How yawning might help clear dirty fluid from the brain

[MUSIC PLAYING] FLORA LICHTMAN: Hey, it’s Flora, and you are listening to Science Friday. A scientific mystery right under your nose– yawning. Almost every animal with a backbone does it. Hopefully, you are not doing it right now as you listen to this show. And here’s the thing. It actually remains a mystery exactly why we yawn. But a listener awakened us to some new research that offers up some clues.

EILEEN: Hello. My name is Eileen. I’m from Houston, Texas. And I found some new findings about yawning, and something about maybe whenever people yawn, the blood goes to the brain or from the brain or something. Could you clarify that? It sounds pretty fascinating to me.

FLORA LICHTMAN: Yes, Eileen, we can clarify that. On the line with us right now is the scientist who led that research, Biomechanical Engineer Dr. Lynne Bilston at the University of New South Wales in Sydney. She studies how mechanical forces in the body affect our health. Welcome to the show.

LYNNE BILSTON: Thank you, Flora. It’s lovely to be here.

FLORA LICHTMAN: OK, so why focus on yawning?

LYNNE BILSTON: So we were doing some other research at the time that was looking at how breathing affects the flow of the fluid that sits around your brain and spinal cord. And I had a student who was particularly interested in yawning. And we put the two of them together and thought that since the breathing really changes that fluid flow, that perhaps yawning also will affect that fluid flow of the cerebrospinal fluid, as we call it, around the brain and spinal cord. That’s where this all started.

FLORA LICHTMAN: What does this fluid do?

LYNNE BILSTON: So Cerebrospinal Fluid, or CSF for short, is the fluid that your brain and spinal cord kind of float in. So it has two main functions. The first is basically to protect your brain and spinal cord when you’re moving around. Otherwise, your brain would just be banging around from one side of your skull to the other as you move your head. The other function it has is to transport various metabolites and other waste products away from your central nervous system.

So when you think and when you move and do all the things that your brain does, you generate byproducts, waste products from all of the chemical processes going on in your brain. And the cerebrospinal fluid helps to transport those away from the brain, and then eventually out into what’s called the lymphatic system, which is the body’s waste disposal system from around the body.

FLORA LICHTMAN: So does yawning change the flow of that fluid?

LYNNE BILSTON: It really does. It was really fascinating. So what we did in this study was to actually use some sophisticated MRI techniques that have only recently been developed to look at the flow of both blood and cerebrospinal fluid into and out of the skull during a yawn. And what we saw was that the cerebrospinal fluid flows quite differently during a yawn, so it encourages the CSF and the blood to flow in the same direction during the yawn, which will increase the flow of those fluids out of the brain.

So you get fresh blood flowing into your brain during the yawn and the deoxygenated blood, so the blood that’s had all the oxygen taken out by your brain. And the CSF tends to flow out of the brain at the same time during the yawn.

FLORA LICHTMAN: Is it, like, squeezing it out like a tube of toothpaste? How should I picture it?

LYNNE BILSTON: So it really just flows back and forth. The bottom of your skull where the spinal cord comes out, there’s sort of a hole, which we call the foramen magnum. Just means big hole. And the fluid flows up and down as your heart beats and you breathe through that. And what the yawn does is it increases that flow out of the brain as you breathe in during that early phase of the yawn.

So it’s interesting because you would think, OK, well, if it’s just to do with breathing, then it would be the same as if you just took a deep breath. And it turns out that it’s not. So we had subjects actually take a deep breath and move their mouth exactly the same. So deep, wide open, gaping breath, we called it. Yes, exactly.

FLORA LICHTMAN: I’m making the face, if you’re listening on the radio–

LYNNE BILSTON: It’s a great face. So try it at home if you want. And we thought that if it was just like a normal breath, if there was nothing special about the yawn, that the pattern of the blood flowing into and out of your skull and the CSF flowing into and out of your skull would be the same, because everything else was looking the same.

But actually, that was not the case. For the normal breath, the deep breath with the wide open mouth, the blood and the CSF were flowing in opposite directions. And during yawn, they were flowing in the same direction.

So that means that it’s more efficient during a yawn to get the deoxygenated blood, and the, if you like, dirty CSF out of the skull and down to other places where it can be absorbed both in the spinal canal, but there are other places in the skull that you also absorb that fluid. So it’s helping to mix that fluid around, a little bit like a washing machine. It’s mixing all the dirt around in your clothes, and then you have to wash it out at the end.

FLORA LICHTMAN: Hmm. That’s fascinating. So there must be something going on with the muscles of the yawn where you’re engaging different ones or it’s a different movement?

LYNNE BILSTON: Yeah. Well, we think so. So what’s interesting is when we were looking at the MRI videos in real time, we both measured the CSF and the blood flow. But we also took videos during the MRI just to see what was happening, because nobody had ever looked really at what the mechanics of the yawn were. And what we noticed is that for everybody, for a start, what goes on inside your mouth when you yawn is really weird. It looks really funny.

FLORA LICHTMAN: Like what? What’s happening?

LYNNE BILSTON: Well, people– open your mouth up and then your tongue has this really complicated motion. Some people have– the tongue flicks up and down in a really complicated way.

But what is interesting is that it’s really consistent for each person. Looks different, but it’s very consistent for each person. And in physiology, we know that when we see a very consistent movement like that, a very complex pattern of actions of the muscles, that usually means that there’s what we call a central pattern generator, which is a little circuit usually in the brain– in this case in the brainstem, we think– that actually drives that pattern.

And that means that rather than having to actually generate every little motor instruction to each of those muscles, all the brain has to do is to trigger the pattern, and it goes automatically. We have similar circuits for a lot of things like breathing and swallowing and even walking further down the spinal cord. So that’s what we observed, and that hadn’t really been seen before. And that explains why everybody has exactly the same pattern.

It’s even really curious, because if you’re sitting in a busy meeting or school or something and you start to yawn, what do you do? You try to close your mouth and stifle it. But we got people to do that as well. And interestingly, if they just closed their lips and tried to pretend that they weren’t yawning, the same exact pattern was still happening inside their mouth. So it really is quite hardwired for everybody, which is just really fascinating.

FLORA LICHTMAN: That is fascinating. It’s like a yawn fingerprint.

LYNNE BILSTON: Absolutely. That’s a good way of describing it.

FLORA LICHTMAN: Hmm. We know that some of the brain’s bio waste is implicated in diseases, neurodegenerative diseases like Alzheimer’s. Does that mean that yawning might play a role in disease?

LYNNE BILSTON: Well, we don’t know that, of course. We do that there are some neurological disorders, including some neurodegenerative conditions like Alzheimer’s, Parkinson’s, and a few other conditions where people do pathologically yawn. So they yawn a lot.

And the current thinking on that, which is not directly related to our research, but is that that’s because of damage in those diseases to these same areas of the brainstem where that we think that pattern generator is. So it’s not quite clear yet whether that’s cause or effect. In that case, in the pathological yawning case, it’s probably the effect of the disease rather than the cause of the disease.

However, we do know that this clearance process inside the brain itself– so not the CSF around the brain, which, if you like, is the washing water. But actually inside the dirty clothes themselves when we put the detergent in to loosen up the dirt and get that out into the water, if you like that analogy, then we do know that that part of the system, the system of loosening up and getting rid of all the stuff inside the brain and sending it out to the CSF, is more active during sleep.

Now, we know that people yawn the most both just before they go to sleep and when they wake up. So it’s possible– we haven’t proven it yet, but it’s possible that this is a mechanism for increasing the speed of that clearance and really clearing out the CSF system. So the system is empty or clear for the stuff that comes out during sleep, but also to help get rid of stuff at the end.

FLORA LICHTMAN: Hmm. It’s like opening up the drain on your washing machine.

LYNNE BILSTON: That’s a really nice analogy, opening up and clearing the drain out so that it’s ready to take a lot of stuff. The other piece of evidence towards that, in fact, is that a lot of the fluid goes out through the spinal canal, as we talked about before, but also a lot comes out in a region just behind your nose, which, of course, is where all those muscles are that are really active during a yawn.

So we think that they might act as a sort of squeezing the fluid out like a toothpaste– out of a tube of toothpaste, like you said before, and actually really helped do that with the activity of those muscles. That remains to be proven, of course, but it’s a really interesting idea.

FLORA LICHTMAN: Hmm. Were you guys– were you all yawning constantly while you were working on this.

LYNNE BILSTON: Absolutely. So in fact, in my lab meeting when we used to discuss this, we used to make the students and the people who were working on this actually always report back and tell us what they were doing at the end, because otherwise, we’d all be yawning the whole meeting. [LAUGHS]

FLORA LICHTMAN: This is like another yawning mystery, right? Why it’s contagious.

LYNNE BILSTON: It is. And our study didn’t really directly address that. So there are some theories out there that contagiousness of yawning is a social cue. Again, we do it before we go to sleep and when we wake up. So maybe it’s a way of trying to synchronize groups.

It is associated with empathy. So people who score higher on empathy traits in psychological testing tend to contagiously yawn more. And conversely, people who score more highly on psychopathic traits tend to contagiously yawn less. Just to clarify, this, of course, is not contagiously– yawning is not a measure that somebody is a psychopath, so don’t go out and accuse your friends of that because they didn’t catch your yawn.

[LAUGHTER]

FLORA LICHTMAN: Lynne, thank you so much for taking the time.

LYNNE BILSTON: My absolute pleasure. Thank you, Flora.

FLORA LICHTMAN: Dr. Lynne Bilston, Biomechanical Engineer at the University of New South Wales, Sydney. And if you want to see an MRI video of what yawning looks like inside the body– that’s what Lynne’s team captured– head to our website, sciencefriday.com/yawning. And I will stop saying that word now. But we can’t stop and won’t stop with the sleepy time subjects. After the break, a new report on sleep trends. Stay with us.

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Speaking of the Y word, according to a new CDC report, about a third of Americans aren’t getting the recommended seven hours of sleep a night. Here to break down the findings and why they matter is Dr. Stuti Jaiswal, a sleep researcher at Scripps Research Translational Institute in La Jolla, California. Hey, Stuti.

STUTI JAISWAL: Hi.

FLORA LICHTMAN: I mean, a third of Americans aren’t getting enough sleep. I feel like I’ve seen this headline before, like maybe recurring for decades, actually. Is it just me? And if not, why does it stay so consistent?

STUTI JAISWAL: Yes. So I think that number where it’s about 30%, one in three, I mean, it may vary a little bit here and there. But really, that is very consistent across the board from year to year.

But I think one thing that you want to think about too, it’s really easy to say, oh, we’re not sleeping enough. But really, I mean, sleep is a luxury, right? So if you’re working two jobs, if you are in the middle of graduate school, for example, if you are at a different point in your life where you are not able to get those hours of sleep.

So I think the question really too becomes, if you are someone who cannot get that recommended number of hours, then how bad is that for you, right? And is there anything that you can do to mitigate those risks?

It was interesting because this study broke it down also by age and race. And you did start to see some of the disparities in there, which, a lot of these are understudied. Black Americans reported short sleep duration of about 40%, so that’s about 10% to 11% higher than the average population. And so that’s really important to think about, and it’s an underexplored area in sleep, unfortunately.

I think the other thing in there that was interesting is that younger adults– so that 18 to maybe 30, 40-year range– they’re sleeping worse than we thought. I think we have an assumption that the younger you are, the better you’re sleeping, but that may not be the case.

FLORA LICHTMAN: Right. I mean, and what are the health impacts of not getting enough sleep?

STUTI JAISWAL: So there are a lot of them. I think one of the most worrisome is mortality is actually associated with short sleep. And so that’s problematic. The other ones are probably ones that are very common. So metabolic disorders like obesity, type 2 diabetes, and of course, cardiovascular outcomes. So hypertension, heart disease itself. So all of the ones that we’re trying to avoid seem to be associated with short sleep.

FLORA LICHTMAN: Do we know why that’s the case?

STUTI JAISWAL: You know, there are a lot of theories on that, but if you’re not sleeping enough, you are putting stress on your body, right? So there is stress that happens when you are not getting enough sleep for a lot of different reasons, and so you potentially are increasing inflammation in other parts of your body as well. And that inflammation in and of itself can lead to a lot of the different health outcomes that we see, the longer term chronic health outcomes.

FLORA LICHTMAN: It seems like these watches and trackers are also giving people sleep scores now. How accurate are they?

STUTI JAISWAL: So I actually don’t know how accurate the sleep scores are, because each one seems to be based on a different algorithm, potentially. I know from my own experience, I don’t know that I feel like I match what the number says necessarily. But then also, if I look at my own sleep, it stresses me out. But I think that probably remains to be seen on how accurate those are. So the quality score, not sure how reliable that is, but the duration score is probably pretty reliable.

FLORA LICHTMAN: The duration score is reliable. What are the questions you’re asking about sleep?

STUTI JAISWAL: So I think one of the most interesting things about sleep is how we’re sleeping from night to night. And that is also coming out in a lot of the research that’s being done. So sleep consistency we’re finding also is mattering a lot.

So you can have an average of eight hours of sleep, for example, but are you someone who’s getting 10 hours of sleep on the weekends and six hours of sleep on the weekdays? So compared to someone who has an eight-hour every single night from night to night, seven days a week. So that variation or that variability– and also timing– like, what time you’re falling asleep and what time you’re waking up, those pieces of sleep consistency are important for health. And both play a role. So duration definitely plays a role, but that consistency also plays a role.

FLORA LICHTMAN: Do you feel like you get enough sleep? Like, is your sleep hygiene A-plus because this is what you do?

STUTI JAISWAL: I actually– I’m a shift worker for my clinical job, so I work overnight shifts and I work really late evening shifts. My sleep is all over the place. [LAUGHS] I have no sleep consistency, and so this is why my sleep stresses me out if I look at it on my activity tracker. So no–

FLORA LICHTMAN: You know too much.

STUTI JAISWAL: Terrible. I do. It’s bad. But no, I think some of the things– I know what tricks work for me, and I think that’s important for people too. Like, some people, temperature is really important. I mean, all these things in theory–

FLORA LICHTMAN: Keep it cold, I heard.

STUTI JAISWAL: –sleep hygiene. Yeah. Keep it cold. No blue light or screens, like, 30 minutes, 60 minutes before bed. No caffeine.

FLORA LICHTMAN: But that’s impossible, actually.

STUTI JAISWAL: Yeah. [LAUGHS]

FLORA LICHTMAN: So we can just take that off the table.

STUTI JAISWAL: I know. It is so impossible, and I mean, I’m totally guilty of it. But I think some things work better for some people than others. Some people are not affected by the caffeine, and they feel like they can fall asleep normally. If I have any caffeine after noon, I just– [LAUGHS] I’m awake forever.

FLORA LICHTMAN: Dr. Stuti Jaiswal, Assistant Professor at Scripps Research. This episode was produced by Dee Peterschmidt. If you feel renewed and refreshed after listening to it, consider leaving us a review on your favorite podcast platform. And if there’s something you’d like us to look into, some new science or new research, give us a call. 877-4SCIFRI is our number. And thank you to Eileen for setting us down this path. I’m Flora Lichtman. Thanks for listening.

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