Understanding Sleep Stages Sleep in humans and mammalian species is broadly classified into two main types: non-rapid eye movement sleep (non-rem) and rapid eye movement sleep (REM). Non-rem sleep consists of four stages, with stages three and four being the deepest. REM sleep is characterized by rapid eye movements and is the primary stage for dreaming, although mental activity can occur in almost every stage of sleep if defined loosely. Transcript: Speaker 1 So sleep, I think in some ways you can define as at least in humans. And in fact, in all mammalian species is broadly separated into two main types of sleep. On the one hand, we have some think that many people will have heard of called non-rapid eye movement sleep or non-rem sleep for short. And non-rem sleep has been further subdivided into four separate stages. And they are unimaginatively called stages one through four, increasing in their depth of sleep. So stages three and four, that’s the really deep sleep that we can speak about. And I should explain a little bit at some point what happens during that state within the brain. It’s stunning. It’s astonishing. So you’ve got stages one and two, light non-rem sleep. When you sort of look at your sleep track, isn’t it? It has light non-rem, deep non-rem, then REM. Stages one and two, let’s light non-rem. Stages three and four, that’s deep non-rem. And that’s non-rem encapsulated. On the other hand, we have rapid eye movement sleep or REM sleep. And it’s named not after the popular Michael Steitband of the 1990s, but because of these bizarre, horizontal, shuttling eye movements that occur during this stage of sleep, hence The rapid eye movements. And REM sleep is the, depending on your definition, and we’ll probably come to this in later episodes, it’s the principal stage in which we dream. But if your definition is quite loose, which is any reported mental activity when I wake you up or when you wake up, then it turns out that we dream in almost every stage of sleep. (Time 0:06:45)
Understanding the Battle of Non-REM and REM Sleep Transcript: Matthew Walker REM sleep is the, depending on your definition, and we’ll probably come to this in later episodes, it’s the principal stage in which we dream. But if your definition is quite loose, which is any reported mental activity when I wake you up or when you wake up, then it turns out that we dream in almost every stage of sleep. But I’ll describe REM sleep from here on Inners, perhaps, you know, dream sleep, and I’ll make that faux pas. So you’ve got these two types of sleep, non-rem and REM sleep. They will then play out in this beautiful battle for brain domination throughout the night. And that cerebral war is going to be won and lost on average for the average adult every 90 minutes. And then it’s going to be replayed every 90 minutes. (Time 0:08:07)
Fascination with Brain Activity during Sleep The speaker expresses excitement when observing sleep traces in the lab, particularly during stage two non-REM sleep where sleep spindles occur. The measurement of sleep in a lab involves tracking electrical brain activity, muscle activity, and eye movement to differentiate between wakefulness and sleep stages. Sleep spindles are described as brief synchronous bursts of electrical brain activity lasting for a few seconds during stage two non-REM sleep. Transcript: Speaker 1 This just gets so exciting to me. And even now when I go into the lab where I look at sleep traces from my sleep center, I’m still in awe, in bewildered awe of what the brain does. So as we start to fall into those lighter stages of sleep, once you get past stage one sleep, which is sort of almost the shallows where you’re just wading out, then you go into stage two Sleep. And one of the hallmarks of stage two non-REM sleep are something called sleep spindles. And the way that we measure sleep in a laboratory, by the way, is that we place, you look like a spaghetti monster. You’ve got all of these electrodes on your head. You’ve got things above your eyes and you’ve got things on your body. And we’re essentially measuring three main signals, electrical brain activity. We’re measuring muscle activity and we’re measuring eye movement activity. And I’ll explain why those three things are necessary for me to know, are you awake? Are you in sleep? And if you’re in sleep, which stage of sleep you’re in? So going into that stage two non-REM sleep, we’ve got these sleep spindles. And at that point, I’m looking at the electrical signals from your brain, what we call the EEG or the electroencephlegram. And these sleep spindles are these beautiful short synchronous bursts of electrical activity. And they last for about a second to two seconds, maybe a little longer. (Time 0:19:47)
The Beauty and Significance of Sleep Spindles in Brain Activity Sleep spindles are short bursts of electrical activity in the brain lasting for about a second to two seconds, occurring at a frequency of 12 to 15 hertz. At this stage, brain waves slow down from the fast and frenetic wake state of 20-40 times per second to 4-8 times in stage two non-REM sleep. These sleep spindles showcase a deceleration of brain activity and are described as a beautiful, synchronized pattern of brain waves that resemble slow, rhythmic throbbing and rolling sounds, adding to the charm and wonder of the brain’s transition into the sleep state. Transcript: Speaker 1 And these sleep spindles are these beautiful short synchronous bursts of electrical activity. And they last for about a second to two seconds, maybe a little longer. And they are bursting out of what we call a frequency of somewhere between 12 to 15 hertz. And what that means is that these brain waves are going up and down 12 to 15 times per second. That’s what our measure is, 12 to 15 hertz. And then you go back and your brain at that point just started to slow down. Now when we’re awake, your brain wave activity can be going up and down, maybe 20, 30, 40 times per second. It’s very fast and frenetic. It’s actually very chaotic electrical brain activity. But as we’re going into these lighter stages of sleep, then the brain starts to slow down. And at that point in stage two, non-REM, it’s maybe going up and down just four to eight times per second. So a huge deceleration in terms of brain wave activity. But occasionally you’ll get these sort of going, and then, you’ll get these beautiful bursts of these sleep spindles. I actually did. I’ve never published it publicly or we did a project called the Sonification of Sleep. We took these electrical signals and then we turned them into sound waves. And you can actually hear this beautiful sort of, it’s almost this beautiful throbbing of a slow down in your brain. And then you’ll hear these spindles. Almost sounds like that beautiful, delicious rolling are in Hindi. So it’s just wonderful. Speaker 2 I’m not sure I can do that, R. How’s it go? You getting off to bed? (Time 0:21:00)
The Wonders of Slow Brain Activity During Sleep During deep non-REM sleep, the brain slows down its oscillation to just one or two times per second, leading to whole brain or localized activity. This can be measured through electrodes which capture the activity of hundreds of thousands of brain cells. The slow oscillation is compared to a football stadium where the activity is akin to a game of football being played. Transcript: Speaker 2 I’m not sure I can do that, R. How’s it go? You getting off to bed? Speaker 1 Not too bad. I mean, we’re airing on the side of feline, but that’s okay, Andrew. Speaker 3 So coming back to, I’m so sorry, coming back to sleep, we’ve gone into light stage two as I’m trying to desperately hold it together. Speaker 1 And we’re going down into deeper, non-REM sleep. Now something spectacular happens. And this is where I just almost lose it every time I see it. The brain now goes back down and its speed of oscillation of going up and down is maybe just one or two times per second. Speaker 2 It’s incredibly slow. And this is whole brain activity or localized activity. Speaker 1 This is, so we’ll come onto this. At first, the way we would measure it is just from these electrodes, which are measuring hundreds of thousands of brain cells underneath them. So a good analogy would be, let’s say you’re at a football stadium and it’s stand for playing Berkeley in American football. (Time 0:22:41)
Deep Slow Brain Activity Analogy Deep slow brain activity during non-REM stages 3 and 4 is not indicative of dormancy but rather of significant brain wave size increase compared to wakefulness. It is like picking up the combined activity of hundreds of thousands of neurons, resembling a single microphone capturing the voices of thousands in a football stadium, and with multiple electrodes, it is possible to measure localized brain territories. Transcript: Speaker 2 It’s incredibly slow. And this is whole brain activity or localized activity. Speaker 1 This is, so we’ll come onto this. At first, the way we would measure it is just from these electrodes, which are measuring hundreds of thousands of brain cells underneath them. So a good analogy would be, let’s say you’re at a football stadium and it’s stand for playing Berkeley in American football. And what we’ve got is a single microphone dangling over the middle of the stadium. And that microphone is picking up the some devices of the 60, 70,000 people underneath. It’s the same thing with when we place an electrode on your head, you’re measuring the summed activity of hundreds of thousands of neurons underneath. But we’ve now started to use maybe a hundred, 200 electrodes on your head and we can pick these up in local territories of your brain. But that beautiful, powerful, slow brain waves that we’re getting during deep non-REM stages three and four, it’s not just slow activity. You would think, okay, that sounds like the brain is dormant. No, no, the brain at that point, the size of the waves is almost quadruple, maybe 10x the size of the brain waves when you are awake. (Time 0:23:22)
The Deep Slow Waves of Sleep Deep slow wave sleep can be likened to epic waves in Hawaii, where huge waves slowly roll in. These waves are followed by sleep spindles. The slow waves are characterized by a synchronized firing of hundreds of thousands of brain cells in the cortex, which fire and go silent together, unlike the scattered activity during wakefulness. Transcript: Speaker 1 But deep slow waves sleep are these kind of epic things that would happen in Hawaii, where you just get these 20, 30 foot waves. And they’re coming in very slowly, but they are epically big. That is deep slow wave sleep. And then what happens is riding on top of those big slow waves are these sleep spindles. They just keep coming. So according to the sort of the sleep sonification project, what you would hear now, these slow waves would be shh, shh, shh, shh, shh, shh, shh, shh, shh, shh, shh, shh, shh, shh, shh, That’s the slow wave and the sleep spindle. What is it that happens in your brain, though, to your question to produce these slow waves? Well, let’s go back to the football stadium analogy. There before the game, that’s wakefulness. Everyone is having a different conversation and a different part of the stadium. And you just get this kind of incoherent sort of blabber that’s going on. That’s wake. Your brain is doing different things at different sort of locations of the brain processing, different information at different moments in time. And that’s the fast frenetic activity of wakefulness. When you go into deep sleep, all of a sudden, for reasons that we still don’t quite understand, hundreds of thousands of brain cells in your cortex all decide to unite in their singular Voice of firing. And they all fire together and they all go silent together. (Time 0:24:54)
Importance of Different Stages of Sleep for Body and Brain Understanding the various stages of sleep reveals that each stage plays a vital role in maintaining the well-being of both the brain and the body at different times during the night. Deep sleep triggers a shift to the parasympathetic nervous system, inducing a calm state, in contrast to the activating sympathetic nervous system. The coordination of slow waves and sleep spindles during deep sleep sends signals to the autonomic nervous system, transitioning the body to a parasympathetic state, emphasizing the significance of all stages of sleep for overall health. Transcript: Speaker 1 And that begs the question, what is it doing? So it turns out that all of these stages that we’ll describe, different stages of sleep, do different things for your brain and your body at different times of night. And it’s very understandable that people sort of in the public will come over to me and say, you know, how do I get more deep sleep or how do I get more REM sleep? My question back to them firstly is, why do you want more REM sleep? And they’ll say, well, isn’t that the good stuff? And I will say, well, it turns out that they’re all important. You need all of them. But we can come on to let’s I’ll speak about non-REM sleep functions first and then I can probably I should unpack REM sleep and then explain its functions. But as an overview, what we know is that during deep sleep, first you switch over in terms of your body’s nervous system to what we call the parasympathetic nervous system that you’ve Spoken about a lot before, which is this kind of very quiet and calming state of your body’s nervous system, the sympathetic nervous system, which is very poorly named because it’s Anything but sympathetic is very aggravating and activating. When we’re awake, that seems to be somewhat more dominant depending on what state you’re in. But in sleep, especially in deep sleep, you shift over into this very strong parasympathetic quiescent calm state and that instigates together with other things. And we’ve demonstrated by the way that we published a paper probably about a year and a half ago that these slow waves and these sleep spindles and the coordination of them, are well that They’re coordinated, seems to instigate a signal down into your bodies, what we call the autonomic nervous system, which carries both the sympathetic and the sympathetic nervous System inside of it and forces you over into a parasympathetic state. (Time 0:27:31)
Quality over Quantity in Sleep Stages Different stages of sleep - non-REM and REM - play essential roles in body and brain functions during the night. Deep sleep triggers the parasympathetic nervous system, promoting a calming state. Each sleep stage holds significance, highlighting the importance of a balance between non-REM and REM sleep for overall well-being. Transcript: Speaker 1 And that begs the question, what is it doing? So it turns out that all of these stages that we’ll describe, different stages of sleep, do different things for your brain and your body at different times of night. And it’s very understandable that people sort of in the public will come over to me and say, you know, how do I get more deep sleep or how do I get more REM sleep? My question back to them firstly is, why do you want more REM sleep? And they’ll say, well, isn’t that the good stuff? And I will say, well, it turns out that they’re all important. You need all of them. But we can come on to let’s I’ll speak about non-REM sleep functions first and then I can probably I should unpack REM sleep and then explain its functions. But as an overview, what we know is that during deep sleep, first you switch over in terms of your body’s nervous system to what we call the parasympathetic nervous system that you’ve Spoken about a lot before, which is this kind of very quiet and calming state of your body’s nervous system, the sympathetic nervous system, which is very poorly named because it’s Anything but sympathetic is very aggravating and activating. When we’re awake, that seems to be somewhat more dominant depending on what state you’re in. But in sleep, especially in deep sleep, you shift over into this very strong parasympathetic quiescent calm state and that instigates together with other things. (Time 0:27:31)
Understanding REM Sleep and Muscle Atonia In REM sleep, the brain paralyzes the body through muscle atonia. This paralysis ensures that the dreamer doesn’t physically act out their dreams while asleep. Muscle tone decreases gradually as one moves from non-REM to REM sleep, with complete muscle atonia occurring just before entering REM sleep. Transcript: Speaker 2 I know we’re going to talk a lot about dreaming in a later episode of this series. But what you just told me forces me to ask at this moment whether or not in dreams where we sense we are flying is that possible because of the absence of proprioception. We’re sort of, we’re on the mattress or on whatever surface we sleep on. But according to the brain, there’s suspended in space. Is that right? Speaker 1 And so it’s one possibility as to why we have those experiences. In some ways though, it does bring us on to REM sleep. During REM sleep and I’ll explain what happens in the brain. But what you’re talking about is something that is even more unique about REM sleep. As we go into REM sleep, your brain paralyzes your body. So you are physically locked into the incarceration of your body. Why would your brain do this? And it’s what we call muscle atonia. Now I was telling you that we measure your electrical brain activity and we measure eye movement activity. But we also measure your muscle activity. Why do we do that? Well, as you’re going into non REM sleep, that muscle tone decreases. But there’s still some muscle tone there. But as you go into REM sleep, in fact, just a few seconds before you enter REM sleep, I already know you’re going into REM sleep. Because bang, you become completely absent of muscle tone. (Time 0:42:00)
Safety in Dreaming Muscle atonia during REM sleep is a protective mechanism by the brain to prevent physical actions based on dreams that might harm the body. This muscle paralysis ensures that while the mind is immersed in dreams during REM sleep, the body remains physically still and safe. The absence of muscle tone is a distinguishing factor between REM sleep and awake states, as the brain activity is similar during both periods. Transcript: Speaker 1 But sorry, I know what you’re referring to. So this muscle, and as we call muscle atonia, and I think in sort of medicine, usually with an A before it means the absence of something. So sort of if you have A with me, absence of normal A with me. Aphasia, yeah, sort of, or, and here it’s atonia, absence of the tone in your muscles. Why would the brain do this? Well, the brain paralyzes your body so your mind can dream safely. You would imagine how quickly you could be popped out of the gene pool if just like you described, you thought I can fly. So you get up out of your bed and you go to the window and you launch. Maybe not going to end well, depending on what floor you’re on. So this absence of muscle tone, this physical incarceration that we have is one of the things by the way that defines REM sleep from when you are awake. Because if all I was doing in my sleep lab was recording your electrical brain activity, and I was in the other room and I was just looking at your brain waves, as you go into REM sleep, I Would not be able to tell are you in REM sleep or are you awake? Why? Because the electrical brain activity is so similar when you are in REM sleep relative to when you’re awake. (Time 0:44:02)
The Paradox of Muscle Inactivity During REM Sleep During REM sleep, although muscles are temporarily paralyzed, the brain remains highly active. Just before entering REM sleep, there is increased brain activity to light up the cortex, while simultaneously, a signal is sent from the brain stem to inhibit voluntary skeletal muscles. However, this inhibition does not affect involuntary muscles such as those responsible for breathing and heart function, ensuring our survival. Despite this muscle paralysis during REM sleep, the extraocular muscles and another unknown set of voluntary muscles still retain movement for reasons yet to be understood. Transcript: Speaker 1 It’s utterly inactive, but your brain is fervent with its activity. By the way, people should not worry when I say that your muscles are shut down. And what happens is that just before you go into REM sleep, there’s a bursting activity that will go up into your brain to light up your cortex. But there’s another signal from the brain stem that’s sent down all the way down the spinal cord to the alpha motor neurons in the spinal cord that will essentially create this inhibition. It’s only your voluntary skeletal muscles, meaning that your involuntary muscles, things, for example, such as your respiration that helps you breathe in your heart. That’s the reason that we survive and live another day after sleep. So don’t worry about that too much. With two exceptions, though, there are two sets of voluntary muscles for reasons that we still don’t know either that are spurred from the paralysis of REM sleep. One of them is the extraocular muscles. (Time 0:45:57)
Yawning and Social Behavior Theories Yawning is contagious among cooperative species like lions, leading to a sequence of actions that promote cooperative group behavior. Additionally, yawning may serve as a way to balance the temperature differential between inhaled cooler outside air and the body’s core temperature, rather than solely for gaseous exchange of oxygen and carbon dioxide. Transcript: Speaker 3 So when your dog yawns, there’s a higher probability that you will yawn. Speaker 1 And we’ve got this stage and it’s very clear. One of the other interesting theories though is that when species that are cooperative species, for example, a pride of lions, when one of those lions yawns, firstly, many of the other Lions will yawn in a contagious fashion. But then consequently, there is a collection of actions that happen after that contagious yawn. And so some people have suggested that the yawning is a way to enact cooperative group behavior. That’s another theory. The final theory, number four, which I think has the best evidence for, is not the gaseous exchange balancing of carbon dioxide and oxygen. But when you inhale oxygen from the outside, it’s usually cooler than your core body and brain temperature. (Time 1:02:10)
Understanding Afternoon Sleepiness in Meetings People may feel sleepy in afternoon meetings due to a drop in core body temperature caused by the warmth of the room, leading to drowsiness. Additionally, a genetically hardwired drop in afternoon alertness, known as postprandial dip, occurs between 1 to 4 p.m. regardless of meal intake, contributing to the tendency to feel sleepy during afternoon meetings. Transcript: Speaker 1 So why would people be falling asleep sort of, you know, in an afternoon meeting when it starts to get a little warm? Well, in part, it’s because the warmth of the room is starting to make their sort of face a little bit more rosy. It’s drawing the blood out to the surface. So what’s happening, the core of your brain and your body temperature are starting to drop. And at that point, that’s why you’re going to start to feel a little bit more sleepy. That’s reason one. The second that you described is that afternoon, you know, you’re in meetings around a table and you start to get, as you said, those wonderful headnuts and people listening. You all know that whether head goes down and snaps back up. It’s not that people are listening to good music and sort of doing this head bobbing. It’s that they’re falling prey to what we know is a genetically hardwired pre-programmed drop in your afternoon alertness. It’s called the postprandial dick in alertness. And that infers that it’s after some kind of a meal. It turns out it’s not really related to a meal. People say, well, I had a heavy lunch. I had sort of pasture at lunch and I always feel sleepy afterwards. Maybe impart. But if I remove it, I prevent you from having lunch and we’ve done these studies too, your brain still shows this very reliable drop in alertness somewhere between quite wide but somewhere Between about 1 to 4 p.m. In the afternoon. (Time 1:06:06)