Ep50 "Why do we spend 1/3 of our lives asleep?" (Sleeping & Dreaming Part 1)

Cosmos with me David Eagleman. I'm a neuroscientist and author at Stanford and in these episodes we sail deeply into our three pound universe to understand why and how our lives look the way they do. Today's episode is about sleep. Why do we do it? Why do we spend so much of our lives in this weird doppelganger state? When you're asleep, it still looks like you, but you are

essentially shut off. And even though we're not typically aware of being in this state, because we have no sense of the passage of time there, we spend so much of our lives there, more time than you spend doing almost anything else, whether that's eating or showering or being with friends. Or listening to podcasts. All of those presumably take up a smaller fraction of your life. Sleeping is at the top of your list of activities, and certainly at the top of the list of activities that you

do consistently. Other hobbies come and go, sleep remains. So in today's episode, we're going to talk about everything you've ever wanted to know about sleep, and we're going to uncover what the brain is really doing during this time. And this is the first part of a three parter. Next week we're going to dive into dreams. Why do we dream? What's that about? Is it a form of consciousness?

Do all animals dream? And finally, two weeks from now, will dive into lucid dreaming, which is where you become consciously aware that you are in a dream state and you are able to take control of the plot. So join me for those episodes. They build on this one about sleep, and I promise you there will be no

end of mind blowing surprises there. So for today's episode, we're going to start in the early morning of May twenty fourth, nineteen eighty seven, when a man named Kenneth Parks is watching TV on the couch and he fall sleep, and he gets up. He drives across the city of Toronto where he lives, and he enters his in law's home, and there he stabs his mother in law to death and attempts to murder his father in law as well, and then he drives himself to the police station and

turns himself in. Now everyone agrees that Kenneth Parks had no motive. He loved his in laws and he had a close relationship with them, and even more strangely, he appears to have been asleep the whole time. Now, that claim sounded outrageous to most people, especially as Parks had to drive fourteen miles to get to his in law's home. But as the case was investigated, the sleepwalking story began

to take shape. When he arrived at the police station, he had looked down in confusion at his bloodied hands and he said, quote, I think I may have killed some people. He claimed to have no memory of what had happened, and he appeared horrified when he learned the details. He testified that he wasn't awake and he wasn't conscious during the crime, and by the way, his testimony never

wavered throughout the whole trial. So his legal team argued that the case represented homicidal somnambulism, which is just a fancy way of saying killing while sleepwalking. So electrode recordings in a sleep lab showed that the electrical activity in his brain was highly unusual and consistent with sleepwalking. So I'll quote the expert testimony given by a psychiatrist named

Ronald Billings. The lawyer asks, is there any evidence that a person could formulate a plan while they were awake and and then in some way ensure that they carry it out in their sleep, And Billings says no, absolutely not. Perhaps the most striking feature of what we know of what goes on in the mind during sleep is that it's very independent of waking mentation in terms of its objectives and so forth. There's a lack of control of

directing our minds in sleep compared to wakefulness. In the waking state, of course, we often voluntarily plan things, what we call volition. We decide to do this as opposed to that, and there is no evidence that this occurs during the sleepwalking episode. So the lawyer asks, and assuming he was sleepwalking at the time, would he have the capacity to intend and Billing says no, And the lawyer says,

would he have appreciated what he was doing? And Billing says no, he would not, And lawyer says, would he have I've understood the consequences of what he was doing, And Billing says no, I do not believe that he would. I think it would all have been an unconscious activity, uncontrolled and unmeditated. So what would your opinion be if you were a juror on this case? Does sleepwalking sound like an excuse? Is one either asleep or awake? Or

can a brain be caught between those two states? So in today's podcast, we're going to explore the fundamental differences between the states of wakefulness and sleep. We'll look at the purpose of sleep and the way that transitions between sleep and wakefulness can go awry, and then, armed with our new knowledge, will return to the Parks case at the end of this episode to see what the jury concluded.

To my mind, one of the most astonishing facts of guroscience is that adult humans we spend a third of our lives in the strange world of sleep, and by the way, newborn babies spend about two thirds. So it's insanely difficult to stay awake for more than a full day night cycle, and if you somehow manage to stay awake for a while, you're gonna be a bit cognitively impaired. And in animal studies, if you deprive a rat of sleep for weeks, the rat will die. So we can

see that sleep really matters. It's not just a choice we make and we could do otherwise. And the massive importance of sleep can also be gathered from the fact that sleep is conserved through evolution. All mammals sleep, reptile sleep, birds sleep. Even in the fruit fly, you can measure sleep wake cycles and three days of sleep deprivation result in the fli's death. The fact that all animals sleep

suggests that sleep is not an accident but something really fundamental. Now, the first question we're going to ask is what is happening under the hood, Because when you look at somebody who's sleeping, it looks like everything is shut down. The eyes are closed, the muscles are relaxed, breathing is regular, there's no response to sound or light. So a century ago, people thought that wakefulness was the natural state of the brain, and that sleep represented a phase in which brain activity

was just decreased or shut down. But in the last century, scientists discovered that the brain is not so much naturally awake as it is kept awake by a whole system of brain areas. For example, if you injure the brain stem, that can cause you to fall into a sleep like state that we call coma. So you need special machinery to run in order to be awake, and as it turns out, you also need special machinery to sleep. If you damage part of the hypothalamus, you get long lasting

sleep difficulties. In other words, for both waking and sleeping, you need particular networks to run. You need the activity of certain cells. Waking is a neurally active state, and so is sleeping. Now, people noted these things about sleep by observing brain damage, but you can really only tell so much from that. So how did the field make

progress from there? Mostly it was due to a technology people had developed some time ago called electro and cephalography or EEG, which is a technique where you stick electrodes onto the scalp and you try to surface clues about

what's happening on the inside of the skull. But what's fascinating is that the EEG was invented in in nineteen twenty four, but researchers only ever measured people who were awake, and it wasn't until almost three decades later that anyone thought took this up to a person while they were asleep. And what did they find. They found that the brain

was screaming along with activity during sleep in retrospect. By the way, just the mere existence of dreams should have alerted people that there must be a lot of brain activity going on, but as I said, just no one had thought of doing that experiment. So they found that the brain was active during sleep, and with a little bit of study, they soon worked out that there were patterns, and these patterns changed, and these changes followed a regular

cycle during the night. In other words, the EEG pattern changes in a predictable way several times during sleep. The sleeping brain has these cycles that last about ninety minutes and the repeat four or five times during the night. Now what immediately became clear is that there are two basic stages of sleep. There's this sleep that you have where your eyes move rapidly back and forth. This is called rem sleep for rapid eye movement. And then everything

else gets lumped into non REM sleep. So let's begin with the non REM stage because that comprises eighty percent of your sleep. Now, during non REM sleep, your heart rate and breathing becomes slow and regular. The deepest stage of this is known as slow wave sleep, and this is named that way because the brain's electrical activity oscillates at a low frequency and with a high amplitude. Think of this like, wooooooo okay. Now, REM sleep is quite different.

In REM, your heart rate and your breathing beat up, and your small muscles twitch like your facial muscles, but your major muscle groups are paralyzed by a very elaborate neural circuitry. So during REM, brain waves become high frequency in low amplitude more like, which reflects more complex cognitive function. REM sleep is sometimes called paradoxical sleep because your brain is highly active and the EEG signature resembles that of the waking state much more so than of deeper stages

of sleep. Now, REM sleep is the stage that usually includes dreaming, and scientists discover this fact very simply by waking people up during this stage and saying, Hey, what were you just experiencing? So people generally report they had story like dreams. During the REM stage. As a side note, you can have dreams during non REM sleep, but this is much less common and when it does occur, it's generally much less visual and more like just having thoughts. Okay,

so dreaming happens during REM sleep. Now, I just want to address the question that comes up a lot. People sometimes say to me, I don't think I dream at night. Well, everyone dreams. This is apart from a few rare cases of patience with specific brain injuries. The thing that happens is that some people have more difficulty remembering their dreams, and therefore they think that they don't dream. But if I awaken you during REM, you will report having just

been dreaming. Now, we're going to talk more about the dreaming phase next week, but for now, what I want to make clear is that your sleep cycle follows a specific pattern. Non REM sleep proceeds into successive phases of depth from stages one to three deeper and deeper, and then that reverses in its shallower and shallower until you reach REM sleep. And as this cycle continues through the night,

you tend to experience less deep sleep and more REM. Now, although we use a single word sleep for this phenomenon. The sleep state emerges from a collaboration of a whole bunch of brain areas interacting in a network. These areas all work in concert to give rise to this state

that we experience as sleep. Now, under special conditions, like if you're deprived of rem sleep or you don't have any light cues, the different areas can get out of sync with each other, and that's in large part how we can figure out the richness of the underlying mechanisms. So let's now look at the most important parts of the brain's sleep network. The most important player involved in sleeping is an area of the brain called the ventrilateral

preoptic nucleus. We'll just call this the VLPO. This is a collection of neurons in the hypothalamus, and when it becomes active, you go to sleep. These neurons inhibit the arousal network, which is all about wakefulness. So if there's damage to your VLPO, you get insomnia. You can't sleep. Now, the arousal network, which it inhibits, also inhibits it right back. These two networks are always locked in mutual inhibition. When either one is active, it uses inhibitory neurotransmitters to suppress

the other one. So, as a general principle of brains, whenever you have two networks set up with mutual inhibition, you get a system that toggles between two stable states, in this case being asleep and being awake. You get one state or the other, but you can't be in both at the same time. Now, the brain doesn't just consist of these two networks, but also other networks that stabilize or destabilize the balance such that the brain can hold onto one state and then make a transition into

the other state. And we'll talk about this again in a minute when we talk about disorders like narcolepsy. So the VLPO squelches the arousal system and you sleep. Then the arousal system squelches the VLPO and you wake up. But as we'll explore more next week, you've also got dream sleep. And you get this from a principle that puts the brain in a state that shares some qualities of both sleeping and waking. Okay, so you have a sleep wake cycle, and this determines two things, when you

sleep and how much you sleep. As for how much not everyone needs to leap the same amount seven to nine hours is recommended for adults, although a lot of people only catch six and a half seven hours of sleep. If you look across the whole population, you find a really big range between four to eleven hours. Now, how much sleep do you need? The suspicion is that that has at least some genetic basis to it, but people generally find out through experience about how much is required

for them to feel good. Now, how many hours you sleep correlates with what you have done while you were awake. If you do a bunch of activity like hiking or weightlifting or public speaking, then you tend to sleep longer. Okay, so that's about how much you sleep. But now let's turn to the other aspect of the sleep wake cycle, which is when you sleep. Why is sleep on such

a regular cycle and what is it synchronized with? The answer involves your circadian rhythm, which is a natural internal pattern that runs on approximately a twenty four hour cycle. Where does the word circadian come from? Circa means about and dion is a day. Now our circadian rhythm is what times are sleep wake cycles? All animals appear to have some form of a circadian cycle that influences not just sleep in wakefulness, but also coordination, blood pressure, alertness,

body temperature, things like that. The circadian rhythm comes from preprogrammed mechanisms in our brain, and it persists even when there aren't any external cues like the sun going up and down. Some animals, like the blind mole rat, maintain their rhythms even though they live underground and don't see the sun. I'll come back to this in humans in just a moment. So the primary clock in mammals is a structure called the super chiaismatic nucleus, which is in

the hypothalamus. So these cells keep a rhythm. And what's amazing is if you culture the cells of the super chismatic nucleus in a dish, they will maintain their own rhythm. So, not surprisingly, when this nucleus gets damaged, that obliterates a regular sleepwake rhythm. So the circadian rhythm is generated by internal mechanisms and runs on its own machinery, but it

becomes entrained to cues in the environment. In other words, it lines itself up with the environment, and the most important cue that it uses is the light dark cycle. In other words, the phase of the circadian rhythm is set by the planet's rotation into and out of light from the sun. The light information gets from the retina to the super chismatic nucleus, which then gets to the pineal gland and tells it to make melatonin, which you've

probably heard of. Melatonin is not directly a sleep hormone, as people sometimes think, but instead it's a darkness hormone. It gets its darkness information from the super chismatic nucleus, and the cycles of melatonin just influence when you get sleepy. So what happens if you are not seeing the sun go up and down, Well, that's a question you can

directly explore. In nineteen seventy two, the French underground explorer Michael Sifra descended a one hundred foot vertical shaft into the Midnight Cave near Del Rio, Texas, and inside he had stashed a campsite with food and water and books enough to last him the six months of the astounding

time isolation experiment that he was about to undertake. Because the bowels of this cave had no light clues, you couldn't tell anything about day from night, and he had no time pieces of any sort, he forbade any incoming contact from the outside world. His telephone only made outgoing calls, and in this self imposed experimental condition, he kept careful track of his sleep and wake times by calling his support team when he woke up, when he ate, and

when he went to sleep. He quickly lost a conscious sense of time. Just imagine what that would be like to have no clues to what time it is. No cell phone, no alexa, no wristwatch, nothing, So what happened without the ability to see the light and dark cycles of the world. The period of his circadian rhythm drifted to about twenty five hours, and subsequent experiments verified that the circadian rhythm is not an exact twenty four hour clock.

It's an approximate clock. It gets nudged into rhythm with the sun's cycle. Presumably, if we had evolved on a different planet with a different light dark period, we would have a different approximate period length to our internal rhythms.

What's cool is that you can use false lighting cues to entrain a different cycle, for example, twenty three and a half hour cycles, or scientists have done a twenty four point sixty five hour cycle and they chose that because that happens to be the period of day and night on Mars, so we'll have no problem adjusting to

Martian day night cycles now. Although the period the twenty four hours is essentially the same for all of us Earth dwellers, the period of this rhythm, in other words, when it hits top or bottom, that can vary across the population. Some people are night owls, meaning that their natural rhythm keeps them up late at night and sleeping late into the morning, while other people are mourning larks

they are early to bed and early to rise. About sixty percent of the population is intermediate between the two. There's a normal amount of variation in the population between night owls and morning larks, but some people on the very extremes have to take really drastic measures to adapt their lives to societal norms. So consider the case of a woman that I'll call Melinda. Every day, Melinda and her two children, who are aged thirteen and eleven. They go to bed at four in the morning and they

typically wake up around noon. Now, some of her neighbors think she's a bad mother, but Melinda and her children suffer from something called delayed sleep phase syndrome, which is a disorder of the circadian rhythm, in which a person has no trouble maintaining sleep, but can't fall asleep or wake up at the same time as the rest of society. The family essentially lives with a constant six hours of

jet lag. Other people have what's called advanced sleep phase syndrome, in which you fall asleep and wake up way earlier than the rest of society. And remember what I said about the circadian rhythm being about twenty four hours. Some people have molecular problems which warp this, so it's much shorter or longer than twenty four hours, and unfortunately, that makes a person incapable of aligning their sleep patterns to

the societal norm. People with all these disorders typically work to adapt their lives and careers around their own rhythms. For example, Melinda has a difficult time holding on to jobs that require her to be present early in the morning. That's not because of a lack of dedication to her job, but instead because of her biological clock. So, the way she describes it, quote, instead of changing myself to keep other people's schedule, I have changed my life so I

can function on my time. She operates a freight brokerage business that she runs from her home. She also homeschools her children through the afternoon and evening. Some companies have recognized that people have different circadian rhythms and that it's not necessary to force everyone into the same time mold. So Netflix, for example, doesn't keep track of their employees' work hours, just so long as their work produces results. One Netflix executive said that rigid office hours are a

relic of the industrial age. Now, statistically, you probably have a fairly normal circadian rhythm, but you know what it's like when you disrupt that rhythm and how tough that can be for your brain. When you fly to a distant time zone, you've presumably experienced the fatigue and disorientation of jet lag, which is just a mismatch of your circadian time with the local day night period. You typically get irregular sleep patterns and fatigue, maybe headaches, irritability, the

disorientation as the rhythm struggles to regain alignment. Now, from a long term health consequence, there's no problem traveling to different time zones once in a while, but long term, repeated disturbances of this synchronization can have a negative impact

on health. One study compared two groups of flight attendants who spent the same amount of time in the air, but one group did lots of east west flights, which means they were always changing time zones, while the other group did north south flights, which meant they didn't have

to change their circadian rhythm. So what the study found is that the east west flying tendance had higher stress hormone levels and the part of their brain called the temporal lobe was physically diminished, which led to demonstrable effects on spatial learning and memory. And these sorts of ongoing disturbances in the circadian rhythm are also linked to mood disorders and obesity. So occasional travel is great, but be

careful about shifting your rhythm around too often. And since we're talking about the rhythm, there's a question that needs to be asked about how many times we sleep in twenty four hours. Most of us sleep only once in the twenty four hour cycle, but having this single long period of daytime wakefulness is not necessarily the way our natural rhythms evolved. You've probably noticed that you experience a

dip during the afternoon when you feel less alert. The basic circadian rhythm that makes you sleep at nighttime contains a bump that nudges you towards a nap in the early afternoon. Sleeping only once at nighttime, the way most of us do, is not the only way to achieve good sleep. As you might guess from watching your cat or your dog, Sleeping multiple times a day is the norm throughout the animal kingdom. This is known as polyphasic sleep, referring to multiple phases, and there are lots of ways

to do polyphasic sleep. For example, one technique is to sleep twice, one for six hours during the night and again in the afternoon for ninety minutes. There's another commonly used pattern, which is to sleep for half an hour every six hours. This was the sleep pattern of Buckminster Fuller or other people nap fifteen to thirty minutes every four hours, which was allegedly the sleep pattern of Leonardo

da Vinci. One advantage of polyphasic sleep is that the total number of hours can be greatly reduced from the usual eight in some cases down to just a few hours, apparently without ill effects. Polyphasic sleep is common in historically older cultures and in current non industrialized societies, especially during the winter months. Even in industrialized countries, many cultures engage in a daily post lunch nap. This is known as a siesta in Spanish speaking countries, a riposo in Italy,

and in Japan it's known as an in emuri. And you know who naturally goes in for polyphasic sleep. Infants, they're up and down all day, and you know who else, the elderly. Polyphasic sleep, also, by the way, is often adopted in the military, especially when you have a situation that doesn't allow for long periods of being down and un alert. But as I said, many adults have a monophasic sleep schedule. And so one question that's been asked is does that indicate that humans have evolved to be

different than most of our animal cousins. And the answer is probably not. It probably just shows that we are merely capable of fighting off sleep when we don't want it, for example with the help of caffeine, or forcing ourselves to sleep when we feel we should be getting it, for example with the help of sleeping pills. Okay, so we talked about the patterns of sleep, like when you

sleep and how often in a cycle. Now let's zoom out the camera to the really foundational question of why brains sleep at all, because the thing to note is that sleep entails lost time and reduced defenses. So it's striking that across all species in the animal kingdom, everyone's doing it. In fact, animals in situations that don't easily allow for sleep, they have evolved elaborate mechanisms to get

that sleep. For example, the bottle nosed dolphin can't just go to sleep because it has to periodically come to the surface to breathe. So what does it do. It has one side of its brain sleep at a time. This mechanism has been found in other dolphin and whale species, and what it does is allow them to come up to the surface to breathe, and it also allows them to swim around and avoid predators and even have social interaction during sleep when the mammalian brain would other wise

be in its own world. So all this underscores an important clue for us, given the missed opportunities while asleep and the dangers of sleeping, as well as the convoluted evolutionary adaptations that come about to allow it. Sleep, clearly seems to perform some function for the nervous system. But what is it. There are lots of theories for why we sleep, and these fall essentially into four categories. The first is that sleep is restorative. It replenishes our energy stores.

In other words, sleep may be a metabolic necessity, helping the body recover from the work it did while awake. And in fact, researchers have long known that the amount of slow wave sleep correlates with the amount of exercise that you do when you're awake, and if you're deprived of slow wave sleep by being woken up every time you enter that stage, you're going to be physically pooped out.

A related idea is that REM sleep allows you to replenish the stalks of certain neurotransmitters that you used when you were awake. But this sleep as restoration hypothesis is not totally clear because you have a lot of activity during REM sleep and even during non REM sleep, And there's a bigger reason why this hypothesis doesn't seem perfect.

Although sleep is necessary to all species, there doesn't appear to be a fixed amount required, and when you look at something like horses, they sleep only about three hours per night, whereas koala bears sleep nineteen hours. Now the high activity of horses and the low activity of koalas doesn't seem to accord with this sleep as restoration hypothesis. So it might have something to do with why we sleep,

but it's certainly not the full answer. Okay, So this second theory about why we sleep is that we do it because it's a survival advantage. So here's the idea. Most animals find the search for food and water to be easier during the sunlight hours. At night, it's best for these animals to save energy and avoid getting eaten and evade dangers such as falling off a cliff. So this framework proposes that sleep protects organisms that can't see well in the dark and would face greater risks if

they were active at night. So the idea is, if you just curl up in the corner of the cave, you stay out of trouble. Now, this framework of sleeping to stay out of trouble. This is also imperfect because darkness as a survival threat can be addressed by the evolution of night vision, and it doesn't apply to nocturnal animals which sleep during the day. And also the theory doesn't shed any light on the unusual sleep habits of

for example, the bottle nose dolphin. So onto the third theory about why we sleep, which is that sleep allows us to simulate rare situations. So let's start with the observation that young animals like babies, spend a much larger percentage of their sleep time in dream sleep. Now that observation has invited these speculations that the nocturnal neural simulations may be a really useful or necessary exercise for animals

to test out activities in the real world. Presumably one wants to practice all these programs while your muscles are shut down, and according to some versions of this theory, the periodic stimulation of the cortex in this sort of semi random manner can maintain circuits that you need for survival,

but they rarely get activated like emergency defense procedures. So a specific version of this framework called threat simulation theory, suggests that sleep exists to simulate threatening events and rehearse threat perception and threat avoidance. The theory suggests that people exposed to more survival threats in waking life should more

commonly experience threat dreams at night. So interestingly, researchers tested that idea by comparing participants in a high crime area in South Africa to those living in a low crime area in Wales, and they obtained detailed dream reports over a really long period of time. Now, contrary to the predictions of the threat simulation theory, people in the high crime area actually reported a lower number of threat dreams

than those in the low crime area. And in South Africa, the percentage of realistic threats and dreams was lower than expected. It was only about twenty percent and escape from these threats occurred in less than two percent, suggesting that it's not really much of a simulation. So tests have so far failed to give much support to the threat simulation theory, and that leads us to theory number four about why we sleep, And this one has the strongest legs, and

the idea is that sleep plays a role in information processing. Specifically, the idea is that sleep allows the brain to cement in important memories and to deprogram the miscellaneous events that shouldn't get stored. Of all the different theories, the data seemed to strongly support this idea that sleep plays an informational role. So I'm going to zoom in on this for the next few minutes. The first part of the

idea here is that we sleep to lock in information. So, for example, let's say I teach you some new task on the computer and you're getting better and better at it. If I look at how much better you got, I'll see you got a lot better if you slept between session one and session two. But if you spent the same number of hours awake in between those sessions, you didn't get much better. So this started suggesting to neuroscientists that sleep had something to do with getting better at

the task. And in part this is because a nap turns out to be as good as getting a night of sleep. So if you get even a sixty to ninety minute daytime nap, you do just as well as someone who slept the whole night. But why does learning improve? The evidence generally suggests that you reactivate memories during sleep and that on lies locking them in. And this is because memories require a consolidation period in which the experience becomes part of long term memory, and the consolidation stage

requires sleep. So let me give you an example. Let's imagine that you train a rat to run around a track for a food reward. Now there are neurons that we call place cells in the hippocampus, and if you record their electrical activity, you'll find that this network of cells has a distinct pattern, a distinct signature of activity, depending on precisely where the rat is in the maze.

So the rat occupies all these different locations, and each of those is represented by a different pattern among the place cells. Okay, so now the rat goes to sleep, and if you continue recording from these neurons, you'll find this same sequence representing this position, then this next position, then this next position. You find the same sequence playing out while the rat is in rem sleep, the same sequence that he ran while he was awake. It's like

he's rehearsing the trajectory that he learned. It's so closely correlated with what he did when he was awake that researchers who study this claim that the animal is dreaming about running the maze. They can take that activity and reconstruct where the rat would be in the maze if it were awake, and they can further guess whether the animal is dreaming about running from position to position or standing still in one spot. Now, this sort of thing

doesn't just happen in rats. This same kind of rehearsal happens in songbirds, whose neural activity during sleep resembles their song production activity. So the idea is that this replay is part of consolidating events into long term memory. In other words, the hypothesis is that the information replayed during sleep determines what we later remember. And this idea is consistent with human studies in which we see that the learning of repetitive tasks relies on REM's sleep to improve

your performance. In some cases, this offline practice session seems to be just as useful as practicing the new task when awake, So both rats and humans perform recently learned tasks better after a period of sleep. And these dreaming rat studies are consistent with a human study in which participants learned to play the video game Tetris. That night, they dreamt of falling Tetris blocks. Now here's the really

wacky thing. When subjects with amnesia were trained on the game, they reported these same dreams of these colorful falling blocks, but they had no idea why they were dreaming about such things. So, just as rats dream about repetitive tasks performed during the day, the Tetris players dreamed about their

new video game experience. Now, although a lot of research has study the effects of rem sleep on learning, I just want to be clear that slow wave sleep is also important, and you find the slow wave activity most strongly in the brain areas involved in some new task. So if I have you do some complex I hand coordination task, we'll see enhanced slow wave sleep activity in those areas that were involved. That's where we find the

most action. And what we find is that the more slow wave sleep you see in a region, the better you get at the task. Okay, Now, if sleep consolidates memories, that raises an important question, which experienced events are the important ones to cement in and which events should be taken out with the neural trash. In other words, if your brain is taking in information all the time, are you ever at risk of cementing in associations that are accidental?

Like you shut the refrigerator door and just at that moment a dog barks outside, and your brain might say, oh, there must be a relationship between those two events. Even though it was totally spurious, it was accidental. As it turns out, people have been thinking about this question for

a long time. I found a quotation from seventeen forty nine where the English philosopher David Hartley suggested that quote the wilderness of our dreams seems to be of singular use to us by interrupting and breaking the course of our associations. For if we were always awake, some accidental associations would be so much cemented by continuance as that nothing could afterward disjoin them, which would be madness. End quote.

Now this idea was taken up by Francis Crick and Graham Mitchison, who in nineteen eighty three hypothesized that we dream to forget. That is, they suggested that rem sleep erases these spurious associations between neurons before they inappropriately lock into place. So this theory posits forgetting as an active nightly practice, which is quite different from the traditional view of forgetting as a slow global degradation of information. Now,

there are various reasons to take this hypothesis seriously. So

look at artificial neural networks. They have this tendency to turn into memory mud when you expose them to too many associations, and Krick and Mitchison were inspired to make their proposal not only by the fact that such networks fail, but the way in which they fail, because when these artificial networks become overloaded, their output can mistakenly associate incorrect inputs, or give the same output irrespective of the input, or

respond to stimuli that wouldn't normally evoke a response. And these types of pathological output loosely seem to parallel fantasy and obsession and hallucinations, all of which occur in humans if they don't get REM sleep. So the idea is that daily experience with the world leads to these rich changes in your synapses, and sleep goes through and erases the spurious associations that could otherwise become fantasies or obsessions

or hallucinations. In fact, I'll just note that some mammals, like the spiny ant eater, don't have REM sleep, and they also have larger than normal brains, and that suggests the possibility that animals without remsleep require a larger brain

to prevent overloading. Now, we talked about rehearsal and forgetting, and those ideas operate hand in hand, but there's something else too, which is that intelligent brains like we find in humans also require something else, not just writing down precisely what happened, but restructuring memories, thinking through things, and that lies at the heart of what we call understanding and insight, and sleep appears to be involved in that process, so people often claim to have gained insight into a

problem through sleep. Otto Levy won the Nobel Prize for discovering neurotransmission with an experiment he did on the frog's heart. Now here's the key thing. He had long believed that chemical transmission occurred in the nervous system, but he had no idea how to prove that until the night when he woke up having dreamt the outline of the experiment. He got up and jotted the experiment down, But the next morning he discovered that he couldn't read his own handwriting.

So the next night he had the same dream, and in his biography he relates of that second night quote, I awoke again at three o'clock and I remembered what the experiment was. I got up, immediately, went to the laboratory, made the experiment, and at five o'clock the chemical transmission of the nervous system was conclusively proved. So he seems to have gotten insight from his dream. And in fact, this connection between sleeping and insight has been supported by

the results of careful experiments. So, in one report, people practiced some cognitive task in which they learned to respond to particular cues with particular responses. Now, practice made them better at the task, but although they didn't know it, there was a hidden rule underlying the order of the sequences. So if you could figure out that hidden rule, that

improved your performance. After the initial training on the task, one group got eight hours of sleep, another group stayed awake during the same hours of the night, and a third group was awake for the same number of hours during the day. So one group slept and the other two did not. Then they were retested and it turns out those who had slept for the eight hours were twice as likely to gain insight into the hidden rule,

irrespective of the time of day. So that result suggests that sleep facilitated the discovery of the hidden rule, perhaps by restructuring memory. So all these are possible reasons why we sleep, to rehearse, to forget, and to restructure memory. Now, one more thing I want to cover in this episode

is the issue of sleep deprivation. So while economists worry about the national debt, sleep scientists have become concerned about the national sleep debt because with the demands of modern life, lots of us get less sleep than the demands of our brains and bodies would call for. And the effects of mild sleep deprivation, for example, losing a few hours of sleep for one night, this is surely familiar to you. You get irritable, you get muscle aches, you yawn, you

have trouble maintains attention. With more severe sleep deprivation, say lasting two or three days or longer, you can also get micro sleeps, which are brief sleep periods in the range of seconds or even a fraction of a second, and these show up as a short moment of cognitive absence. So for drivers on the road this is not uncommon and it's very dangerous. Unfortunately, people who experience micro sleeps

will often be unaware that they were just sleeping. They will believe themselves to have been awake, or maybe they think they've temporarily spaced out. Many people who operate heavy machinery or vehicles will experience these brief moments, and this often leads to dire consequences and long term sleep deprivation often caused by disorders of sleep and wakefulness. This can result in really detrimental health like hypertension or diabetes, or

heart disease or stroke. The studies performed over the decades generally converge on a common theme, which is that the effects of sleep deprivation are often more detrimental to the mind than to the body. So subjects with sleep deprivation often perform fine on physical tasks the next day, although their stamina tends to diminish a bit faster, But the

most striking effects are on cognition and mood. Sleep deprivation is a real world problem because you've got medical residents and pilots and soldiers and truck drivers and lots of others who work long shifts that lead to sleep deprivation, and that can lead to poor decision making, which in

turn leads to avoidable accidents. For example, there have been years of research on doctors which make it clear that sleep deprivation causes them to make more errors when they read charts, and they have lower performance because of a distant inclination to apply effort, and they have decreased mood

and poor attitudes. A review of sleep deprivation studies on physicians showed that doctors could still respond without trouble to novel situations if they have to learn a new routine, but their decreased vigilance led to more mistakes in the routine and repetitive tasks that characterize a night in the hospital. Now, I just want to say that we always need to interpret sleep deprivation studies with an eye towards what else could be going on, because often there's lots of other

stuff too, like stress. So you will feel unwell after a night of red used to sleep, but interpret that with a little bit of caution, because often the poor night of sleep comes from a stress inducer like an important exam or a deadline or whatever, and the poor physical feelings that you have the next day might be caused in part by the effects of distress rather than the sleep deprivation itself. Okay, Now, before moving on from sleep deprivation, I just want to mention that sometimes people

can stay awake for remarkably long periods. In nineteen sixty four, there was a seventeen year old named Brandy Gardner who decided he was going to shoot for the world record for sleep deprivation. So, in collaboration with Stanford sleep researcher William Dement and physician John Ross, he was continuously monitored and he kept himself awake for a record breaking eleven days. He then went to sleep, and he woke up feeling fine fourteen hours and forty minutes later, and on subsequent

night he slept a normal eight hours. In other words, he didn't require catching up on a sleep debt. He

merely fell back into his normal rhythm. Now. Dement reported that the sleep deprivation's main effect was on Randy's mood, involving all the changes that are typical of fatigue, and Dement wrote that the deprivation had little effect on Randy's cognitive abilities, pointing out that on the tenth day, Randy won a game of pinball, and on day eleven he gave a press conference in which he appeared healthy and his speech was unslurred. But it should be noted that Ross,

the physician, presented a somewhat different story. Beyond the mood swings, He reported extreme problems with Randy's ability to concentrate. For example, when Randy was asked to start at one hundred and continuously subtract seven. Randy stopped about a third of the way through. He stated that he had forgotten what he was doing. He also appeared to have paranoia and hallucinations, as well as a delusion by the fourth day that

he was a famous football player. At one point, he mistook a street sign for a person, So the effects of staying awake that long may not have been as harmless as Dement portrayed. In any case, the fact that such an extraordinary stretch without sleep is even possible serves as an important data point for any theory of sleep. Other sleep deprivation records have been set, apparently extending to eighteen days and seventeen hours, but these are less documented

and didn't monitor for micro sleeps. What's interesting is that the Guinness Book of World Records no longer maintains a sleep deprivation record because they have decided smartly that they don't want to encourage people to risk health consequences. By the way, I'll just note that many people report sleeping poor in being sleep deprived, but measurements taken at home or in a sleep lab sometimes reveal that the person

slept better than they believed they did. Sometimes a period of wakefulness in the middle of the night gives the lasting impression of a poor night's sleep, when in fact it wasn't that long. But in general, there are fewer things important than making sure that you are getting a good night's sleep. Finally, an episode about sleep wouldn't be

complete if I didn't talk about sleep disorders. Although lots of people get sleep deprivation because of their circumstances and stressful work, some people suffer chronic sleep disorders, and this, by the way, is the most common medical complaints, second only to pain. There are currently about one hundred different types of sleep wake disorders. Almost all of these can be grouped into four categories. There's insomnia, which is difficulty

falling asleep or maintaining sleep. There's hypersomnia, which is extreme daytime sleepiness, and there's parasomnia's which is complex behaviors performed during sleep. And then there's the circadian rhythm disorders, which I already discussed before. So I'm going to turn to

these other three. So let's talk insomnia first. So this is the most common sleep problem, and it involves not getting enough sleep to feel rested from the brain's point of view, insomnia happens when both your sleep and arousal systems are simultaneously active. By the way, it's an interesting side note, there's often a link between insomnia and psychiatric disorders, and the link is bidirectional, meaning either can lead to

the other. Now, to deal with insomnia, some people take hypnotics, which are sleep aids, and although people have used lots of compounds as hypnotics over the millennia, such as opium and barbituates, these often lead to addiction and sometimes to breathing problems. Over the counter hypnotics that are approved by the Food and Drug Administration, they're all antihistamines, and these can produce next morning sleepiness or several other side effects

and eventually tolerance to the sedating effects. Now unregulated, over the counter sleep aids like valerian or hops or lavender or melatonin generally appear to be of no benefit for insomnia. There are only a few pharmaceutical treatments for insomnia, and most commonly these include drugs that modulate the effect of the neurotransmitter called GABA. Now, insomnia can take a heavy toll on mood and productivity, and in the most extreme and rare cases, it can actually be fatal. I'll tell

you a case that's really terrifying. Think about what it would be like to never sleep again. In nineteen seventy eight, there was an Italian woman in her forties who went to the doctor with terrible insomnia. Sleeping aids were totally useless. She simply couldn't fall asleep anymore, and as the toll of the disorder grew over the course of a few months, she could no longer walk and could barely speak, and within one year of the onset of this insomnia, she

died with her body exhausted. A year later, the woman's sister presented with exactly the same symptoms, and she soon died the same death. For both women, their minds remained fine as their bodies disintegrated. So there was a physician named Ignotio Reuter who was a nephew by marriage to

these two sisters. He was drawn to this mystery and began to plumb his wife's family tree for clues, and he discovered that this fatal insomnia had struck down other ancestors of theirs, and he suspected he was on the

trail A rare genetic disorder. So when his wife's uncle, Silvano, came to visit, Reuter spotted the early onset of the symptoms, and he convinced Silvano to go to an Italian sleep clinic, where his worsening insomnia and progressive decline were documented on video, and then Silvano died at the age of fifty two.

Now Silvano's brain was quickly removed and flown to a lab at Case Western Reserve University in Cleveland, and there a neuropithologist named pure Luigi Gambetti examined the brain and noticed that it was shot through with tiny holes, like a sponge. This reminded him of something that he'd seen before, a brain disease known as kraitzfeld yakub, which is the human form of mad cow disease. So Gambetti called Stanley Prusner, a scientist who had recently suggested that these brain diseases

were caused by abnormal proteins called prions. So Prusner performed studies and confirmed that this insomniac family had a prion disorder, and he won the nineteen ninety seven Nobel Prize for his investigation of prions in part for the work on this family. Now, in the intervening years, more than thirty other families who are carriers of this genetic disorder have

been discovered, and this is now known as fatal familial insomnia. Thankfully, most types of insomnia are typically much milder and more treatable. For example, one of the most common forms is restless leg syndrome, where upon falling asleep, a person feels unbearable discomfort in the legs that calls for relief by stomping or rubbing or twitching the legs. And this restless leg syndrome is experienced by about five to fifteen percent of

the population and appears to have a genetic component. But fortunately, medications can successfully address restless leg syndrome in most cases. Okay, so that's a quick overview of insomnia. But there's also the opposite of insomnia, known as hypersomnia. People with hypersomnia have excessive sleepiness. Now this is different from being tired

because of a late night. This exists at such a high level that one has no choice but to nap frequently during the day, including inappropriate times such as in the middle of a conversation or a meeting. Or lunch. The naps typically provide little relief from the excessive sleepiness, so hypersomnia often comes with anxiety and disorientation upon waking and diminished energy and memory problems. As you can imagine, hypersomniacs often can't participate in the normal settings of fail family,

in work life. Sometimes this disorder occurs as a result of taking medications for depression or having excess body weight or suffering brain damage, and treatment options typically target the symptoms because often the cause is unknown. This hypersomnia this is the main symptom of narcilepsy, which is something you've surely heard of, where people have sudden attacks of sleepiness

and will fall asleep. Aside from having extreme fatigue and frequent naps, narcalptics ironically can also have pretty poor nighttime sleep.

They have other issues too, like sometimes right when they're in between wake and sleep, they'll have hallucinations or they'll do automatic behaviors in which they spontaneously produce purposeless sounds or acts with no conscious intervention or censorships where sometimes in a person with narcilepsy, there muscles will suddenly weaken and this can manifest as a slight sagging all the way to a complete shutdown of the muscles leading to collapse. No,

how do we understand this? Note that narcoleptics fall abruptly into REM sleep directly from the waking state, whereas normally you pass through the stages of non rem sleep first. In REM sleep, the major skeletal muscles are shut down, and in narcilepsy, this is what happens. The body drops straight into REM sleep and causes the major muscle groups

to just stop functioning. So this mistimed activity to the muscles also plays a role in another common symptom of narclepsy, which is sleep paralysis, and here the shutdown of the muscles lasts longer than it's supposed to, such as the brain wakes up but the body is unable to move for a short period. Narclepsi appears to be a genetic condition, and it results from a lower level of a hormone

called ourexin, also known as hypocretan. This hormone promotes wakefulness, so in the late nineteen nineties, researchers discovered that people with narcolepsy have fewer neurons producing this orexin or hypocretin, and they realized that narcilepsy can be an autoimmune disorder in which the immune system attacks these neurons. Now, remember earlier I talked about these two mutually inhibitory networks that

take care of wake and sleep. The current view is that the orexin hypocreting system stabilizes this system in the waking state, and if you don't have the proper level of these molecules, the flip flop of these networks is unstable and can switch states at inappropriate times. Okay, so that's what I want to tell you about hypersomnia, where the balance is disrupted in the direction of too much sleep instead of too little. And there's one more category

of sleep to that's important. We've been talking about the states of sleep and waking as a flip flop system. But keep in mind that the arousal network or the sleep network consists of lots and lots of sub areas, and these areas typically work together in a smooth manner so that when the networks switch, all the appropriate systems come online when the others go offline. But given the complexity and size of these networks, the brain sometimes gets

caught between stages. In other words, some neural areas have completed the switchover and others have not, and in these cases we find Parasomnia's where you get actions performed during sleep that are not under voluntary control, for example, sleep walking also known as somnambulism, like we saw in Kenneth Parks at the beginning of this episode. Parasomnias used to be thought of as a disorder, but they're now understood to represent a mixture of the waking state and some

stage of sleep, either rem or non rem. These parasomnias come into being as the brain transitions from one state to the next. Now, non rem parasomnias are these disorders that happen when a sleeper's brain tries to jump directly from a deep sleep into the waking state and it

becomes caught in between. So you get things like sleepwalking, but you also get things like talking in one sleep called some niloquy, or sleep eating or teeth grinding, or night terrors in which a sleeper will bolt upright in fear, often with a scream or a gasp, and have a temporary inability to regain consciousness. Now, most of these non rem parasomnias exist more commonly in childhood, and they tend

to diminish in frequency with age. Now you also have REM parisomnia's, and the most common of these is REM sleep behavior disorder, in which the paralysis of the muscles that normally happens during REM is not there, so sleepers will act out their dreams, which usually results in injury to themselves or others. Ninety percent of patients with this disorder are male, and they'll sometimes try to deal with this problem by tying themselves directly to the bed or

constructing fortresses of pillows. Happily, this can typically be dealt with with gabba enhancing pharmaceuticals. Now, as the neuroscience of sleep progresses, new parasomnias are being identified. For example, there's a fairly newly described parisomnia, which is sleep sex or sexomnia, in which a person engages in sexual acts while asleep. Several defendants in sexual assault trials have been found not

guilty because of their sex omnia. Now, many people decry this plea as an excuse, and possibly sometimes it is, but sex omnia is a real thing, and people can have full experiences with no conscious awareness or memory of them. And when people do become aware. There's typically a lot of distress and shame, so people often don't even go to the doctor for help, but they should because this can often be dealt with with antidepressant and anti anxiety medications.

So now that you have listened to this episode, you understand that there are detectable differences in the activity of the sleeping brain in people with parasomnias. Although we tend to assume that the brain must be either awake or asleep, many well studied phenomenon like the parasomnia's result from the brain truly caught in an intermediate state between the two. In a network of on hundred billion neurons, there's room for a tremendous diversity of different modes of activity, many

of which were only just beginning to understand. So let's wrap up today's episode. It's not clear why we spend so much of our live sleeping. Arguments from different angles suggest that sleep promotes restoration of the energy you burned during the day, or keeps us out of trouble in the dark, or allows us to practice neural programs that we don't otherwise get the chance to do, and or

it helps us consolidate memories. These suggestions are not exclusive, nor perhaps are they complete, and it remains to be understood how sleep restructures memory and inspires insight. Now, at the beginning of this episode, I introduced Kenneth Parks, the man who killed his mother in law and assaulted his father in law during an apparent sleep walking episode. The idea of murdering in one sleep tends to challenge the belief of the public, and people will typically dig into

the position that the plea is a fraudulent excuse. But whatever the merits of any individual case, we know that sleepwalking is a real phenomenon. We saw that transitions between waking and sleeping result from these opposing networks, and that parasomnia's can result when these networks don't hand off power smoothly. Now, a normal sleep cycle moves from very deep sleep into

more and more shallow sleep and eventually into wakefulness. But Kenneth Parks's EEG revealed that he had a parasomnia about ten to twenty times each night. His brain networks attempted to transition directly from deep slow wave sleep directly into wakefulness. The jury came to understand that there was no way for him to fake EEG results intentionally, and so those measurements proved critical to his defense, and after hours of deliberation,

the jury found Kenneth Parks not guilty. Now, there are other cases that have fared differently. In nineteen ninety seven, Scott Fallader was accused of stabbing his wife forty four times and holding her head underwater. He pled homicidal somnambulism, but he was convicted and sentenced to life in prison. In total, there have been dozens of homicidal somnambulism cases tried in North America going back to the sixteen hundreds, and more recently, parasomnia defenses have been offered in cases

of sleep sex. Can the legal system assume that some of these defenses are fake? Perhaps, but presumably not all of them. In many cases, defendants have been a quit after careful measurements of their brain activity. So back to Kenneth Parks. Most somnambulists are nonviolent. But note that Parks had insomnia, and he was depressed, and he was trying to overcome a gambling problem, and he had financial and marital problems, and he had also been massively sleep deprived

from not having slept the night before that event. So, although it's difficult to know exactly how these factors played a role the physiology of his brain as measured in an EEG laboratory, this was the clinching piece of evidence in convincing the jury that he did, in fact suffer from abnormal transitions from slow wave sleep to the waking state, transitions that made his actions involuntary. So cases like this one highlight the mysterious and poorly understood border lands between

wakefulness and sleep. So the amazing thing to me is that your brain can operate in three very different states. There's the waking state, there's rem sleep, and there's non rem sleep. And these three states differ fundamentally. They represent the same machinery running very different tasks. One of the amazing features of the brain is its ability to transition

between these states, usually without a problem. And although the parasomnias represent cases in which the brain has trouble getting a state fully locked into place, it's amazing that these don't happen more often. So this episode has been an introduction to the main landscape about the sleeping brain. Join me next week as we zoom in the camera to the topic of dreams and the whole wild world of

what we know and don't know about them. And then two weeks from now will come to lucid dream, where you become aware that you're in a dream and you get to grab a hold of the script. Until then, make sure that you're getting sufficient sleep, although you are not consciously there for most of it. It is one of the most important things that your brain does. Go to eagleman dot com slash podcast for more information and

to find further reading. Send me an email at podcasts at eagleman dot com with questions or discussions, and check out and subscribe to Inner Cosmos on YouTube for videos of each episode and to leave comments. Until next time. I'm David Eagleman and this is Inner Cosmos.