The carpet shark lay on the bottom of the tank at the University of Auckland’s Leigh Marine Laboratory. When alert, it usually propped itself up on its pectoral fins like a sphinx or swam around looking for food. But now, it lay flat on its stomach, fins relaxed at its sides, the sinuous creamy back with the chessboard patterns barely moving. Apart from the fact its eyes were open, it looked for all the world like it was asleep.
Does a shark snooze? Until 2020, no-one had proved it either way. Many large sharks have to continually swim in order to breathe, ensuring seawater keeps flowing over their gills. Carpet sharks use a different breathing technique called “buccal pumping”, so they can chill out on the bottom of the sea without suffocating—but are they really asleep, or just resting?
Marine biologists Michael Kelly from Melbourne’s La Trobe University and Craig Radford from the University of Auckland decided to find out, catching seven carpet sharks in the Hauraki Gulf for their experiment.
New Zealand’s carpet shark, Cephaloscyllium isabellum, is not actually a true carpet shark, but a type of catshark. Some fishers, meanwhile, call the animals “dummy sharks” due to their habit of getting stuck in cray pots, but they’re “not a silly animal”, Radford says—the sharks can be trained to respond to stimuli, just like Pavlov’s dog.
Sharks rely strongly on their electric sense, so the researchers used a weak electrical current to test the animals’ arousal threshold: what it took to get them to react. “Just a small electrical current to give them a little shock, like ‘wake up!’” says Radford. If the sharks were lying still on the bottom, it took a more powerful current to startle them or get them to start swimming.
The researchers also found that the sharks’ metabolic rate dropped dramatically when they were in this sleepy pose—more circumstantial evidence that catsharks catnap, and that they do it to conserve energy.
To properly prove sharks slumber in the sense that we humans understand it, though, Radford says he would need to scan their brainwaves, to see if they show similar patterns to sleeping mammals or birds. That’s a feasible experiment to do with a carpet shark, but you can’t slide a great white into an MRI machine, and inserting electrodes into its brain at sea would be, um, logistically challenging. So, for now, we can’t definitively say that all types of shark are truly getting shut-eye—whether their eyes are shut or not.
Still, over the past few years, scientists have been looking for signs of sleep all over the animal kingdom, and found it almost everywhere. Fruit flies sleep; so do both zebrafish and zebra finches, and nematode worms doze in between moults.
Migrating birds and bottlenose dolphins put one side of their brain to sleep while the other hemisphere remains awake and alert, and rely on the collective power of the other half-awake brains in the flock or the pod to scan for threats and navigate. Jellyfish don’t even have brains—but they still sleep.
The ubiquity suggests sleep is an ancient adaptation, evolving even before brains did, says David Samson, an anthropologist and director of the Sleep and Human Evolution Lab at the University of Toronto in Canada. “Whatever sleep is for, it was likely one of the most early adaptations in complex life.”
And yet sleep comes at such a cost, leaving animals vulnerable to predators and with less time for more fun activities like eating and mating. For sleep to have persisted through hundreds of millions of years of evolution, then, it must be important, Samson says. “If sleep doesn’t provide an absolutely critical function, it’s the biggest mistake the evolutionary process ever made.”
Human sleep poses its own paradox, Samson says. We rely on it for our brains to work properly, suffer if we don’t get enough, and we’re (arguably) the smartest species—and yet we sleep the least out of all the primates. “Humans are weird,” Samson says. “Owl monkeys are the marathon sleepers of the primate order. They sleep 17 hours a night.” (That’s sprightly compared with koalas, which sleep 20 to 22 hours out of every 24. Elephants, on the other hand, get by on just two.)
If sleep is so crucial for brain function, why do people do so little of it, compared with our closest relatives? The mystery inspired Samson—who has spent his career watching various primates snooze, including orangutans, chimps, lemurs, and baboons—to look into the evolutionary origins of human sleep.
The ancestral primate, he says, was probably solitary, nocturnal, and slept in nests in trees. Even now, the only other apes that feel safe enough to sleep on the ground are male silverback mountain gorillas.
The only reason humans do is because we’re not all morning people.
Some of us are indeed larks, confoundingly perky before breakfast. Others of us… not so much. There’s a German word for that, of course: morgenmuffel, meaning roughly “morning grouch”—someone who stomps around troll-like for the first few hours of the day, but is quite happy to stay up into the wee hours, long after the larks have retired.
Samson thinks there’s a very good evolutionary reason for this disparity in chronotype, what scientists call a person’s inborn preference for certain times of the day. In 2016, he and his colleagues gave actigraphs—Fitbit-like wristwatches that detect light and movement—to 33 members of a Hadza group in Tanzania, who still live a traditional hunter-gatherer lifestyle. Then they monitored their sleep for a couple of weeks.
The researchers went into the study assuming there would be at least a couple of hours in the dead of night when no-one in the group was awake.
Instead, over the entire 20 days of the study, there were only 18 minutes—in total!—when everyone was deeply asleep at once. During the night, 99.8 per cent of the time at least one person was awake or sleeping lightly (the actigraphs couldn’t tell the difference).
Some people—generally the older members of the group—went to bed early and rose early, while younger people tended to sleep later and wake later, though there was individual variation as well. During the night, people roused from deep sleep at different times, then sank back into dreamland.
“If you were to look at any random minute throughout the night-time period, the median was eight and a half people awake,” says Samson. Those people act as sentinels, watching over the rest of the group so that everyone else can sleep soundly. Other studies have shown that humans get more REM sleep than other animals, increasing memory consolidation and social intelligence and boosting creativity and innovation.
Sleeping in groups made up of people with diverse chronotypes, then, acts as a kind of invisible snail shell, Samson says—a protective “mobile unit” that humans carried with them as they journeyed out of Africa and spread across the globe. “What it allowed was deep, high-quality, short, flexible sleep that allowed humans to become one of the most dominant species on the planet.”
Scientists are still nailing down exactly why sleep exists. There are likely many critical functions, and they probably vary from one species to another.
But broadly speaking, Samson says, animals sleep to conserve energy and regulate their immune system, improve cognition, and in some cases consolidate memories and prune unnecessary neural connections.
It looks like we need to be “offline” in order for our brain to carry out some of these important tasks.
Skipping sleep can have dramatic effects. When humans are alert, our brains produce a protein called beta amyloid, which is associated with impaired brain function and Alzheimer’s disease. Sleeping allows the body to break down the protein, says Samson. “You get about half a shot of this stuff in your brain every day that you’re awake. If you go a couple days without sleep, you’re working on two or three shots of this straight garbage that needs to be processed through the lymphatic system.”
Recent research has shown that animals suffer from sleep deprivation, too. Juliane Gaviraghi Mussoi’s research is tiring for everyone involved—scientists and subjects alike. To test the impact of sleep disturbance on birds’ songs and cognitive abilities, the University of Auckland doctoral student camped out next to a Melbourne aviary, mildly hassling eight wild-caught Australian magpies throughout the night to prevent them from sleeping. Every time one looked at risk of dropping off, she would tap on the cage to jerk it awake—apologising all the while.
In the morning, Gaviraghi Mussoi helped her Australian colleagues to test the birds on a learning task they had been trained to complete a few days before (the magpies had to remember which coloured lid hid a mealworm treat underneath, and then re-learn another colour when it was switched). Then, Gaviraghi Mussoi went to bed, leaving a digital audio recorder facing the aviary to record the sleepy birds’ songs.
The effects were profound. After a night of no sleep, only one of the eight birds managed to finish the memory task. The rest “had little to no motivation—even though they knew they would get yummy mealworms from it”, she says.
They also sang fewer songs, but sang for longer when they did, as if they were trying to make up for lost time. Gaviraghi Mussoi’s latest studies, on mynas, show that tiredness can also make birds sing out of tune—using more high-pitched or low-pitched notes than normal.
In the wild, birds’ sleep may be regularly disturbed by light pollution, fireworks or gunfire, or by living near a nightclub or motorway. If just one bad night can affect their songs and their brainpower, chronic sleep deprivation could be having population-level effects, making it harder for birds to find mates and defend territories.
Studies on bees have shown that when researchers shook a hive all night long—or, in another experiment, kept them in a specially designed “insominator”—they did funkier waggle dances, missing out some of the steps showing the way to the best nectar grounds. They were also more likely to get lost when returning to the hive the following day, with the scientists concluding the bees’ brains had not properly consolidated the day’s memories overnight.
This seems to be an important function of sleep for many animals, at least for those that learn aspects of behaviour and communication throughout their lifetime.
When young zebra finches sleep, for example, they move their syrinx—the avian equivalent of vocal cords—but no sound comes out. At the same time, the neurons in their brain light up exactly as if they were serenading a mate. It’s as though they are silently lip-syncing or rehearsing their songs, says Gaviraghi Mussoi, “as if it was a dream of singing”.
Aristotle and Darwin both speculated that animals dream, but the question was left unexplored by science for much of the 20th century—even as millions of dog owners watched their pets whimper, pedal their paws in the air, and appear to chase imaginary cats with their eyes closed.
In 2019, an octopus named Heidi was filmed sleeping in her tank, her skin changing in milliseconds from smooth luminous white to chocolate brown to a textured, seaweed-patterned mustard—the same camouflage transformations that would flash over her body while she was hunting in real life.
Was Heidi dreaming? The clip went viral, but scientists were divided on the question. Octopuses, after all, are separated from humans by hundreds of millions of years of evolution, and we can’t ask them about their experiences. Still, new studies keep finding evidence of REM sleep and dreamlike states in a surprising variety of animals.
During the first COVID-19 lockdowns in 2020, behavioural ecologist Daniela Roessler was stuck at home in Trier, Germany. Unable to go to the Amazon for fieldwork as she usually did, she turned instead to the literal field behind her house.
There she discovered a host of tiny critters behaving in fascinating ways she had never noticed before. “It was a goldmine,” she says. She picked up a couple of jumping spiders (Evarcha arcuata) and kept them in a box on her windowsill. At night, they hung upside down from the lid, each drifting on a single thread. To check whether this was a common behaviour in the wild, Roessler returned to the meadow after dark.
Jumping spiders were “dangling like Christmas ornaments all over the place”, she says. Sometimes they would sway in the wind, but if Roessler touched their silk, the spiders would spring to life and scramble up the line.
They certainly appeared to be sleeping, and Roessler planned to test the assumption with similar experiments to Radford’s to see if the spiders’ response to a stimulus was slower when they were in the sleep position. But she and her colleagues were instead distracted by something else.
Spiders can’t move their eyes like we can; to shift their gaze they move little tubes inside the eye containing their retinas. When jumping spiders first emerge from the egg, they’re two millimetres long and transparent. That meant the scientists could look right through their glass-like eyes and see the retinal tubes moving inside.
When the baby spiders hung themselves up to sleep, they would mostly dangle peacefully on their threads. Sometimes, though, their little legs would violently curl, their spinnerets would flinch, and their retinal tubes would wave around wildly inside their heads—just like a person in the throes of a nightmare.
“Having watched hundreds of videos of spiders in this REM phase, I do think that they’re experiencing something like a dream,” Roessler says.
REM sleep hasn’t been demonstrated in any terrestrial invertebrate before. Spiders doing it throws the dreaming door wide open, to “a huge chunk of the tree of life”, Roessler says. “We may be looking at something much more ancient, and much more universal.”
She has since found similar behaviour in other genuses of jumping spider, including Portia—famously smart and hairy spiders that can plan complex attacks and possibly even count to three. One spider-lover sent Roessler photos of a pet tarantula in repose, its legs twitching just like the jumping spiders’.
What might spiders dream about? Would their dreams have narrative, and would they confuse them with reality, as we do? Roessler hopes one day to glimpse something of jumping spiders’ interior world by changing elements of the creatures’ environment and seeing whether those changes are reflected in their behaviour when they sleep.
“Our idea was that we would give them a standardised visual input—like a video of a fly that always flies in a loop.” The spiders would have to watch the same film over and over for days, and then the scientists would check to see if their retina recreated the same pattern at night.
In a new book, When Animals Dream, philosopher David Peña-Guzmán suggests that dreaming necessarily implies consciousness and imagination—and that changes our moral obligations towards the creatures with which we share this planet. “To dream is to create a world out of nothing,” he told the Princeton University Press Ideas podcast. “There is something inherently fantastical about dreams—and that means you need imagination of some sort, even if it’s not exactly a human imagination, in order to dream.”
Sharks, reliant on their electric sense, might have electric dreams. Perhaps birds dream in ultraviolet. Near-blind orb-web spiders could dream in vibrations, Roessler points out.
Our own imaginations—bound as they are in language and the visual—may struggle to conceptualise these “worlds without human contours”, as Peña-Guzmán puts it. These wildest dreams could forever be beyond us.