Sleep-flying is a thing!

I assure you that this blog is not about astral projections, but about the mindblowing discovery on how birds can sleep-fly without bumping into trees. So, here you go! I have given away the suspense. Nonetheless, do read along as I cherish my love for sleeping and long-standing collaboration with Ipsa Jain.


Scientists have found that migratory birds can fly for 200 days straight, eating and sleeping while soaring through the sky. Image credit Ipsawonders

A few years back, I got the chance to visit Sultanpur Bird Sanctuary, India. It is a magical place to be. Every winter, around 250 species of birds and 1 enthusiastic Homo species known as Bird watchers confluence in the park. Both playing one’s cards close to their chest; displaying their magnificence, skills and power.

Then there is me fighting off my early morning slumber, and continuously bickering about how long can it possibly take to reach the park through the infamous Delhi-NCR traffic. However, the serenity of this place has something, that allowed me to think about how these nomadic birds sleep while migrating all the way from Siberia, Russia, Turkey, and Eastern Europe.

Alas! I’m not the only one who comes up with such fantastic thoughts. For years, scientists have been suspicious that birds could sleep mid-flight, as several bird species can fly non-stop for weeks. On the other hand, some researchers propose that few birds can forgo sleep entirely while flying for extended periods of up to 200 days straight.

This time scale will lable a human insane, even if s(he) contemplates trying it out, Isn’t it? Humans along with many other species would experience irritability, hallucination, cognitive impairments, paranoia, and psychosis as side effects of sleep deprivation within 3 days or less.

So what makes birds’ brain so special?

Due to the lack of studies monitoring the sleep patterns of flying birds, the above hypotheses had previously been uncharted. Ratthenborg and his team in 2016, were among the first ones to pursue this question as they embarked on a red-eye flight to the Galápagos Islands; monitoring the brain activity of great frigatebirds (Fregata minor).

The great frigatebird is a fascinating model to study these questions as this species of large seabirds can spend weeks continuously flying over the ocean in search for food and shelter, and to my surprise without bumping into obstacles on its way. The team’s work provided evidence that birds do indeed sleep while flying.

The great frigatebird (Fregata minor) is a large seabird in the frigatebird family. Their nesting populations are located in the tropical Pacific (including the Galapagos Islands) and Indian Oceans, as well as a small population exist in the South Atlantic. Image credit Charles J Sharp

How do they know that?

The team attached a lightweight, portable device onto the heads of frigatebirds, to track the brain activity. Their equipment used electroencephalography (EEG) to identify if and when the birds were asleep during the flight. After 10 days of non-stop flight, the birds returned to land, and the researchers recollected the devices to observe the results.

The team showed that flying frigatebirds display unihemispheric slow wave sleep (USWS). It is a unique capability of the brain, that allows the animals to doze off one hemisphere of the brain at a time. This way is allowing them to watch out for potential threats and roadblock through one open eye.

Other animals and birds are also equipped with such a superpower. For example, the Dolphins have been observed to exhibit USWS, letting them sleep while swimming. Also, on land, the Mallard ducks (Anas platyrhynchos) keep one cerebral hemisphere up and running letting the corresponding eye open, directed away from the fellow flock-mates, but toward potential threats. This way it has devised a safety net out of the use of USWS, when sleeping at the edge of their group.

The Mallard (Anas platyrhynchos) is a dabbling duck that breeds throughout the temperate and subtropical Americas, Eurasia, and North Africa. Image credit momentofscience

If now you are thinking that this is the coolest part, wait for it.

Rattenborg and his colleague also found that frigatebirds continue to fly even when both the cerebral hemispheres are asleep, that means both the eyes are entirely closed. For simplicity sake, imagine it as some sort of autopilot mode. The monitored birds in this study, even experienced brief bouts of rapid eye movement (REM) sleep, although they lasted only a few seconds. They observed that during deep REM sleep birds head droops due to relaxed muscle tone, although this did not affect the flight pattern. Suggesting that the frigatebirds did sleep for brief periods in mid-flight (~ 42 min per day), they spent a majority of the flight awake and half-brain awake.

Admittedly, it still remains unclear how birds have adapted to function with such little amount of sleep. Nevertheless it opens up other questions like, why us and many other animals suffer consequences of sleep deprivation dramatically.


The cover image is made by a science communicator friend, Ipsa Jain. She uses arts and design to start conversations about science. Ipsawonders is one woman labor of love. She wants to create beautiful things that speak science


We publish using the Creative Commons Attribution (CC-BY) license so that users can read, download and reuse text and data for free – provided the authors, illustrators, and the primary sources are given appropriate credit.

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