Sharks benefit from some shuteye, and biologists in Australia and New Zealand think they’ve begun to understand why. Their experiments suggest sharks sleep to save energy between meals. The discovery might lead to a clearer model of how sleep evolved and developed among vertebrate animals.
There is plenty of evidence of sleep in vertebrate animals, so it’s likely that sharks snooze, too. However, until recently the only evidence came from scuba divers’ anecdotal reports of dozing sharks, says Michael Kelly, at the time a biologist at La Trobe University. Kelly and his colleagues have spent the past few years putting those reports to the test through formal scientific studies, trying to nail down whether sharks do in fact sleep, and if so, what good it does them.
Their latest research focuses on the draughtsboard shark (Cephaloscyllium isabellum), a 1-meter-long species found in the coastal waters around New Zealand. The draughtsboard shark—named for the checkerboardlike markings on its skin—is nocturnal, and the researchers suspected it sleeps during the day. The team had already demonstrated that sharks in laboratory aquariums periodically show behavioral signs of sleep during the day, moving considerably less and reacting more slowly to mild electrical shocks delivered through the water. The researchers next wanted to see whether sharks that appeared to be slumbering had a slower metabolism, a hallmark of sleep in many animals.
The researchers housed seven sharks individually in sealed aquaria fitted with oxygen sensors. Then, over a 24-hour period, they monitored each shark’s physical activity while also recording the oxygen level in the aquarium water to assess how quickly the animal was using the gas to fuel its metabolism.
Not surprisingly, the sharks metabolized oxygen fastest while actively swimming. The rate dropped when they stopped swimming, and it dropped even further during behavioral signs of sleep—which the researchers defined as remaining motionless for at least 5 minutes with the body held in a near-horizontal position. On average, the oxygen uptake rate when the sharks appeared to be asleep was one-third that of an active shark, and one-half that of a resting shark, the researchers report today in Biology Letters.
The findings close the case on sleep in sharks, Kelly says. “Behavioral evidence for sleep can be misleading and challenged,” he says, but physiological evidence is harder to dismiss. Moreover, the study also hints at a reason why sharks sleep: to conserve energy while resting.
Kelly says confirmation of energy saving would be significant because sharks sit on one of the lowest branches of vertebrates’ evolutionary tree. As vertebrate animals have evolved and diversified into groups including reptiles, mammals, and birds, he says, the features and purposes of sleep have probably evolved and diversified, too. Sharks could provide a model for what sleep looked like in some of the earliest vertebrates, leading to a better understanding of this process of sleep evolution.
Amita Sehgal, a neuroscientist at the University of Pennsylvania’s Chronobiology and Sleep Institute, is convinced that sharks do sleep. The new study helps cement sharks’ place in a broader context of sleeping animals. “Sleep has now been discovered in many organisms, including invertebrates—fruit flies, worms, jellyfish, and hydras,” she says.
But Sehgal thinks the evidence isn’t strong that sharks sleep exclusively to conserve energy. Even in sharks, she says, “sleep is complex and could serve many functions.”
Not everyone agrees. Jerome Siegel at the University of California, Los Angeles, Center for Sleep Research argues all animals that sleep do so primarily to conserve energy. In recent years, many researchers have argued that mammals’ elaborate sleep cycles—which include deep sleep and rapid eye movement sleep—are important for memory consolidation and removal of toxins from the brain. But Siegel thinks the evidence for these additional functions is weak.
Deeper insight into shark sleep will come only with studies of the predators’ brain activity, argues Patrick McNamara, a neuroscientist at Boston University School of Medicine, who says the new study is still “extremely valuable for sleep science.”, In particular, McNamara would like to know whether sharks zonk out completely or keep one half of their brain alert, as most marine mammals do.
Kelly says his team is doing such experiments, although he acknowledges the work won’t be easy. A 2019 study with zebrafish hints at the possibilities: using fluorescent imaging techniques to track brain activity, researchers identified at least two distinct phases of sleep in the fish. But those experiments were made easier by the fact that zebrafish, unlike sharks, live in fresh water. “The logistics of recording brain activity in sharks is rather complex as electricity and saltwater make for a challenging setting for scientific experiments,” Kelly says.