In this episode of the “Sleep Review Conversations” podcast, we hear from Niels C. Rattenborg, PhD, an ornithologist who studies how birds sleep.

Hosted and produced by Rose Rimler, associate editor of Sleep Review

Run time: 19:53

Transcript Below

Rose Rimler, Sleep Review associate editor (RR): Hello and welcome to Sleep Review Conversations, a podcast from healthcare media company Allied 360. This episode is an expanded interview with Niels C. Rattenborg, PhD, who is leader of the Avian Sleep Group at the Max Planck Institute for Ornithology in Germany. Dr. Rattenborg was awarded the Outstanding Scientific Achievement Award by the Sleep Research Society this year. I interviewed him in advance of the Sleep 2017 Conference, which he will be attending as an award winner.

I included a shortened version of that interview in our SLEEP conference preview podcast, which is also available on Here I’m presenting the interview in its entirety for those who are interested in hearing more about Dr. Rattenborg’s work. I started by asking him to give an overview, a bird’s eye view if you will, of his most recent work, which was published Nature Communications last year.

Rattenborg: People have suspected for centuries that some birds fly nonstop for days, weeks, months, maybe even longer. Several recent studies have confirmed that many birds do this. They fly non-stop for long periods of time and this has been shown with  GPS trackers and various other devices. Once we were pretty sure that some birds could fly for long periods of time, the obvious question was, “Well, when do they sleep? Or do they sleep at all during these long flights?” Until recently, we didn’t have the technology needed to be able to actually record sleep in a bird flying in the wild.

What we did was, a colleague of ours had developed a small EEG or brainwave data logger that was small enough for a bird to carry on its head while flying out in the wild. We attached this to what are called frigatebirds. These are very large black sea birds, and as a sea bird, they’re a bit strange because they actually cannot land on the water. If they land on the water, they get wet and then they can’t take off and then they drown. Despite living over the ocean, they can’t land on the ocean.

Yet they go out and fly over the ocean for weeks or months at a time, so we went to an island where these birds were nesting, caught them, put the device on their head and then let them go. Then we waited for one to two weeks and then when they came home, we caught them and took the little EEG recorder off of them and looked at what they were actually doing while they were flying out over the Pacific Ocean.

RR: You measured sleep. Did you measure anything else?

Rattenborg: They also carried a GPS device that told us where they were going and how high they were flying and the little device that recorded the brain waves also recorded head movement so we could see what they were doing. This also helped us seen when they were sleeping. We found many exciting things. First of all, we showed for the first time that birds could sleep while flying, but what was interesting was previously most people had expected that they had to sleep with just one half of their brain at a time. What we found is that although that was the typical type of sleep, they could also sometimes sleep with both halves of the brain and still keep on flying.

RR: How do they do that? That sounds so dangerous. It’s like driving a car with your eyes closed.

Rattenborg: Yeah, it does sound dangerous. For frigatebirds, they were far out over the ocean and there aren’t many other birds out there, so perhaps for a minute or two closing both eyes and sleeping with both halves of the brain isn’t a big problem. But I should say that that wasn’t the typical type of sleep. Most of the time they did sleep with half their brain at a time.

What was interesting is we found that which half of the brain was sleeping related to which way the birds were circling in the air. They usually slept while they were floating on rising air currents, and so they would circle up much like a hawk does over land, and when they did this, they kept the eye connected to the wake half of the brain facing the direction they were turning as if they were watching where they were going.

RR: Wow, so they were kind of using these air currents as a little bit of a break. They’re able to take a break by riding some of these updrafts and when they did that, they had a little bit more freedom to pay less attention to what was going on around them and could half-sleep.

Rattenborg: Yeah, so some people that thought, well they might sleep while they’re gliding down, but actually we found that they usually slept while they were riding these air currents that were taking them up higher, and that might be a particularly safe time for them to sleep because they know they’re going up and not down.

RR: If you had gotten the results of the EEG back and you had seen no evidence that they slept at all, would you have been surprised?

Rattenborg: Well, actually that’s a good question because that was another exciting component of our results was that although they can sleep in flight and they can do so with one half of the brain or both halves of the brain, we actually found that they slept remarkably little during these long flights. In fact, they only slept on average 42 minutes per day.

RR: How does that compare to what some other large birds sleep? Do we have that information?

Rattenborg: Most birds that we’ve studied sleep probably most of the night, in birds that are normally active in the daytime. But with the frigatebirds, we were actually able to compare how they slept during these flights to how they slept when they came back to land. What we find was that once they were back on land, they slept almost 13 hours a day.

RR: Wow.

Rattenborg: Perhaps they’re actually recovering from the sleep loss during these long flights over the ocean when they were sleeping so little.

RR: How long are they are land for, typically?

Rattenborg: We study them while they’re taking care of a small chick and when they’re doing this, one of the parents always has to be guarding the chick while the other one is out over the ocean looking for food. They take turns, one fishing and one guarding, and these rotations on average were about every six days. The longest time one of our birds was away was for 10 days, well, flying out over the ocean.

RR: They are home tending the chick, but they are able to sleep while they tend the chick?

Rattenborg: Yeah, and actually they’re literally confined to the nest because when the chick is small, if one of the parents leaves, even just for a minute, the other frigatebirds will eat the chick.

RR: Oh.

Rattenborg: Yeah, it’s not very nice-

RR: That’s incentive to stay put.

Rattenborg: So much of the time they’re just stuck there sitting on top of the small baby, covering it, so they can’t go get food. They can’t do anything. It’s probably a good time to recover the lost sleep from their previous flight.

RR: When they are sleeping on the wing and most of the time, when they are sleeping it’s with one hemisphere, is it correct to say that they sleep with half a brain awake, half a brain asleep? Because I’ve heard people talk about dolphins sleeping that way. I don’t know if this is the same mechanism or the same kind of thing, but I wonder if that language is correct.

Rattenborg: Yeah, it’s correct, and it’s a very similar phenomenon to what dolphins do. In the case of dolphins, they seem to sleep this way in order to continue surfacing to take a breath of air. Dolphins may also use it to watch for predators while obtaining some sleep in the ocean.

RR: Are the frigatebirds, are they sleeping with half a brain and keeping one brain awake and then connected to the eye in order to … you said there aren’t a lot of other birds where they are, so it’s not really so much about navigating traffic. Why do they need to be so aware? Are they looking for food all the time? Are they trying to find the next draft to take advantage of? Or what do they need to be so aware of?

Rattenborg: Not only do they sleep primarily with half of the brain when they do sleep, but as I said, they sleep very little. Even at night, when they don’t seem to spend much time foraging. Then the question is, “Why aren’t they sleeping more? What are they looking for?” We don’t really know, and actually this was a surprising aspect of the study. It’s possible that even though they’re not feeding, they may be following features of the ocean that are predictive of good foraging opportunities.

The ocean has these eddies of moving water, and certain parts of the eddies have more fish and there are previous studies showing that frigatebirds follow these ocean eddies even at night when they don’t feed. The idea is that this allows them to be at a good spot when the sun does come up and they can start to feed. It’s possible that’s what they’re doing but how they’re doing it at night is a complete mystery.

RR: Right, because they shouldn’t be able to see that.

Rattenborg: No. One possibility is that they are feeling these eddies create differences in temperature above the water and perhaps they’re sensing this and that’s how they maintain their position over these good feeding sites.

RR: Do we understand how it works physiologically to switch on and off half of your brain?

Rattenborg: No, we don’t have a very good understanding of it. People have been looking at the brains of dolphins and seals to try to understand what differences there are that might account for this ability to sleep with one half of the brain. But so far there’s no clear result that explains this phenomenon.

RR: Do we know what other animals sleep in this way? We mentioned dolphins and these are frigatebirds. Do we know about other birds or other animals?

Rattenborg: Yeah, some of the seals, the fur seals can sleep this way and they do it more so when they’re in the water, and when they’re back on land, they primarily sleep with both halves of the brain at a time. It’s also been observed in manatees. Then it seems to be a general phenomenon in birds.

RR: No matter if they’re sea birds or birds that are terrestrial?

Rattenborg: Right. We’ve seen it in songbirds and pigeons and ducks and a variety of species. I should say it was the most prevalent and pronounced that I’ve ever seen in the frigatebirds.

People often might think, “Well this kind of research is interesting, but what can it really tell us about sleep in general and perhaps sleep in humans?” I can give an example of how this work can influence our understanding of human sleep, and it relates to the work I did during my PhD dissertation.

During that study, we showed that when ducks are sleeping at the edge of a group, that they spend more time sleeping with half their brain at a time. When they’re sleeping this way, they direct the open eye away from the other ducks as if watching for approaching predators. That’s interesting and nice and all. Just last year, a group published a paper that was directly motivated by this study of ducks, and they were interested in understanding why people have problems sleeping during the first night in a new environment.

They wondered if there was something going on similar to what I had shown in the ducks. They compared sleep in humans on their first night in the laboratory to that on the second night. Surprisingly, they found on the first night, there was an asymmetry in how deep the two halves of the brain were sleeping. Both were sleeping, but based on EEG activity, one was sleeping deeper than the other. They took this a step further and they presented sounds to the ear connected to the deep-sleeping half of the brain and the ear connected to the lighter-sleeping half of the brain.

When the sound was presented to the lighter-sleeping half, they were more likely to wake up. Then on the second night, the asymmetry was gone. This suggests that although we can’t sleep strictly speaking unihemispherically, we show something like it and we use it under a situation that is to some extent similar to what the ducks did, that perhaps when we sleep in a new environment, we perceive greater risk and so we sleep more asymmetrically and this allows us to have a better chance of detecting bumps in the night.

RR: Wow, that’s really interesting and it makes me wonder if this ability to dampen the sleep in one half of the brain and increase the sleep in the other half of the brain, I mean birds do it very dramatically, one half off, one half on. We seem to have some ability to do some very light version of that. This is something very ancient that ancient ancestors of animals maybe was able to do. How deep does this go in our evolutionary history?

Rattenborg: Yeah, it’s quite possible that the ancestors to living mammals, including ourselves, and birds also could do this. There’s been a little work in reptiles. Reptiles will often sleep with one eye open. There’s some debate over how this relates to asymmetries in brain activity, but it suggests that some form of this half-sleep or asymmetric sleep is in fact quite evolutionarily old. Occasionally, even modern humans can tap into this capacity.

RR: Yeah, it reminds me of my roommate in college who was a really light sleeper, and I’m a heavy sleeper. I remember saying to her, “It’s almost like you’re a gazelle on the African plains, and you need to be a little bit awake all night to make sure a lion doesn’t come and eat you.” I would say, “You’re not a gazelle. You can just go to sleep.” I was joking, but now I wonder if there was a little bit of truth in that. Could there be some people who are a little bit more tapped into this primal ability to sort of be partially awake throughout the night?

Rattenborg: It’s quite possible that there is variability and maybe it even links to differences in personality. Perhaps more anxious people you could imagine could sleep this way, or maybe even mothers with newborn babies, perhaps they sleep more asymmetrically and thereby can hear when the baby needs tending to.

RR: Well, what do you think you’re going to do next?

Rattenborg: At this point, in the short term, we’re interested in understanding if this capacity to perform adaptively on very little sleep is a feature unique to the frigatebirds or can other birds do this as well. We plan to do similar studies in a number of species that are also known to fly for long periods of time, such as swifts or albatrosses, and this should give us a better understanding of how prevalent this is and what kinds of ecological circumstances demand a continuous wakefulness.

In the longer term, and this will be much more challenging, I think if we can understand how some of these birds are able to perform adaptively, in the real world, despite sleeping so little, that this might inform us about the general functions of sleep and the consequences of its loss in general and in humans in particular.

RR: Dr. Rattenborg will be at the Sleep Meeting in Boston this June, so we should tell people to be on the lookout for you if they want to talk to you further about your work and maybe they can buy you a cup of coffee.

Rattenborg: Yeah, feel free to grab me and ask the questions.

RR: Sounds good. Thanks for listening to Sleep Review Conversations. Go to and click on “Resources” to find this and other podcasts and their transcripts.