Mystery Behind Shark Sleep Solved: They Nap With Both Eyes Open, Researcher Says

One of the great mysteries of the shark world has finally been solved, as an Australian researcher proved these underwater predators do sleep.

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, answered several questions about how sharks sleep.

At least one species of shark – and they suspect many others – regularly descends into sleep, Dr Michael Kelly and his colleagues found in the world-first discovery, according to 7News. The study also revealed sharks like to sleep lying down.

Draughtsboard sharks, also known as carpet sharks, even adopt a sleeping position as they relax and lie flat instead of sitting up on their pectoral fins.

The study comes amid beliefs that sharks don't sleep at all.

To conduct the study, seven draughtsboard sharks were collected from Hauraki Gulf in northeastern New Zealand. They were housed in an outdoor aquarium, and their movements were recorded.

The draughtsboard shark is native to New Zealand waters and lives among rocky reefs and can be found at depths of up to 13 feet.

Dr Kelly said they could unravel the mystery when they were able to match up behavioral signs of sleep, such as five minutes or more of inactivity. While swimming and resting, the sharks always had their eyes open. He stressed the findings were far more than a fun fact.

"We still haven't gotten our heads around why the hell it is that we spend one third of our lives in this weird state, where we are just kind of dead, kind of unconscious," he said. "We know it clears brain waste, and it helps synaptic connections.

"But why are we tired? And why do you have to go into this weird state? Why couldn't you say, just sit on the couch and chill?

"Why do you have to completely switch off, have to lose consciousness? Why is it that your sensory input is basically switched off? We don't know that," he added.

Marine animals, like whales and dolphins, and birds also share the ability to rest one half of the brain, while the other half remains alert, 7News reported.

"We actually don’t know whether there’s one all encompassing reason that all animals sleep. But the weird thing is every animal we’ve ever studied so far sleeps and it seems to have persisted across all of evolutionary time," Dr Kelly noted.

Draughtsboard sharks are able to sleep because of being able to breathe with their cheeks – an ability known as buccal pumping. Dr Kelly wants to further the study by analyzing what sharks without that power do to sleep.

"We started with the buccal pumping species because they stop and move. That’s an easier starting point than an animal that moves all the time," Kelly explained.