Swimming, hunting, and digesting is hungry work and for sharks, and no doubt quite tiring too, but do sharks sleep? It’s a seldom-studied topic, but researchers believe they have found the first physiological evidence of sleeping sharks.

“Investigating sleep in animals is very challenging,” lead author the new paper published in Biology Letters Mike Kelly, a PhD research fellow from the Circadian Rhythms and Sleep Lab & Translational Neuroscience Lab at Simon Fraser University, told IFLScience.

“Doing it in a saltwater medium with animals that are very unlike us is even trickier. It might surprise you that of the circa 35 animal phyla recognized, sleep has only been described in representatives from six. That should not be taken as evidence for lack of sleep but rather a lack of research!”

It seems sharks aren’t alone in being something of a mystery when it comes to sleep (though it's possible someone caught a great white napping on camera). As they don’t do well in captivity and often receive less attention and funding due to public opinion around them, it’s been particularly tricky establishing if sharks sleep.

Draughtsboard sharks (Cephaloscyllium isabellum) were employed for the study. Researchers aimed to identify metabolic signatures of sleep and look out for sleep indicators like eye closure and laying down (aka postural recumbency). They observed the sharks during what looked like sleep, as well as while restful and actively swimming over 24 hours.

Their results showed that what was expected to be the sleeping phase of a draughtsboard shark did indeed link up with a lower metabolic rate and a flat body posture, supporting the idea that sleep is a way to conserve energy for these animals.

Sharks don’t sleep with their eyes closed, however, at least not in the case of draughtsboards. While their sleep might not look exactly how we imagined, their position in evolutionary history makes the findings highly significant for the evolution of sleep in general.

“I think it is fascinating to get a glimpse into the possible beginnings of sleep in vertebrates,” said Kelly. “Sharks have been swimming in our oceans for over 400 million years and have changed very little in that time. They are the oldest living jawed vertebrates (a trait they share with us)."

“Knowing that they sleep (and maybe how) gives us an amazing peek into the evolution of sleep. I also get a kick out of animals that sleep with their eyes open! […] It feels amazing discovering something so convincing for the very first time.”

Sleep is widely considered to be a behavior that promotes energy conservation through extended restfulness, and for that reason is expected to exist among a lot of animals even though it has only been scientifically proven in so few. So, if it works for draughtsboards, do other sharks sleep?

“I would say it's extremely likely if not almost certain,” said Kelly. “Let me put it another way: if we find that there are sharks out there that don't sleep, then they would be the first animals that we would have ever found to not require sleep. For this reason, I believe we will find in time that all species we investigate sleep. The question then becomes: do they all sleep in the same way?”

Kelly believes the work has “only just touched the tip of the iceberg” and hopes, with his team, to now investigate other sharks including obligate constant swimmers like makos and tiger sharks.

Sleep-swimming sharks? Now there's sci-fi in the making.