What is Sleep?

What exactly is sleep? If you’re like me, you just take sleep for granted as the thing that happens for 8 hours every night after you get in bed and lose consciousness. Well, actually, if you’re like me, sleep is that thing that happens to you only occasionally, never lasting quite long enough on any given night. Hooray for coffee.

Sleep is a recurring physiological state of the body and brain. Scientists define it by either physiology or behavior. Technical definitions of sleep—and there are several—include some combination of unconsciousness, limited body movement, distinct brainwave patterns and eye movements, reduced response to external stimuli, body posture, and a few other things. 

Some form of sleep is present in pretty much every type of animal. Sleep-like states have been documented in everything from jellyfish and nematode worms to bees, fish, and land vertebrates like you and me. And that includes birds, of course.

Sleep is driven by things that happen in the brain. Chemical and electrical processes in the brain differ between waking and sleep. Brainwaves provide an important measure of these differences. These waves are groovy rhythmic patterns made by millions of neurons communicating with each other. Certain brainwave patterns dominate while an animal is awake and others occur during sleep. Brainwaves are commonly measured with an electroencephalogram, an EEG.

You’ve heard of REM, which stands for “rapid eye movement.” During REM sleep, our eyes wiggle back-and-forth and our brainwaves oscillate rapidly. Most of our dreams happen during REM sleep.

Then we have slow-wave sleep, SWS, during which—you guessed it—the brainwaves pulse more slowly. Slow-wave sleep is what we call “deep” sleep. In humans, REM and slow-wave-sleep typically alternate over the course of a night. 

Other mammals as well as birds have both REM and slow-wave-sleep phases. And there’s evidence that some fish and reptile species display similar patterns.

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Functions of Sleep

When a trait or behavior like sleep is widespread across the animal kingdom, we can infer why that might be. It could be a primitive behavior that was present in the common ancestor of all these critters. Or maybe the trait serves a really important function. In that case, it might have evolved independently in each lineage. Or, of course, the behavior could be both primitive and important.

We’ll come back to the evolution stuff in a moment. But we do know that sleep serves a couple critically important functions for birds.

First, sleep allows for healing, restoration, and the removal of wastes from the brain and body. 

Second, sleep allows for memory processing and consolidation.  

Memory has many important functions in a bird’s life. For example, memory is crucial when a juvenile bird learns how to sing. This is true for songbirds, which account for about half of the world’s 10,000+ bird species. This is how songbirds learn to sing: they need to listen to a tutor first, whether a parent or another adult bird. 

Biologists have studied the relationship between sleep and memory in Zebra Finches. These are perky little songbirds native to Australia. They’re used as model organisms in bird research. In several lab studies, juvenile Zebra Finches — they’re 40 days old and they have never heard the song of an adult up to this point — were played a recorded “tutor” song and this was the first song they ever heard. Researchers found that brain patterns during sleep changed significantly after the juvenile finches heard this first song. While asleep, their tiny brains lit up with bursts of neuron activity in the regions associated with singing. 

So it seems like the recent memory of having heard an adult song is being processed in the sleeping brains of these young birds. The memory is being consolidated.

Hummingbirds and parrots also learn their songs. These bird groups are distantly related to songbirds like the Zebra Finch. As far as I know, biologists haven’t yet figured out if sleep is important for baby hummingbirds and parrots as they learn how to vocalize. 

Chickens, too, are distant relatives of songbirds, but there is some data on them regarding sleep and memory. Juvenile chickens—chicks, if you will — need to sleep in order to imprint properly on their mother. Well, they’ll visually imprint on anything that moves, really. Could be a person, a dog, a bag of potato chips… whatever. In any case, This natural process is called filial imprinting. Studies of chicks found that they were not very good at filial imprinting if they didn’t get some good sleep soon after seeing their mom or whoever for the first time. 

I don’t know about you, but I’m picturing fuzzy, yellow chicks huddled together and drifting off to sleep with their tiny eyelids slowly closing. Cuteness factor 1000. 

Other research shows that sleep is also important for memory processing in adult birds. Adult Zebra Finches mentally replay their songs while they sleep. These birds might be practicing in their sleep to maintain their refined songs. We don’t know this for sure and research is ongoing.

And here’s a puzzle: it seems these practice songs that play in the finch’s brain during sleep are super variable. They’re all over the place compared to the highly stereotyped song performed while awake. Researchers aren’t sure what this means.

Adult European Starlings get clear memory benefits from sleep. Unlike Zebra Finches, starlings are open-ended learners. This means they can learn new songs or song phrases throughout their lifetimes, not just when they’re very young.

Starlings are way better at learning new song segments when they have a chance to sleep afterward. Those that heard songs in the morning and were tested later that day, before getting any sleep… Well, they weren’t so good at remembering the new songs. The birds that had a good night sleep, however, were significantly better at reproducing the novel songs.

Despite what we know about the functions of sleep in birds and mammals, such as memory consolidation, sleep is still a bit of a mystery. It’s complex, and it varies a lot across species and types of animals. We still don’t know why sleep is so necessary.

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Evolution of Sleep in Birds

Theodosius Dobzhansky said:  “Nothing in biology makes sense except in the light of evolution.” Evolution is the unifying perspective of the biological sciences. So… Sleep. Birds. What’s the evolutionary deal?

Sleep as we know it in birds has similarities to sleep in mammals. For example, both have REM and slow-wave sleep. These two animal lineages shared a common ancestor 300 or so million years ago. Is sleep in birds and mammals similar because of their shared ancestry? Or did sleep evolve independently in the two groups?

Some biologists used to make the case that sleep evolved to be so similar in birds and mammals because these animals are warm-blooded and have large, complex brains. REM and slow-wave sleep must serve necessary functions for such creatures. But since the complex brain structures of birds and mammals are different and have evolved independently, it seemed sleep, too, probably evolved independently.

But in recent years biologists have discovered that at least some reptiles and fish have mammal and bird-like brain patterns during sleep.

Zebrafish—another model organism; no relation to Zebra Finches—have brain signatures during sleep that look a lot like REM and slow-wave sleep. You know, this makes me wonder if anyone has studied sleep in the Zebra Hound or the Zebra Weasel or how about the good ol’ Zebra Zebra?  

But in all seriousness, sleep patterns very much like REM and slow-wave sleep have been recorded in the Central Bearded Dragon, a lizard from Australia. This means we need to consider the common ancestor of not just birds and mammals, but also of fish and modern reptiles. That wriggling ancestral creature lived more like 400 million years ago.

Some features of sleep that we once thought were unique to humans, REM and slow-wave sleep, might have existed in some form way back when vertebrates hadn’t even crawled onto land for the first time. Perhaps. But I think some biologists would still argue that these aspects of sleep evolved independently through convergent or parallel evolution in birds and mammals.

Interestingly, research shows that ostriches, which belong to an ancient group of birds, spend much more time in REM sleep than more recently evolved birds. This is like what we see in the Duck-billed Platypus compared to more modern placental mammals. There’s also less of a distinction between REM and slow-wave sleep in these more primitive animals, the Ostrich and Platypus. 

This suggests that the similarities we observe in sleep between songbirds and mammals might have evolved on their own, independently, over hundreds of millions of years.

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Bird Brains and Sleep

There are several major differences between what sleep is like for birds and what it’s like for humans. First, birds sleep in really short bouts. Instead of sleeping for 8 hours at a time like we do—or at least try to do—birds sleep only a few minutes at a time. But they repeat these short bouts of sleep up to hundreds of times over 24 hours.

In only a few minutes, a bird goes through one or more full cycles of REM and slow-wave sleep. These alternate, with maybe 10 seconds of REM and a couple minutes of slow-wave sleep. Only about 10% of their sleep is REM, compared to the 25% we need. 

The second big difference between the way we sleep and the way birds sleep is that they can sleep with only half their brain at a time. You know the saying, “Sleep with one eye open,” meaning be cautious and attentive even when you’re trying to sleep? Well, humans can’t really do that. But birds can!

Birds can let one half, or hemisphere, of their brain fall asleep while the other half stays alert. This amazing ability is called unihemispheric slow wave sleep.

You’ve probably seen ducks adrift on a pond as they sleep with their bills tucked under their feathers. Have you ever noticed one of those ducks having one eye open and the other shut? That bird is sleeping with half of its brain while staying vigilant with the other half. It's keeping an eye out for any signs of danger. Ducks floating at the edge of a flock tend to keep their open eye facing out away from the center. So they’re looking in the direction where predators are more likely to sneak up. Ducks in the middle of the group tend to feel more secure. They often allow themselves to close both eyes and get some shut-eye with their entire brains. 

Kind of changes the meaning of “sitting duck,” doesn’t it?

Unihemispheric slow-wave sleep has been documented in species across the avian tree of life, from ducks and chickens to parakeets, falcons, and sparrows. A few years ago unihemispheric slow-wave sleep got some press when a scientific paper was published about this phenomenon in frigatebirds. Researchers from the Avian Sleep Group at the Max Planck Institute for Ornithology were interested in how birds deal with sleep on multi-day non-stop flights. By the way, I seriously love that there’s an Avian Sleep Group! 

The bird of choice for this study was the Great Frigatebird. This seabird is a master of gliding and soaring over the open ocean. Frigatebirds routinely fly for days at a time without ever landing on the sea surface. In fact, they can’t swim or float and never land on water if they can help it.

The researchers connected tiny EEG devices to 15 female Great Frigatebirds in the Galapagos Islands. These devices measured the birds’ brainwaves and were connected to little data logger backpacks. Data were collected on non-stop flights of up to 10 days.

This study had a couple of cool results. First, it turns out that frigatebirds do indeed use unihemispheric slow-wave sleep on their epic flights. This was long predicted but never proven until this study. Pretty awesome.

Second, these birds sleep less deeply and much less often while flying than when perched on land. This result surprised the researchers. While flying, the birds sleep about 45 minutes a day on average, which is less than 10% of the time they sleep on land. So even though frigatebirds can cruise the skies using unihemispheric slow-wave sleep, they don’t use it very often on these long flights. It seems they need to be fully alert most of the time.

If you think frigatebirds are impressive, let me tell you about the Common Swift. This species now holds the record for the longest uninterrupted flight. They can stay in the air for 10 months without ever landing! They eat, drink, and mate while on the wing. But scientists don’t yet know how or how often they sleep. 

These little swifts are too small to carry the data loggers used in the frigategbird study. But as this technology continues to be miniaturized there’s a good chance we’ll someday have some data. 

Birds aren’t alone in having the superpower of sleeping with one eye open. We’ve known for a while that many marine mammals use unihemispheric slow-wave sleep. These include manatees, seals, sea lions, dolphins, and possibly some baleen whales. 

There’s even a wee bit of evidence of some asymmetry between the hemispheres of human brains during sleep. One recent study showed that when a person sleeps in an unfamiliar place for the first time, there is a weak, but statistically significant difference in responsiveness between the two halves of their brain. And they sleep less deeply overall. You’ve probably had the experience of not getting much sleep in a strange bed. I don’t mean to suggest anything by that, so don’t take it the wrong way.

Maybe this parallels what birds are doing, just not as dramatically. Our ancient ancestors might have benefitted from keeping one half of their brains a little more alert when sleeping around the fire at a new campsite. Like drowsing ducks on the lookout for danger, they’d be faster to respond when the proverbial saber-toothed tiger skulks into the firelight.

The third and last major difference I want to point out between sleep in birds and humans-slash-mammals is in seasonal changes. Humans need the same amount of sleep each night, whether it’s summer, winter… whenever. Birds, however, aren’t on such an unchanging schedule.

Birds normally sleep a fair number of hours within a 24-hour cycle. Diurnal birds sleep at night, nocturnal birds sleep during the day. But during certain times of year, many species have wildly different sleep patterns. These changes happen during migration and on the breeding grounds.

Species that migrate long distances every spring and fall rarely have the luxury of getting their normal nightly dose of Zzzs while they migrate. For example, Swainson’s Thrush is a neotropical songbird that breeds in North America and spends the winter in Central or South America. A 2006 laboratory-based study of this species shed some light on how it deals with sleep during its long migration.

Like many songbirds, Swainson’s Thrushes migrate at night. We don’t know how or if they sleep on these night flights. But at least in the lab conditions of this study, thrushes in their migratory state slept at night only one-third as many hours as they normally sleep when not migrating. In this state, they also took a bunch of naps during the day. That’s probably the more interesting finding of the study. These mini siestas last a few minutes each and appear to involve some regular sleep and a few seconds of unihemispheric slow wave sleep. It seems migrating Swainson’s Thrushes don’t suffer from sleep deprivation because they catch up on the rest they need during the day.

When humans don’t get enough sleep, there are some lousy consequences. We get irritable, our cognitive ability is impaired, and we suffer lapses in memory. When we’re really sleep deprived, we can have hallucinations and even organ failure. Birds—at least some of them—can deal with sleep deprivation so extreme that it might be deadly to a human.

And when you need to know about sleep deprivation in birds, who you gonna call? The Avian Sleep Group! 

Scientists from the Avian Sleep Group published a study in 2012 on the Pectoral Sandpiper. This shorebird breeds in the Arctic. Males set up and defend territories, for the purpose of mating with as many females as possible during the 3-week breeding season. In the 24-hour sunshine of the Arctic summer, these males display, chase, and fight for days or weeks at a time. They sleep very little. Unlike long-haul truckers, these feisty birds can’t just use cocaine and loud music to stay alert. Instead, they rely on a physiological ability to get by on very little sleep. It’s quite impressive.

This ability is an adaptation in the evolutionary sense. The 2012 study revealed that the male Pectoral Sandpipers that slept the least ended up having significantly more offspring that season. Those birds got more of their genes into the next generation. So this is a winning strategy in terms of natural selection. For these sandpipers, the motto should be: “If you snooze, you lose."

‍Where and How Birds Sleep

We’ve talked about the functions of sleep in birds as well as some similarities and differences it has with sleep in mammals. Now let’s talk a little about where and how birds sleep. They clearly must sleep—at least most of them do, most of the time. Sleep has to be super important, otherwise evolution would favor birds that just stay awake all the time. Such birds could use the Pectoral Sandpiper playbook to make more babies and leave behind more genes. And, importantly, ever-wakeful birds would be far less vulnerable to predators.

The danger of being attacked by a predator while sleeping seems to be an enormously important factor in the sleeping behavior of birds. This was demonstrated by a 2006 analysis of sleep data from 23 bird species across a wide diversity of families. Species ranged from owls and turkeys to penguins, magpies, and sparrows. The researchers tried to find any links, any correlations, between sleep duration and quality with variables such as body mass, brain mass, and metabolic rate.

In the end, the only significant relationship they found was between the amount of deep sleep a bird gets and a variable they called the “sleep exposure index.” This index captured the amount of predation risk faced by a particular bird species, based on where it typically sleeps.

What this analysis showed was that bird species that tend to be more exposed to predators while they sleep get less slow-wave sleep, less of what we think of as deep sleep. This makes sense. When birds feel safe, such as when they sleep tucked into a burrow or tree cavity, they can let their guard down and get some deep sleep.

The tradeoff between a bird’s need to sleep and its risk of being a midnight snack for a predator is apparent in many situations. For example, migrating Garden Warblers that are well-fed and in good shape sleep with their heads up and facing forward. With this posture, these birds are more alert and able to react quickly if a housecat or other predator enters the scene. Conversely, Garden Warblers in poor metabolic condition during a migratory stopover need to conserve more energy while they sleep. They tuck their heads under the feathers on their backs. You‘ve no doubt seen this common sleeping posture in birds. But these little warblers are less alert and thus more vulnerable in this position. It’s a tradeoff between energy conservation and the risk of being killed by a predator.

Research on pigeons gives us another example. In experimental conditions, pigeons on perches close to the ground sleep less deeply than pigeons on high perches. The idea here is that sleeping closer to the ground is riskier.

So birds are out there sleeping in all sorts of places, some of which are pretty safe and snug, while others leave birds exposed to danger. Some sleep on the ground and depend on their camouflage plumage. Nightjars and nighthawks are good examples of this. 

Other birds sleep while hidden in grass or among rocks. Some sleep on the water, like the ducks we were talking about earlier. And of course, many, many species sleep in trees or bushes.

Perching birds can sleep on a tree branch without falling because when their legs are folded under them tendons in their ankles automatically cause their toes to clench. No muscles are involved or any conscious control. Birds do not sleep in their nests. This seems counterintuitive but it’s true. Nests are built for protecting eggs and baby birds. I’m sure there are some exceptions out there, because, well, nature is crazy like that. There’s always variation.

Lastly, I want to return briefly to sleeping posture. As I mentioned, a near universal sleeping posture in birds is where they place their head on their back, usually with the bill tucked under their scapular feathers. Ducks, swans, sparrows, penguins… many types of birds do this. For birders trying to identify ducks on a pond this head tucking thing can be kind of frustrating. 

This sleeping posture has apparently been a feature of birds for over 125 million years. In 2010 a new bird-like dinosaur was discovered in China. Its fossil was curled up in a sleeping position just like that of modern birds. This little dinosaur was given the scientific name Mei long, which in Chinese means “sleeping dragon.”

The birds that share the Earth with us today are living dinosaurs. There are more than 10,000 species. At any given moment, there are billions of birds out there somewhere, sleeping and dreaming.