Sharks sleep? Scientists find ocean predators take naps with their eyes open!

BURNABY, British Columbia — If you think ferocious sharks are always on the hunt for food, think again. It turns out these predators love to grab a nap when there’s nothing going on. However, researchers say some of these sharks actually sleep with their eyes wide open!

The new study is revealing the first pieces of evidence that sharks really do take a break from tracking prey and spend time sleeping or conserving energy. Comparative sleep physiologist Mike Kelly and a team of researchers examined the metabolic and behavioral signals of sleep among New Zealand’s draughtsboard sharks.

After 5 minutes of boredom, sharks go to sleep

Study authors found that the sharks had a lower metabolic rate and went into a “recumbent body posture” after being inactive for more than five minutes. Simply put, the sharks laid down on the ocean floor and went to sleep.

Kelly and the team studied the sharks for 24 hours at a time, tracking their metabolic rates and behavior while swimming, resting, and possibly sleeping. They believe their findings support the idea that sharks conserve their energy through sleep, just like other species.

“The sharks showed a dramatic decrease in their metabolic rate and an obvious postural change following five minutes of inactivity, which demonstrated a distinct separation between periods of quiet wakefulness and sleep,” Kelly, from Simon Fraser University, says in a media release.

Although scientists know that many animals sleep just like humans, including other fish species, Kelly notes that the new study provides the first evidence that elasmobranchs sleep as well. This is a fish group which includes sharks and rays.

Sleeping with one eye open!

Although sharks may sleep, they’re making sure their deep-sea neighbors don’t find out. Study authors found that the draughtsboard shark — or “carpet shark” — is capable of staying stationary, resting on the sea floor, with their eyes wide open while asleep!

Researchers note not every species of shark will have this ability. Others must swim constantly to stay alive.

carpet shark
Carpet shark (Photo: Paul Caiger)

Kelly previously studied the behavior of crocodiles for his Honors thesis at Australia’s La Trobe University. That research found that crocs also sleep with one eye open.

“I find the best way to overcome my sheer terror of these creatures of the deep is to get up close and personal with them,” Kelly adds. “Besides, better to be around them when they’re sleeping than when they’re ready to feed!”

Researchers are now learning even more about sharks through the first ever wireless recordings of brain activity from both anesthetized and free-moving sharks.

The findings are published in the journal Biology Letters.

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