Some fish do, while others, such as some sharks, don’t. In humans and many land mammals, sleeping would mean eye closure and the presence of typical patterns of electrical activity in the brain, including the neocortex. But fish lack eyelids and a neocortex. So, the fish that sleep do not sleep in the same way that land mammals do, they simply rest with their eyes open.
Researches show that fish may reduce their activity and metabolism while remaining alert to danger. Some fish float, some stuff themselves into a secure spot in the mud or coral, and some even locate a suitable nest. These periods of suspended animation are comparable to that of sleep in people. The nature of fish “sleep” is still an area of active research.
What about shark? There are over 400 species of sharks. Some need to move pretty much all the time to keep water moving over their gills so that they can breathe. These sharks are obligate ram ventilators. These include the great white, the mako and the whale shark. Though there have been some accounts that vouch that these sharks too rest, such ‘sleep’ has not been confirmed through studies.
Many fish species, such as bluefish, Atlantic mackerel, tuna, and bonito, also swim continuously and do not show signs of sleep.
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