Many fish owners have tapped on their aquarium and hoped little Bubbles was merely taking a nap inside the tiny castle.

It can be difficult to determine whether a fish is sleeping, and even scientists have struggled to understand the sleep process of fish in the past.

In recent years, technology has made it possible for researchers to scan fish brains and learn how they snooze.

Do Fish Sleep?

“Fish sleep, but not the same as humans or mammals,” says Jared Guthridge, the aquarium curator at the Alaska Sealife Center (ASLC) in Seward.

The ASLC is home exclusively to Alaskan species, including many types of rockfish that like to hover near the bottom of their exhibit among the substrate. Others like to rest on top of the substrate on one side so it looks like they are lying flat.

 Guthridge says sleep can look different from one fish species to the next.

“Many species are catching little catnaps, so to speak, throughout the day,” he says. “Some species that associate with substrates that are in a ‘deep sleep’ will find a crevice at night and appear washed out or paler in color. When a light is turned on or shone on them, they take a bit to react.” 

Read More: Do Fish Feel Pain?

Recognizing Sleep Patterns in Fish

Sleep is a near-universal behavior, but how a living creature sleeps and for how long varies greatly from one species to the next. Fish sleep behavior has been difficult for scientists to study, particularly because fish lack eyelids. Fish don’t yawn, stretch, and close their eyes for a well-needed nap. Fish sleeping behavior is far more subtle, and in the past, scientists couldn’t be certain about what they were observing.

In the past two decades, scanning technology has allowed scientists to identify fish sleeping behavior through brain activity. Zebrafish (Danio rerio) have been preferred sleep subjects for scientists because they are translucent in their larval state, which is helpful for scanning. In their larval state, they only have 100,000 neurons, which makes neuron mapping much easier than with other species. (In contrast, the human brain has about 86 billion neurons). 

Using zebrafish, scientists have been able to better understand fish sleep states. In a 2019 study in Nature, scientists analyzed neuronal recordings and found that zebrafish have two distinct sleep states. They described these states as “slow bursting sleep” and “propagating wave sleep.”

The propagating wave sleep could be compared to the rapid eye movement (REM) sleep stage in humans. Fish, however, do not have a neocortex in their brains so the propagating wave sleep is not the same as REM sleep.

Read More: Meet the Mudskipper: The Fish That Walks on Land

Do Sharks Sleep?

Researchers have also been able to identify sleep and rest behavior in sharks. In a 2022 study in Biology Letters, researchers analyzed metabolic rates to determine when sharks were snoozing.

The scientists caught seven draughtsboard sharks (Cephaloscyllium isabellum) off the coast of New Zealand. The sharks were kept in natural light conditions for two weeks prior to the start of continuous data collection for a 24-hour period.

Looking at the sharks’ metabolic rates, the scientists were able to distinguish between sleeping and resting periods. Once the sleep patterns were established, the researchers analyzed data regarding the sharks’ physical behavior to identify which behaviors were consistent with sleep.

The draughtsboard shark, for example, has a third eyelid. But they don’t always close their eyes to sleep, and researchers found that sharks were more likely to close their eyes to sleep during the day than at night. This meant that open eyes at night weren’t an indicator the shark was awake. 

Read More: These Animals Get Creative To Get Some Sleep

Comparing How Fish and Sharks Sleep

Although sharks with a third eyelid can get some shut-eye, fish don’t have that option. “They lack eyelids, so light stimuli can have an impact. They don’t necessarily need darkness to sleep,” Guthridge says.

Regardless, the ASLC turns off the lights at night, and because Alaska has long summer days and long winter nights, they are mindful of having the light schedule mirror what their aquatic residents would experience in the wild.

“We try to stay consistent with what the natural temporal photoperiod would be,” Guthridge says.

Keeping the light patterns consistent with the outside world is important for a fish’s biological clocks. “Many species spawn only once per year, and disruptions in photoperiod can significantly throw the timing off,” he says.

Read More: Animals That Sleep the Least and the Most

How to Know if Your Fish Is Sleeping

Without metabolic measures or scanners that measure brain activity, the average aquarium owner might not be able to tell what exactly their goldfish is doing when it goes inside the little castle and just stays there.

To determine when their fish is snoozing, aquarium owners will have to do what scientists have done for centuries — old-school observation.

When fish are resting, they will have little movement. They also might not respond immediately to stimuli like light, a tap on the tank, or a few food flakes appearing at the water line. Identifying a pet fish’s patterns can help an aquarium owner better predict their behaviors and avoid those scary moments when Bubbles appears motionless inside that tiny castle.

Read More: Meet the Sheepshead — The Fish with Human Teeth

Article Sources

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Emilie Lucchesi has written for some of the country’s largest newspapers, including The New York Times, Chicago Tribune and Los Angeles Times. She holds a bachelor’s degree in journalism from the University of Missouri and an MA from DePaul University. She also holds a Ph.D. in communication from the University of Illinois-Chicago with an emphasis on media framing, message construction and stigma communication. Emilie has authored three nonfiction books. Her third, A Light in the Dark: Surviving More Than Ted Bundy, releases October 3, 2023, from Chicago Review Press and is co-authored with survivor Kathy Kleiner Rubin.