Do fish sleep? Here’s what scientists have discovered so far

If you’ve ever gone snorkeling on a coral reef during the day, you likely saw waves of brightly colored fish of all shapes and sizes. But return to the same spot in the middle of the night, and most of those same creatures will have vanished. 

Which might make one wonder, do fish sleep ?

“All animals that have been carefully examined show sleep,” says Philippe Mourrain, a neuroscientist at Stanford University. 

Where things get complicated is when you start to ask questions about what we know about that sleep, says Mourrain. 

For instance, in 2019, Mourrain and his colleagues were able to observe zebrafish beneath a microscope as they snoozed in petri dishes of goo. This allowed the team to measure sleep patterns in the fish for the first time—patterns that looked surprisingly similar to our own deep sleep. The zebrafish even exhibited what looked like rapid-eye movement or REM.

(These slumbering fish may offer clues to the origins of sleep.)

With at least 20,000 species of fish known to science, behaviors vary widely across the many different kinds of fish. Some may exhibit features more like those we’re used to seeing from mammals, while others perform rest-like states that are unlike anything we’re used to.   

Most research suggests that brains need downtime, says Michael Heithaus, a marine biologist at Florida International University. Whether that’s to clear toxins or simply to rest is less understood.

But given that fish brains are very similar to our own, it’s no surprise that fish need to power down from time to time, too. 

Do sharks sleep?

You might have heard that if a shark stops moving, it will suffocate and die. And while that is true for some sharks, the rule is not absolute.

For starters, there are two ways that fish breathe, says Craig Radford, a sensory biologist at the University of Auckland in New Zealand. There’s what’s known as ram ventilation, in which the fish must swim to force water across its gills. However, some fish can breathe through buccal pumping, which is when the animal moves part of its jaw or body to pump water across its gills.

One example of buccal pumping can be seen from footage collected by divers in 2017 that revealed about 20 whitetip reef sharks huddled together in mostly motionless pile.  

(Watch how sharks ‘sleep’ in large groups.) 

“Our work on some sharks, mainly the carpet shark, shows that they shut their eyes, and we can arouse them using a stimulus,” says Radford. 

However, carpet sharks and most other kinds of fish have not yet been submitted to the kinds of thorough, empirical sleep testing as Mourrain’s team was able to conduct on the zebrafish. 

But even sharks that rely on ram ventilating have tricks.

“Some of these reef sharks may just kind of hang out in the currents during the day and get the oxygen flowing over their gills,” says Heithaus. “They’re just kind of hanging out.”

Other species appear to use a different sort of workaround.

Researchers in California have noticed that “baby white sharks will just swim in this big circle, very slowly,” says Heithaus. The patterns are so regular, the sharks leave markings in the sand on the sea floor which “kind of look like crop circles.”

While scientists can’t say for sure, it seems likely that the sharks are going into a sort of auto-pilot mode where they continue to swim and breathe, but in a way that allows their brains to recharge. This makes Heithaus believe that the sharks may be able to perform what’s known as unihemispheric sleep—also known as slow wave sleep—which means they sleep with one half of their neocortex at a time.  

Dolphins have been shown to do this, by the way. “They are kind of still thinking about breathing, coming up to breathe, and they’re pseudo-alert,” says Heithaus, “but that’s kind of sleeping for them.” Obviously, dolphins are mammals, not fish, but it’s possible whites and other sharks have evolved a similar behavior.

(Here’s how Wels catfish sleep in France’s Tarn river.)

Do fish sleep at night? 

Just like life on land, animals below the water’s surface keep all sorts of different hours. While many fish are active during the day and rest at night, the opposite can also be true.

Many kinds of reef shark are active at night, says Heithaus. 

What’s more, every single night, there’s a massive, global movement of plankton which sense the setting of the sun and rise out of the depths to feed near the surface. It’s known as the diel vertical migration, and it’s considered to be the largest migration of animals on Earth. 

As the plankton ascend, so do their predators, and the predators of those predators, in a parade of biomass that plays an enormous role in the way nutrients are cycled throughout the planet’s oceans.

Parrotfish use their mucus to sleep peacefully

While many fish simply find a quiet spot to drift off at night, parrotfish take bedtime to the next level—secreting themselves inside a layer of mucus like underwater sleeping bags.

“Parrotfish make these kind of mucus bags, and they just sit in them at night on the reef,” says Heithaus. 

The mucus layer seems to protect parrotfish against predators and parasites while the hefty turquoise fish relax. There’s also evidence that the ooze contains antibiotics that ward off pathogens, and the sealed chamber may also prevent the fish’s scent from attracting unwanted attention.

Interestingly, disturbing the mucus cocoon will alert the fish and send it swimming off at high speed—almost like an alarm clock.

(Read about more surprising animals sleep habits.)

Do deep sea fish sleep?

Even in the ocean’s depths, where sunlight never reaches, fish still probably undergo some form of rest—though it may not look like anything we’re used to.

“I suspect most deep-sea fish still have circadian rhythms as these don’t need to be entrained by light,” says Radford. “There are clock genes that can regulate circadian clocks, so light is not the only regulatory mechanism.”

In another example, blind cavefish in Mexico were found to perform a kind of sleep during the day—the animals stopped moving until they were poked, at which point, they’d start swimming again—while remaining more active at night. 

Yet more evidence that fish can be either diurnal, or day-active, as well as nocturnal, or even crepuscular, with activity peaking near dusk and dawn.

In the end, fish are more like humans than we often give them credit for. 

“Fish were the first vertebrates on this beautiful earth,” says Mourrain. “So it’s really not, ‘Do they sleep like us?’ It’s ‘Do we sleep like them?’”