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Chapter 1: What is the main topic discussed in this episode?
This is The Guardian.
Chapter 2: Why do humans sleep and what are the mysteries behind it?
For thousands of years, humans have pondered the question, why do we sleep?
Sleep is still one of the biggest mysteries in biology, and we still do not know why it is so important to sleep.
It puts us in such a vulnerable state, and yet it's vital to life for almost all animals.
I think we all feel on our body on a daily level how much sleep actually matters for our performance and our well-being the following day.
Hypotheses about sleep's purpose have changed as we learn more, from rest and recovery for the body to consolidating memory in the brain. But according to neuroscientist Maiken Nedegaard, it goes even further.
We now realize that sleep is so much more than that, and that probably many both immune action, many housekeeping function that's going on in sleep, and it's not all about memory performance.
As we understand more about the biology of sleep, its importance is becoming clearer. And it turns out that sleep disruption could have major consequences for our future brain health. So today, the incredible science of the sleeping brain and why it all comes down to good housekeeping.
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Chapter 3: What groundbreaking discovery did Maiken Nedergaard's lab make in 2012?
From The Guardian, I'm Ian Sample, and this is Science Weekly. Michael Nadergaard, you're Professor of Neuroscience at Rochester Medical School and at the University of Copenhagen.
Chapter 4: How does the glymphatic system function during sleep?
And back in 2012, your lab was behind the groundbreaking discovery of this mechanism the brain has for clearing out its waste, which you named the glymphatic system. So first of all, what does this cleaning system have to do with sleep?
All other organ and tissue can do the housekeeping, the cleaning when we are asleep, when we are awake at any time point. The brain appears to only be able to do the basic housekeeping when we are asleep.
Right, so unlike every other organ, the brain can only clean out its waste molecules, proteins like amyloid and tau and metabolic byproducts while we're sleeping, which provides a clue as to what sleep might be for. And this process all centers on what's happening to the fluid that surrounds the brain, the cerebrospinal fluid. How did you discover this mechanism?
Yeah, what we found in a nutshell was that if we injected colored substances in the cerebrospinal fluid that surround the brain, we found that if the animal was asleep, they were pumped straight back into the brain. And this was a very unexpected finding because the cerebrospinal fluid is supposed to be basically produced by the brain and then protect the brain and leave the brain.
And then when we started to do awake mice, we saw, wow, it doesn't happen. So, of course, we mistrusted it. We repeated it many times, used different techniques, and we published it.
And so this was your big discovery, the glymphatic system. Essentially, these channels running alongside blood vessels in the brain, which cerebrospinal fluid can flow through and flush out any waste. But has it actually been observed in the living human brain?
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Chapter 5: What role do neurotransmitters play in sleep and brain waste clearance?
Yes, so that is a big deal. So all the basic concept of the glymphatic system was defined in mice and rats. But very quickly, just a few years later, people start to replicate this finding and could confirm the core principle of the glymphatic system. For example, that it turns on when we sleep, it turns off when we awake, and it follows a certain pattern or organization of fluid flow.
So you and other scientists have pieced together what is going on with the glymphatic system, what it seems to be doing and how it seems to be working. Tell me what the impact of this discovery has been.
Yeah, there has been about 3,000 papers published on the glymphatic system. And what people have done so far is to describe the drivers of it, that it is significantly reduced activity. in all neurodegenerative diseases. So this is Alzheimer, Parkinson, frontotemporal dementia, and so on. It's decreased, and aging in itself would decrease it.
It's also been shown that it's suppressed by wakefulness, and if you keep awake for just one night, it takes at least two days or more to clean out the waste that accumulated during that night.
Mike, and over the past five years or so, we've seen further advances in understanding how this system operates, and particularly the role of neurotransmitters in the operation of the glymphatic system. Can you tell me what the neurotransmitters are doing in our bodies during the day, in our waking hours?
Yes, so they know transmitters or modulators. So these are noradrenaline, dopamine, serotonin. They are basically determining when we awake who we are because they're driving emotion, cognition, and desire. So they have very different functions. Sometimes they work in parallel, but most of the time they work in different brain regions and at different time points.
Okay, so when we're awake, these neurotransmitters like dopamine and serotonin are acting individually. But now we understand that they're doing something surprising when we're asleep. What's that?
So the biggest surprise was that they all synchronize activity and they oscillate. So they increase about once every minute and fall. So very, very slow oscillations. What it comes down to is that during evolution, the brain probably decided to use the neurotransmitters for two different purposes.
One is to decide our awake desire and cognition, and the other is these are the driver of fluid flow when we fall asleep. So this is very common in evolution, that the same building block is used for multiple purposes, often two or three.
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Chapter 6: What impact does sleep quality have on brain health?
Yes, so this slow wave of activity actually appeared in evolution about 600 million years ago when the very primitive organisms started to have a gut and a vascular system. And you need to move fluid around in these systems.
Chapter 7: How do mental health issues affect sleep and dementia risk?
These organisms did not even have a heart, but they still have very simple vessels that could move oxygen from one place to another by this very slow oscillation mechanism. And what probably happened is that evolution decided this is actually a great function to basically constrict and dilate blood vessels about once every minute. And that is maintained in the human brain.
Now we know this is what moves cerebrospinal fluid around when we sleep. And we also know why it doesn't happen when we are awake, because these neurotransmitters are not synchronized. They move a little here, a little there, so there's no concerted movement of fluid flow.
When they're all synchronized when we sleep, we get this highly organized movement of not only blood, but also cerebrospinal fluid, and that is what drives brain clearance.
So when we're sleeping, and specifically it's during non-REM sleep, we fall into this state where the release of these neurotransmitters, these chemicals like serotonin, dopamine, norepinephrine, they're synchronized into slow waves. And that seems to be driving this kind of pump that moves the cerebrospinal fluid through the brain.
And as it moves through the brain, it's sort of picking up the waste that is accumulating
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Chapter 8: What are the evolutionary aspects of sleep and brain function?
in the brain. That's how it all works in a healthy situation. And then there are things that can disrupt that.
So there's many things that can disrupt it. As the brain age, the blood vessels get stiffer, they do not move as much, and therefore cleaning is not efficient any longer. And that is probably one of the reasons we accumulate proteins and we get Alzheimer's, Parkinson's disease, and so on.
But the new discovery that these neurotransmitters are such a key for driving this cleaning process opened up for the question, if you are mentally sick or you are chronic stressed during daytime, you know that the signaling for noradrenaline is not functioning normally.
And if no adrenaline signaling is messed up when we are awake, it's certainly also messed up when we are asleep, and therefore the cleaning is not as efficient as it should be.
Coming up, how the quality of your sleep today can impact your brain's health in the future. Mike, and you mentioned that age, chronic stress and some mental illnesses can disrupt this really important waste clearance system. But we also know that there's a connection between these conditions and our risk of dementia. And I'm really interested in what the link is there.
Why and how might these be related?
So it's very well documented in many, many studies that if you suffer from depression, schizophrenia, you are chronic stressed, you have an increased risk of premature dementia. And that has not been possible to really define why that happened earlier. And I think with the dual function of these neurotransmitters, we have an answer.
We know that the neurotransmitters are basically disturbed the signaling during wakefulness, and it remains to be shown that it's disturbed during sleep, and therefore, glymphatic clearings is reduced. But we already know that all of these diseases are associated with very significant sleep disorders.
And that in itself would very significantly reduce glymphatic flow because glymphatic flow needs healthy, long-term sleep. Short, fragmented sleep is basically returning some of the waste to the brain. So having normal sleep, normal neurotransmitter oscillation is probably the key to restorative sleep.
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