Fish do not sleep in the same way as humans and other mammals. Fish do however enjoy the same biological benefits of sleep. Similar to what we call sleep, fish enter an extended rest period where their brain activity lowers, and their metabolism and respiration slow down. Sleep research in fish is an active field of study and there is still much to be discovered about the details of how and why fish sleep.
Fish sleep could be classified as a lighter form of the deeper sleep humans experience. There are lots of similarities between fish and mammal sleep, but many of the deeper sleep functions of mammals are not exhibited in fish.
Fish sleep is different from other vertebrates for several reasons. Some of the major physiological reasons why fish sleep is different are as follows. (1) Fish do not have eyelids. This does not seem to prevent a good night’s sleep amongst studied zebra danios. (2) Some fish, like sharks, must remain in motion while sleeping so oxygenated water can continue to pass over the gills. (3) Most fish sleep at night, like most humans, but there are several nocturnal species that sleep during the day and are awake at night. (4) There are even some blind, cave dwelling species of fish that still sleep, but do not follow day-night sleeping cycles because their environment is always dark. (5) Fish do not have a neocortex, which is the main part of the brain associated with slow wave and Rapid Eye Movement (REM) sleep in mammals. Although fish still exhibit a rudimentary form of slow wave and REM sleep.
Fish Sleep Research
Most research into fish sleep is conducted by monitoring behavior, including eye-movements, sleeping position, and difficulty of stimulation when sleeping or increased arousal threshold (Zhdanova 2011). Huang and Neuhauss in 2008 found the eye-movements of zebrafish in sleeping positions were prominent and could be easily monitored and quantified. Sleeping positions were found to be consistent with horizontal, near the bottom of the tank and floating with the head elevated as the most common.
An increased arousal threshold refers to the increased difficulty in waking up, stimulating, or arousing a resting or sleeping individual. A feature of sleep in mammals is a higher arousal threshold. This means it is more difficult to stimulate a sleeping animal than one that is awake. This has also been found to be true in fish (Zhdanova 2011).
These sleep behaviors are triggered by several neurological systems in the brain. Many of which fish share with mammals. The neuroanatomical structures and neurotransmitters and their receptors involved in sleep regulation in mammals are also present in fish (Zhdanova 2011).
Just like in mammals, zebrafish have a pineal gland which produces the sleep hormone melatonin, the primary neurotransmitter responsible for sleep in most vertebrates and fish. Fish use melatonin to regulate sleep within homeostatic and circadian mechanisms, and the hypocretin/orexin system; all important parts of the sleep-wake modulator (Zhdanova 2011).
Melatonin directly influences the circadian system, a mechanism of sleep regulation in most animals. Animals that exhibit sleep behavior also possess genes and proteins which regulate an individual’s circadian rhythm and consequently, their sleep-wake cycles (Zhdanova 2011).
Zebrafish also appear to be diurnal (awake during the day), a common pattern of animals that sleep for extended periods.
Adult zebrafish sleep less overall than juveniles. Most sleep occurs at night. There are sleep periods where fish will lose their buoyancy and gradually sink to the bottom of the tank (Zhdanova 2011).
We can conclude that any animal with melatonin also has a circadian system and therefore experiences a sleep-wake cycle. How pronounced that cycle is (light or deep sleep) depends on the species.
More evidence that fish sleep is their response to sleep deprivation. We know fish have homeostatic responses to sleep deprivation; a phenomenon that would not occur if they did not have homeostatic regulation of sleep (Zhdanova 2011).
When zebrafish were kept under prolonged light periods, they exhibited signs of sleep deprivation after the periods end. This would only occur if fish had a sleep-wake cycle. After the prolonged periods ended, the fish returned to normal sleep cycles, further signifying the presence of a circadian regulatory system (Yokogawa 2007).
Even more interesting is how during the prolonged light periods, fish exhibited hyperactivity, suggesting an evolutionary adaptation of some kind (Yokogawa 2007). This phenomenon is not seen in mammals.
Fish Do Sleep
There are major similarities in the sleep behavior of mammals and fish. The degree to which fish sleep as defined by mammals, vs undergoing an extended rest, is yet to be determined. Science is not sure if fish exhibit the increased depth of sleep or distinct stages of sleep like some other vertebrates (rapid eye movement in mammals). However, reduced or absent movement, specific restful postures, and increased arousal threshold suggest fish do experience and benefit from extended periods of rest. Whether we call it sleep or not is a matter of semantics. However, there is strong evidence fish benefit from an extended rest period just like most other vertebrates.
Fish have developed mid and hind brains, brainstems, and sensory and motor circuits. They do not have as highly developed forebrains as mammals. Mammalian forebrains are responsible for many different sleep functions such as slow wave and REM sleep. Fish still exhibit a rudimentary and alternating form of slow wave and REM sleep, but without the eye movement.
The nature of sleep and its function is most likely as diverse as the taxa that exhibit it. Mammals, birds, reptiles, fish, and other vertebrates may exhibit different types of sleeping behaviors because sleep provides different functions for each taxa. The array of similarities would suggest practicing extended rests and sleeping has an evolutionary advantage.
The future of fish sleep studies is in new imaging technology which has allowed science to observe the neuron behavior in the brains and vertebrae of sleeping fish. This allows further insight and greater detail into the exact mechanisms behind how fish sleep (Zhdanova 2011).
The conclusion seems to be that while fish do sleep, they do not experience the depth of sleep and variety of sleep patterns of an evolutionarily later taxa, like mammals.
You can ensure your fish get a good night's rest by making sure your light schedule is consistent. As we've learned, fish have a circadian rhythm. When the photoperiod for each day is consistent, your fishes' sleep will be as well. Even better is ramping the light intensity up and down to simulate dawn, noon, and dusk. This brings about circadian responses over time and is more realistic to your fishes' natural environment.
Huang, Y., & Neuhauss, S. (2008) The optokinetic response in zebrafish and its applications. Frontieres in Bioscience (13) 1899-1916.
Yokogawa T, Marin W, Faraco J, Pézeron G, Appelbaum L, Zhang J, et al. (2007) Characterization of Sleep in Zebrafish and Insomnia in Hypocretin Receptor Mutants. PLoS Biol 5(10): e277
Zhdanova, I.V. (2011) Sleep and its regulation in zebrafish”. Reviews in Neuroscience. Vol. 22(1): 27-36.
Zhdanova, I.V., et al. (2001) Melatonin promotes sleep-like state in zebrafish. Brain Research. 903 (1-2): 263-268.