fish species are the sleep of sharks, Zebrafish, Spanish Hogfish and parrotfish. Sharks move while they sleep because they need forced gill ventilation (they are "obligate ram ventilators"), which most other species don't need. Since sharks cannot stop to deeply rest, their sleep doesn't approach a recognizable concept of sleep; it is described as more of a "snooze." Zebrafish drop their tails and either float just below the water's surface of sink to the bottom. Their sleep cycles can exhibit sleep disorders, like insomnia and sleep deprivation. The Spanish Hogfish, on the other hand, goes into such deep sleep that they can be lifted from the bottom without being aroused until they break the surface of the water. Parrotfish create a sleeping pouch by spewing mucus that surrounds them as they rest at the bottom. ["Do fish sleep?" The Fisheries Blog.]

While the transparent eyelid fish have--called the adipose, having a small slit for the pupil--is significant in adaptation to their aquatic environment and ultra-violet light conditions, no well publicized studies have as yet made a link between the adipose and sleep in fish. It serves a protective purpose without any known purpose linking it to sleep in those fish that possess the adipose, as some species do not have the adipose. For example, the Mugil cephalus in the waters off Acapulco is reported by the U.S. Fish and Game Commission as having no adipose eyelid. (The adipose eyelid in fish is transparent; other animals, like reptiles, have a similar inner eyelid that is translucent, rather than transparent, and is called a nictitating membrane.)

Research has shown that some fish that do not sleep have unique physical requirements or live in unique sensory environments that either prohibit or make unnecessary the function of sleep. Unique physical requirements, specifically forced gill ventilation, preclude "sleep," making rest-while-in-motion a necessary substitution. If sleep is a restorative function--restoring metabolic, mental and physical systems--as it is defined as being, then species with uniquely reduced sensory input, it is theorized, have reduced sensory load, thus no need to sleep. Sharks (and tuna) need continually stable activation of gill function even while they rest, so do not sleep. Species that live in caves and are blind have decreased sensory input, and don't sleep. Species that swim in schools don't sleep because, it is theorized, the fish in the center have reduced sensory and physical input, so don't need to sleep ("Do Fish Sleep?").

Fish sleep characteristics:

• regularly occurring cycles of activity and reduced activity


• reduced physical activity


• reduced metabolic activity


• reduced memory activity


• release from sensory input from environment    


• restorative function for physical, metabolic and memory activities

Some identifiers of sleep in fish are:

• more similar to day dreaming than to true sleep (except in species like the Hogfish)


• no REM cycle


• no eyelid closure (no eyelids, except for sharks, and these are not used during shark's moving "sleep")


• minimal movement beyond what is needed for stabilization in the water


• some species float, some burrow or nest, some sleep deeply, some do not


• sleep quantity varies per species and according to individual activity


• sleep cycle in some species is related to light and dark cycles, some with a diurnal (daytime) activity cycle and some with a nocturnal (nighttime) activity cycle


• fish species (except the deep-sleepers) are thought to remain alert to danger while sleeping


• sleep cycles are disrupted by spawning and migration 


• fish can suffer sleep deprivation if disruptions occur