Is it true that sharks never sleep?

It is widely believed that sharks are unfamiliar with the sleep state because they need to constantly remain in motion for their gills to function. We decided to check what scientists know about the rest of these fish.

About how sharks never sleep write like regular internetusers, and major media. For example, RIA Novosti asserts: “Sharks don’t sleep at all. They are in motion from the first to the last second of life.” "Teacher's Newspaper" in the article "It's all about..., or Why birds don't fall and fish don't drown - top 10 children's "whys"" tells, that sharks “have to move without sleep or rest, so as not to take water into their mouths and suffocate.” And the magazine "Around the World" reports, that “sharks generally need to be on the move all the time, since their relatively imperfect gills must be constantly washed by water.” Questions "why sharks don't sleep" and "why sharks never stop" also interested users various sites questions And answers.

Sharks, together with rays, are a superorder of cartilaginous fish. Scientists know more than 500 species of sharks - from the deep-sea small Etmopterus perryi from the genus of black spiny sharks, size which only 16 cm, to the whale shark, reaching 19 m long. Sharks existed another 400 million years ago. Most known species live in sea salt water, but there are also those that are found in freshwater bodies. 

Like other fish, sharks breathe by extracting oxygen from the water they pass through their gills. Externally in front of the pectoral fins located from five to seven gill slits, and inside, in the walls of the pharynx, are rows of gill filaments that are located on the gill arches. It has long been thought that due to weak jaw muscles, sharks can only use passive, or flow-through, ventilation of the gills - water enters the body when the animal moves. Accordingly, if the fish stopped in place, including to take a nap, it would have to suffocate. 

However, not everything is so tragic. Off the coast of the Yucatan Peninsula there is an entire cave called the Cave of Sleeping Sharks. This is a unique place discovered in 1969 by local fisherman Carlos García Castilla. He noticed that sharks swim into this cave and stay inside for a long time. He told Ramon Bravo, an ecologist and diver, about his observation. After diving into the cave, they discovered a blunt-nosed shark, also known as a bull shark, lying motionless on the bottom and appearing to be sleeping. 

Bravo managed to photograph this unique phenomenon and provided the images to scientists specializing in shark research - Shelton Applegate and Eugenie Clarke, famous under the nickname Lady Shark. The world scientific community became interested in the unusual location, and in 1970 Ramon Bravo became a guide to the cave for the famous French ocean explorer Jacques-Yves Cousteau.

Scientists think, that due to the influx of fresh water from the land, the water in this place is less salty, and underwater springs from the bottom of the cave saturate it with small bubbles of oxygen, which can have a hypnotic effect on the fish. Also a system of underwater canals does the water current is more active, and the sharks can relax into stillness. However, divers note that some fish follow people with their eyes, so they cannot be considered sleeping. At the very least, the Cave of Sleeping Sharks clearly demonstrated that sharks can stay in one place for a long time and not die from suffocation.

Scientific evidence of sleep in sharks has only recently emerged. A team of scientists from the University of Western Australia, led by ecophysiologist Michael Kelly, studied different species of sharks endemic to the region, which were placed in special aquariums that simulated their usual environment. In 2020, researchers shared the first conclusions. During certain periods, Australian bull sharks and New Zealand cathead sharks (also known as checkersharks) required more electrical stimulation to respond to a stimulus than during others. Since one of the signs of sleep in animals is an increased threshold of arousal, this observation allowed scientists to declare that animals sleep during such periods. 

In 2022, the same team of scientists shared Another important discovery is that during the rest period, sharks have a reduced metabolic rate, which also applies to signs of sleep. During this time, the sharks also consumed less oxygen - this is consistent with the fact that breathing slows down during sleep in all animals, including person. At the same time, sharks did not always close their eyes during periods of rest - in bright light, simulating daylight, almost all individuals preferred to sleep with their eyes closed, but at night, 38% of the fish kept their eyes open, although all other indicators indicated that the animals were sleeping. “Eye closure is most likely due to an external factor, such as the presence of light, rather than sleep,” suggest scientists. Thus, Kelly and team's findings completely overturn the belief that no species of shark can sleep.

During the course of evolution, different species of these animals have developed their own ways of adapting the body to both rest and pumping oxygen. For example, katrans, or spiny sharks, have nerves that provide coordination of movements during swimming, are not in the brain, but in the spinal cord, which allows fish to swim (hence, breathe) and sleep at the same time. There is also hypothesisthat some species of sharks can swim and sleep at the same time, turning off only one hemisphere of the brain. In the same way, for example, sleeping dolphins and fur seals when in the water. 

Thus, the belief that sharks are doomed by nature to insomnia and perpetual motion is nothing more than a myth. Scientists observed motionless sharks that breathed due to a special water flow, and sharks that showed the usual signs of sleep - an increased threshold of excitability, slower breathing and slower metabolism. Evolutionary nature has provided various mechanisms to give this type of fish a rest.

Cover image:  Image by Andrea Bohl from Pixabay

Not true

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