The Just Plain Odd Ways Birds Sleep — And What It Means for Us All

Humans do very little when we sleep. This might not seem like a big deal, until you look to the dolphins as they sleep and swim with the pod. Or fur seals, who can sleep while watching out for sharks. Best of all there are birds, who can rest part of their brain to sleep while, yes, flying.

While sleep for humans may seem straightforward — lie down, close eyes, count sheep — how it works is quite mysterious, even to the experts who study it. Why do we need to sleep at all? And what evolutionary advantage could our eight hours of unprotected snoozing give us? Looking to the wider animal kingdom only complicates matters. Most animals sleep — and in all sorts of strange ways that, on the surface, are quite baffling.

Such discoveries have “inspired lots of thoughts about sleep,” says Niels Rattenborg, a researcher at the Max-Planck-Institute for Biological Intelligence in Germany who specializes in avian sleep. By looking at unusual sleep patterns in the animal kingdom, we can start unlocking mysteries of how our brains work.

One Eye Open

The ability of some animals to put half their brain to sleep at a time, called unihemispheric slow wave sleep in researcher parlance, was first described in bottlenose dolphins in the 1970s.

In this phenomenon, one half of the brain shows the large, slow brainwave patterns indicative of sleep while the other half shows the small, fast waves that occur when the brain is awake on something called an electroencephalogram, which measures electrical activity in the brain. When the left hemisphere is asleep, the animal’s right eye remains open and vice versa, with the right hemisphere and left eye (if you remember from high school, each hemisphere of the brain tends to control the opposite side of the body, so the left hemisphere controls the right body.)

“It allows them to monitor their environment while still getting some sleep,” says Rattenborg.

Unihemispheric slow wave sleep has been studied in bottlenose dolphins, harbor porpoises, Amazon river dolphins, belugas, pilot whales, and some types of seals — and, more recently, birds.

The great frigatebirds Rattenborg studies, for example, are also capable of sleeping with one eye open for short periods of time both on the wing and when settled on land. Birds experience unihemispheric sleep and something called asymmetric sleep, where one hemisphere is asleep and the other shows brainwave activity somewhere between sleep and wakefulness, for short bursts.

While the phenomenon hasn’t been studied in many species, Rattenborg hypothesizes putting half their brain to sleep at a time is a common skill for birds. “It seems birds in general have this capacity,” he says. “Some do it more than others, but they all seem to be able to do it at least a little bit.”

Scientists still don’t know how animals do this or why only some animals can sleep this way. But the advantage seems clear, allowing these animals to rest their brains when they’d otherwise have to be fully on guard.

Sneaking In A Few Winks

One thing is clear: Sleep, in some form, is necessary for survival. There are plenty of hypotheses as to why, but it’s one of the great biological mysteries that is still being unpacked. The biggest problem with sleep, at least in the wild, is that it puts the animal doing it at great risk. What’s easier prey to catch than one that is taking a nap? This is likely why nature has found many innovative ways to sneak in some sleep.

Take dolphins, belugas, and similar species who usually sleep with half their brain — and continue swimming slowly while doing so. Some species never stop swimming at all, like the Amazon river dolphin, while others, like belugas, have been known to occasionally rest near the surface.

This form of sleep may help these marine mammals in a few different ways. For one, it could help them navigate turbulent waves near the ocean’s surface, regulate their body temperature, and ensure regular breathing all while getting some rest.

Dolphins have been documented sleeping and swimming with their open eye facing their pod, suggesting they’re trying to stay with the group. “Mother dolphins and newborn calves will swim side by side, with the baby and mother looking at each other,” says Rattenborg

Fur seals can also sleep half their brains at a time while floating on their sides, nostrils and tail out of the water with one flipper paddling to keep the seal balanced. The eye facing down into the water remains open, likely to keep a watch out for hungry sharks and orcas.

For birds, the ability to half sleep could serve similar functions.

“Even in the human brain, sleep is often a local phenomenon ... It doesn’t always take over the whole brain at the same time.”

“It enables birds to reconcile the inherent dangers of sleep on land and in flight with the simultaneous need to sleep,” says Rattenborg.

Great frigatebirds, large seabirds that spend months at a time winging over the open ocean without landing, will occasionally rest half, or even all of, their brain at a time while flying. The birds tend to do this while spiraling on air currents, keeping the eye open that faces into the turn, possibly so they don’t go crashing into any fellow travelers.

On the ground, ducks sleeping at the edge of a group tend to sleep with one eye open, while the ducks in the middle go fully to sleep and close both eyes. Those on the edge could be standing watch, keeping one eye out for danger.

First Night Effect

Humans clearly don’t have the ability to keep going while sleeping, but understanding bird and dolphin sleep has helped researchers gain a deeper understanding human sleep as well.

“Now we fully recognize that even in the human brain, sleep is often a local phenomenon,” Rattenborg says. “It doesn’t always take over the whole brain at the same time.”

These sleep patterns in animals inspired researchers in 2016 to look more closely at why people generally sleep poorly on their first night in a new environment (like in a sleep study lab), a pattern called the ‘first night effect.’ They found people tended to sleep less deeply with the left half of their brains compared to the right half on their first night, and were more likely to respond to sounds played by the right ear. (A follow-up study published in 2024 indicated the first night effect can affect visual plasticity in the brain.)

“It was serving some ability to monitor the environment,” says Rattenborg of the study (he was not involved in the research). By the second night, however, both halves of the brain tended to sleep together.

Studying sleep in marine mammals could also help us better understand sleep disorders in humans. “Studying these sleep patterns may help explain what goes wrong in human sleep disorders in which the brain fails to fully ‘switch off,’ such as insomnia, parasomnias, and sleep-related movement disorders,” says Oleg Lyamin, Ph.D., a leading researcher at the A.N. Severtsov Institute of Ecology and Evolution in Russia who has studied sleep in marine mammals for decades.

And some researchers are applying the idea of sleep as a local phenomenon to studying recovery after traumatic brain injury. Damaged areas often generate slow brainwaves indicative of sleep even when the person is awake, as shown in recent research from a group in Italy. Understanding this phenomenon better could help improve recovery and cognition.

“It’s a neat example of how studying sleep and unusual animals without any clear direct connection or utility for humans can lead to new thoughts that cause us to think about human sleep in a new way,” says Rattenborg. One of many that may help us unpack the many unexplored mysteries still out there in sleep science.