How Do Sharks Sleep? With Both Eyes Open, Scientists Discover

Sharks sometimes sleep with both of their eyes open, scientists have discovered.

Scientists already know that sharks do not sleep or rest in the same way mammals do. For example, some sharks must swim constantly, even during sleep, in order to keep oxygen-filled water flowing over their gills. This means they probably enter a state of "rest" rather than sleep.

But a study from the School of Life Sciences at La Trobe University in Melbourne, Australia, and the Institute of Marine Science at the University of Auckland, New Zealand has shown that sleep in sharks differs from mammals more significantly than originally thought.

In mammals, sleep serves the purpose of conserving energy levels. The study published by the Royal Society set out to assess whether sleep in sharks plays the same role.

To do this, seven draughtsboard sharks were collected from Hauraki Gulf in north eastern New Zealand, and were housed in an outdoor aquarium and recorded. The draughtsboard shark, also known as the carpet shark, is native to New Zealand waters. It lives among rocky reefs and can be found at depths of up to 13 feet

Researchers then compared the metabolic rates and behaviors of the draughtsboard sharks when they are thought to be "sleeping," or resting.

While swimming and resting, the sharks always had their eyes open. Scientists found that when the sharks were inactive for more than five minutes at a time (indicating sleep) they usually had their eyes closed, but about 38 percent of the time they had their eyes open. Eye closure during sleep was also more common during the daytime, the authors said.

A stock photo shows sharks swimming. Some sharks have to keep swimming while sleeping. DigtialStorm/Getty Images

The study said: "This suggests that eye closure is more likely associated with an external factor, such as the presence of light rather than sleep."

It appeared that the shark's metabolic rate, body posture and time spent inactive are a more reliable way to tell if a shark is sleeping.

The fact that the sharks' metabolic rates were lower during sleep also supports the conservation of energy as a function of sleep in sharks, the authors said.

"Overall, lower metabolic rate and a flat body posture reflect sleep in draughtsboard sharks, whereas eye closure is a poorer indication of sleep. Our results support the idea for the conservation of energy as a function of sleep in these basal vertebrates."

Before this, concrete physiological indicators of sleep in sharks have never been recorded.

Co-author of the report, Michael Kelly at La Trobe University told Newsweek that this discovery is massive: "Until now, sleep in sharks was completely unstudied and unknown. Sharks are a particularly important group as they are the oldest living jawed vertebrates—a trait they share with us.

"They've been swimming in our seas for over 400 million years and have evolved very little in that time providing us with a peak into the past. Understanding how and why these animals sleep will provide important insight into the function of sleep and how it has evolved over time."

Kelly said that the next step is for researchers to understand how they sleep, as many shark species swim continuously.

"The best way to achieve this is to start looking at what is going on in the brain of these animals when they engage in sleep. This, in fact, is something we are currently working on," Kelly said.

This article has been updated to include quotes from Michael Kelly