Every animal with a brain needs sleep – and even a few without a brain, too. Sleep is universal, “even though it’s actually very risky,” Paul-Antoine Libourel, a researcher at the Neuroscience Research Centre of Lyon in France, said.
When animals nod off, they are at their most vulnerable to sneaky predators. But despite the risks, the need for sleep is so strong that no creature can skip it altogether, even when it is highly inconvenient.
Animals that navigate extreme conditions and environments have evolved to sleep in extreme ways – for example, stealing seconds at a time during around-the-clock parenting, getting winks on the wing during long migrations and even dozing while swimming.
For a long time, scientists could only make educated guesses about when wild animals were sleeping, observing when they lay still and closed their eyes. But in recent years, tiny trackers and helmets that measure brain waves – miniaturised versions of equipment used in human sleep labs – have allowed researchers to glimpse, for the first time, the varied and sometimes spectacular ways that wild animals snooze.
“We’re finding that sleep is really flexible in response to ecological demands,” Niels Rattenborg, an animal sleep research specialist at the Max Planck Institute for Biological Intelligence in Germany, said.
You could call it the emerging science of “extreme sleep”. Take chinstrap penguins in Antarctica that Libourel studies. These penguins mate for life and share parenting duties – with one bird babysitting the egg or tiny grey fluffy chick to keep it warm and safe while the other swims off to fish for a family meal.
Then they switch roles, keeping up this nonstop labour for weeks. Penguin parents face a common challenge: getting enough sleep while keeping a close eye on their newborns. They survive by taking thousands of catnaps a day, each averaging just 4 seconds long.
These short “microsleeps,” as Korea Polar Research Institute biologist Won Young Lee calls them, appear to be enough to allow penguin parents to carry out their caregiving duties for weeks within their crowded, noisy colonies.
When a clumsy neighbour passes by or predatory seabirds are near, the penguin parent blinks to alert attention and soon dozes off again, its chin nodding against its chest, like a drowsy driver. The naps add up. Each penguin sleeps for a total of 11 hours per day.
To remain mostly alert, yet also sneak in sufficient winks, the penguins have evolved an enviable ability to function on extremely fractured sleep – at least during the breeding season.
Poets, sailors and birdwatchers have long wondered whether birds that fly for months at a time actually get any winks on the wing. In some cases, the answer is yes, as scientists discovered when they attached devices that measure brainwave activity to the heads of great frigatebirds, large seabirds that nest in the Galapagos Islands.
While flying, frigatebirds can sleep with one half of the brain at a time. The other half remains semi-alert so that one eye is still watching for obstacles in their flight path. This allows the birds to soar for weeks at a time without touching land or water, which would damage their delicate, non-water-repellent feathers.
Frigatebirds cannot do tricky manoeuvres – flapping, foraging or diving – with just one half of their brain. When they dive for prey, they must be fully awake. But in flight, they have evolved to sleep when gliding and circling upwards on massive drafts of warm rising air that keep them aloft with minimal effort.
Back at the nest in trees or bushes, frigatebirds change up their nap routine. They are more likely to sleep with their whole brain at once and for much longer bouts. This suggests their in-flight sleeping is a specific adaptation for extended flying, Rattenborg said.
A few other animals have similar sleeping hacks. Dolphins can sleep with one half of the brain at a time while swimming. Some other birds, including swifts and albatrosses, can sleep in flight, scientists say.
Frigatebirds can fly 255 miles (410 kilometres) a day for more than 40 days before touching land, other researchers found – a feat that wouldn’t be possible without being able to sleep on the wing.
