Written By: Lana Adler
Updated March 26, 2021
From birds and bears to seals and sloths, all animals need rest. Although snoozing appears to be essential for the survival of most species (1), it can look quite different from one animal to the next. Human newborns can sleep up to 18 hours every day (2). On the other end of the spectrum, African elephants may only sleep for two hours per day (3)
Fish are a fascinating and diverse group of animals. It’s estimated that there are over 32,000 species of fish on earth (4) and there may be even more left to discover. While scientists understand a lot about fish, there are still many questions left to be answered about the biology and behavior of these aquatic animals.
Do Fish Sleep?
Sleep, or sleep-like periods of rest, are likely necessary for all animals. Although fish don’t sleep like humans, scientists believe that they do need regular periods of rest. To understand the sleep habits of fish, it’s important to first consider that there are two ways of defining sleep, one based on brain activity and another based on behavior.
- Sleep as Brain Activity: One way to define sleep is based on what occurs in the brain when we doze off. In a definition based on brain activity, sleep is measured by specific changes in the brain’s electrical activity. Evidence of sleep through measuring brain activity has only been found in mammals (including humans), birds and reptiles (5).
- Sleep as Behavior: Another way of defining sleep is based on an animal’s behavior. A behavioral definition suggests that sleep is a reversible state of reduced responsiveness (6), usually associated with a lack of body movements. Sleep rebound, meaning the tendency to sleep more after a period of sleep loss, is another behavioral sign of sleep. Many fish meet the behavioral definition of sleep.
While learning about the sleep habits of fish and other animals may help researchers more fully understand why we sleep (7), there are many challenges when studying fish. First, it’s difficult to measure brain waves underwater (8). Second, fish lack a part of the brain called the neocortex, which is where brain waves are measured in humans. Without the ability to measure the brainwaves of fish for signs of sleep, some researchers prefer to use the term “rest” for the sleep-like behaviors commonly seen in fish.
Why Do Fish Sleep?
Researchers know surprisingly little about why fish, humans, and other animals need sleep (9). While much about the purpose of sleep is still a mystery, common hypotheses can be separated into two categories: restorative and adaptive.
- Restorative Sleep Hypotheses: Restorative theories about the purpose of sleep focus on how sleep allows the body time to recuperate. While resting, the body repairs itself, processes new information and memories, and enhances the immune system.
- Adaptive Sleep Hypotheses: Adaptive theories suggest that sleep allows animals to better adapt to their environment. Sleep may have evolved to help animals avoid dangerous conditions like predators and to avoid having to compete in both daytime and nighttime conditions.
How Do Fish Sleep?
While the need for rest appears to be universal in the animal kingdom, the quantity and timing of sleep varies considerably. Like other animals, when fish sleep, and how much sleep they need, depends on a fish’s anatomy, diet, and the demands of their environment. Here are some examples of the sleep habits of different fish species:
- Zebrafish: Zebrafish are fresh-water minnows, often used in medical research (10). Like humans, zebrafish sleep at night. When zebrafish rest, they stop swimming and their breathing slows.
- Mexican Cavefish: The Mexican cavefish, also called Mexican tetra, has adapted to two different environments: rivers and caves. Scientists suggest that cave-dwelling populations of cavefish evolved to meet the demands of their unique environment (11), losing their eyesight and reducing their need for sleep during periods of starvation.
- Sharks: Sharks are an extremely diverse group of fish and live in a range of different habitats from salty oceans to freshwater rivers. Nurse sharks, a salt water species, appear to rest during the day and are slow to respond when disturbed during sleep.
Where Do Fish Sleep?
Water covers over 70% of the earth’s surface (12). With so many environments to inhabit, it’s understandable that fish sleep in a lot of different places. Some fish float in place, some wedge themselves into a secure spot in the mud or coral, and some even locate a suitable nest before getting some rest. Below are a few examples of the creative places that fish find to sleep:
- Sleep Swimming: Many fish need to stay moving in order to keep oxygen-rich water moving over their gills (13). Certain types of damselfish living in coral reefs actually increase the speed at which they move their fins while at rest.
- Floating in Place: Some fish don’t need to keep swimming while sleeping. Buccal pumping sharks, like the bullhead and Port Jackson sharks, breathe by using their mouth to push water through their gills. Without having to swim in order to get oxygen, buccal pumping sharks can stay still for long periods of time. The nursehound, a freshwater shark, sleeps with its eyelids half-closed and sometimes even rests its head against a rock.
- Tucked into Coral or Sand: Fish often take advantage of their environment to find protection while sleeping. Lemon sharks rest by lying motionless on the sandy ocean floor for extended periods of time. Parrotfish find a protected spot on a coral reef before sleep, then create a cocoon-like structure made of their own mucus (14) to protect them from predators while they rest.
Scientists continue to learn about the sleeping habits of this fascinating group of animals. While they may not sleep the same way as humans, fish need rest just like we do.
- https://pubmed.ncbi.nlm.nih.gov/22654183/ Accessed on March 22, 2021.
- https://medlineplus.gov/ency/article/002392.htm Accessed on March 22, 2021.
- https://pubmed.ncbi.nlm.nih.gov/28249035/ Accessed on March 22, 2021.
- https://pubmed.ncbi.nlm.nih.gov/26338871/ Accessed on March 22, 2021.
- https://pubmed.ncbi.nlm.nih.gov/31292557/ Accessed on March 22, 2021.
- https://pubmed.ncbi.nlm.nih.gov/18752355/ Accessed on March 22, 2021.
- https://pubmed.ncbi.nlm.nih.gov/29895581/ Accessed on March 22, 2021.
- https://pubmed.ncbi.nlm.nih.gov/31775150/ Accessed on March 22, 2021.
- https://pubmed.ncbi.nlm.nih.gov/32269596/ Accessed on March 22, 2021.
- https://irp.nih.gov/blog/post/2016/08/why-use-zebrafish-to-study-human-diseases Accessed on March 22, 2021.
- https://pubmed.ncbi.nlm.nih.gov/29405117/ Accessed on March 22, 2021.
- https://www.usgs.gov/special-topic/water-science-school/science/how-much-water-there-earth? Accessed on March 22, 2021.
- https://www.blm.gov/or/resources/recreation/mcgregor/files/how_a_fish_works.pdf Accessed on March 22, 2021.
- https://oceana.org/marine-life/ocean-fishes/queen-parrotfish Accessed on March 22, 2021.