Sharks can sleep, and often opt to keep their eyes open while they do, according to new research published in Biology Letters.
Because some sharks must swim constantly to keep oxygen-rich water flowing over their gills, it has long been rumored that they don't snooze at all. Scientists in Australia have now documented a species of bottom-dwelling shark sleeping for the first time, upending the long-standing debate.
"Until now, sleep in sharks was completely unstudied and unknown,” says study author Michael Kelly, an ecophysiologist at La Trobe University, to Robyn White for Newsweek. “Sharks are a particularly important group as they are the oldest living jawed vertebrates—a trait they share with us.”
In their study, the team of scientists observed draughtsboard sharks, a nocturnal shark native to New Zealand, apparently sleeping during periods of rest. Unlike great whites and tiger sharks, which must keep swimming to ventilate their gills, draughtsboard sharks are a species of buccal pumping sharks, which manually push water over their gills to take in oxygen while stationary.
To see if the animals were in fact asleep, the team analyzed the metabolism and posture of seven draughtsboard sharks over 24 hours. When the sharks were resting for five minutes or longer, their oxygen consumption dropped, suggesting the animals are drifting off to sleep.
“We know that a drop in metabolism is a telltale sign of sleep in many, many other animals,” Kelly tells the Guardian’s Donna Lu.
Because draughtsboard sharks are ambush predators, the three-foot-long animals usually position themselves with their fins out and head up. But when the sharks fell asleep, the researchers noticed the fish changed their posture by flattening and nestling closer to the ground.
The sharks occasionally slept with their eyes shut, though mostly during the day. At night, they opted to keep their eyes open more often, leading researchers to suspect the sharks’ eye closure may have more to do with light than the sleep state itself, according to Science Alert’s Tessa Koumoundouros. Around 38 percent of sharks kept their eyes open at night, even when they appeared to be asleep.
"We have provided the first physiological evidence of sleep in sharks," the team writes in their paper.
The work builds on an earlier study by the same research team that investigated behavior signals of sleeping sharks. In that study, they found it was more difficult to spur the sharks into movement if they had been still for a long time, suggesting the animals were sleeping, Veronique Greenwood reports for the New York Times.
"Not only do sleeping sharks have reduced responsiveness to stimulation, they also have lower metabolic rate," the authors note in their study.
The team notes more research will be needed to see if other species sleep like the draughtsboard shark. Next, they plan to analyze the sharks’ brain activity while they sleep to learn more about their wake and rest states.
"[Sharks] have 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,” Kelly tells Newsweek. “Understanding how and why these animals sleep will provide important insight into the function of sleep and how it has evolved over time."