The Science of Everything Podcast - Episode 131: Sleep Science

Episode Date: September 11, 2022

I discuss the mysterious phenomenon of sleep, outlining the different stages of sleep, how the brain controls sleep and wakefulness, and the various theories for the functions of sleep. I also consider sleep in animals, the effects of sleep deprivation, and some major sleep disorders. Recommended pre-listening is Episode 38: Neurons and Synapses.   If you enjoyed the podcast please consider supporting the show by making a PayPal donation or becoming a Patreon supporter. https://www.patreon.com/jamesfodor https://www.paypal.me/ScienceofEverything

you're listening to The Science of Everything podcast episode 131, sleep science. I'm your host, James Fodor. Now, this is an episode I've been wanting to do for quite some time, and finally, well, here we are. I'm going to be talking about sleep, which is something that is near and dear to all of us, I think. We spend about a third of our lives sleeping, but we perhaps don't think very much about exactly what it is or what it is for. So this episode, I'm going to talk about what we know about the science of sleeping, including some of the different stages of sleep, how sleep is regulated by the brain. We'll talk about circadian cycles and so forth, and how sleep and wakefulness are regulated.

I'll talk about some of the effects of sleep deprivation and how you can get better sleep by looking at some techniques for sleep hygiene. We'll also talk about some of the functions of sleep, which is a highly disputed area, but still very interesting since we know relatively little about why we actually sleep. Recommended pre-listening is episode 38, Neurons, and synapses, where I give some introduction to the neuron, which will be helpful for some of the material today. And without further ado, let's make a start then, and I'll begin by talking about what sleep is and the stages of the sleep cycle. So sleep is a naturally occurring state of mind and body, which is characterized by an altered consciousness and inhibited sensory activity, reduced muscle

activity and diminished interactions with our surroundings. So we sometimes talk about sleep as if it's a state of unconsciousness. We talk about, you know, returning to consciousness after sleep. But that's technically not correct because sleep is a state of consciousness, because you are still, for example, able to perceive stimuli and able to move your muscles while asleep, at least to some degree, as we'll talk about, that varies a bit between different stages of the sleep cycle. So it's not like you're unconscious when you're asleep. I mean, it's sort of obvious if you think about it, right? Because a bright light or a loud noise can rouse you out of sleep. So obviously, your brain is still able to perceive and integrate sensory signals from the environment. Otherwise, that wouldn't

happen, right? So sleep is an altered state of consciousness, which is naturally occurring. And its primary distinction from wakefulness is by a decreased ability to react to stimuli and much reduced bodily activity and muscle activity. However, there is much greater activity than in a coma. Perhaps we'll talk about other altered states of consciousness in another episode. So a sleep is quite different from a coma, whereas a coma is a pathological state, sleep is a natural state, as well as sleep is characterized by a much higher level, generally, of brain activity than you would have in a coma.

So sleep occurs in repeating periods, as I think everyone knows. So we generally sleep once every day. and during sleep the body alternates between two distinct modes of sleep. In fact, the two different modes of sleep are so distinct from each other that some sleep scientists have actually suggested that we really shouldn't even think about sleep in quite that way. We should really talk about the two different types separately as two distinct modes of altered consciousness from wakefulness.

So there's sort of wakefulness, and then there's REM sleep and non-REM sleep. So REM stands for rapid eye movement, R-E-M. and it's something you may have heard of before. It gets brought up sort of in movies or popular culture periodically since it was discovered, I think, in the 50s or so. It's a relatively recent discovery. Rapid eye movement sleep is characterized by, well, you guessed it, rapid movement of the eyes.

So the eyes kind of dart around. Typically, we sleep with their eyes closed, of course, so you perhaps don't notice this. But if you look at a video of someone while they're asleep, during REM sleep, you can see that their eyes are often darting around. very very rapidly which is which is how it gets its name. Rem sleep is also distinguished by different patterns of brain activity which is how it was originally studied and discovered but I'll come to that in a moment before we get to the details of REM sleep

let's talk about the other stages of sleep as well but just at the outset remember that it's important to keep in mind the distinction between REM and non-REM. That's the most important distinction. The other stages of sleep are all parts of or aspects of non-REM sleep. Okay so let's talk about what happens when you fall asleep starting from wakefulness? So while you're awake, the EEG activity of your brain, EEG, which stands for electroencephalogram, is a technique which measures the overall patterns of electrical activity

across the surface of the skull. It doesn't directly measure brain activity in the sense that it's measuring neurons firing. What it does is that it measures the overall average electric fields at different points on the surface of the skull. Now, those electric fields are, generated by the polarization and depolarization of neurons, mostly in the cortex, but it's a bit misleading to think of it as a direct measurement of brain activity, because it's only a somewhat

indirect measurement of the fields produced by the aggregate firing of many millions or really billions of neurons. So it's not a very precise measurement either, because a given electrode on the skull will measure very large areas of the core activity over very large areas of the cortex. But nevertheless, it is useful for many purposes, including studying sleep. So when people talk about, you know, measuring brain waves or you see these graphs of the squeakly lines that go up and down, these are electroencephalograms or electroencephal graphs for the graph of it. And this is a common way of studying the different types of brain activity in different parts of the sleep cycle. So during wakefulness, EEG activity is typically characterized by what's sometimes called alpha waves. So this is sort of

the standard wakefulness type of brain activity. And it consists of sort of moderate series of spikes. It doesn't necessarily look like much by itself, but it's important in regards to how it compares to brain activity in other stages of sleep. We start with alpha waves while we're awake. The first stage of sleep then is called stage one sleep. So it's basically stage one through stage three. There used to be a stage four. I believe that recently the stage four, the stage four, four and stage three of non-REM sleep were consolidated into a single stage. So some older texts you might see four stages referred to, but I'll just talk about three stages here.

So we have stages one through three of non-REM sleep, and then we have REM sleep. So we'll get to REM sleep in a moment. So what generally happens as we go from wakefulness through stages one, two, and three of non-REM sleep is that the amplitude of the EEG waves progressively increases. but the frequency progressively diminishes. So in other words, the waves become longer wavelength, but higher amplitude. Now, there are a few particular unique characteristics of the EEG. In particular, stage two is characterized by particular patterns called sleep spindles,

which are periodic clusters of more rapid frequency waves. Remember that in stage two we're beginning to reduce the frequency of the EEGs, but sleep spindles kind of has a rapid clustering of more rapid frequency waves. And also K complexes, which is essentially a very large upward deflection followed by a slower downward deflection in the EEG. I'm not entirely sure what the significance are of these phenomena, but they're well known from studies of stage two of non-REM sleep. By the time we get to stage three, we observe delta waves in the EEG, which are quite high amplitude and long wavelength variations in the EEG. So just to recap, we go from alpha waves in wakefulness through theta

waves in stage one, stage two showing the sleep spindles and K complexes, and by the time we get to stage three, we see the high amplitude long wavelength delta waves as the wavelength of the EEG is gradually increasing and amplitude also gradually increasing as we move from wakefulness to stage three sleep. So stage three is also called deep sleep. It's the most difficult state of sleep to row someone from typically, and in a sense furthest away from wakefulness with respect to say brain activity and degree of responsiveness to stimuli. Now, so far that kind of presents a fairly regular pattern, right? Is that, okay, well, you go relatively more deeply into sleep, and the brain activity changes more with respect to what it looks like during wayfulness, and then you become harder

to rouse and so forth. But then something weird happens. It usually takes about an hour or maybe an hour and a half to get to stage three sleep, but then what happens once you've got to stage three sleep and been there for, I don't know, 10, 20 minutes or something, is that you'll actually start to go up again. So we go from a wake down to stage three. But then maybe after 60 to 90 minutes, you'll start to go back up. You'll go from stage three to stage two and then up to stage one again. And then what happens is that you go from stage one to entering REM sleep. So remember I said the big distinction is between non-REM and REM sleep?

And stage one to three is all non-REM sleep, right? Well, REM sleep is only entered after you've gone from Wakefulness down to stage three and then back up, so to speak, to stage one, and then you enter REM sleep. So, but typically you don't fall asleep and enter straight into REM sleep. You have to go down, so to speak, through stage 3 first, and then come back up and enter REM sleep. And the interesting thing about REM sleep is that the brain activity, the EEG, looks quite similar to the alpha waves characteristic of wakeful EEG. It looks a little bit different, but it's pretty similar and much more similar than any of the non-REM sleep EEGs. So it's almost like when you've entered REM sleep, you're kind of awake again.

And that's also characterized by the rapid eye movements, which are much less common in non-REM sleep. So there's so much that we don't know about REM sleep, one thing that we do know is that during REM sleep, there's large or complete loss of muscle tone. So you lose control over skeletal muscles, which means that you can't, or at least typically can't move skeletal muscles during REM sleep. And it's thought that one of the reasons for this might be because REM sleep is also the time when most dreaming occurs. again, dreams can occur at other times, but from sleep studies, we know that dreams are very common during REM sleep. In fact, most of the time when people enter REM sleep, they will experience some sort of dream state. However, very frequently you won't remember those dreams when you wake up. So some people say that they rarely, if ever, dream.

That's almost certainly not true, unless perhaps they have an unusual sleep pathology. It may simply be that they don't remember their dreams. A good way to tell, if you're interested, is having someone wake you up when you enter REM sleep and asking you if you recall any dreams. And most people do when they're awoken during REM sleep report that they were experiencing a dream. It's thought that the loss of muscle tone and inability to move gallopal muscles during REM sleep

may be a mechanism to prevent or diminish people from acting out in response to the dream state that they've entered into. Why we dream in the first place is unknown, and I'll talk a bit more about that later when I get to talking about the function of sleep. So now, coming back to the cycle of the different stages of sleep, Remember, we've got starting from wakefulness, we go from stage one to three, and then from three back to one, and then we enter REM sleep.

That whole cycle, if you think of REM as kind of like the closest back to wakefulness, but it is still asleep, that whole cycle will take perhaps two hours or so. And then what happens over the course of a night, say roughly eight hours, which is about the average duration of sleep for adults, we repeat this cycle of going from REM through to stage three and then back up to REM and then so forth. However, there are a few changes over the course of the night. What happens is that the cycle, the frequency of the cycle increases. So that means that you go through the whole cycle in less time. You get back to REM sleep more quickly.

The other thing is that each time you leave REM sleep, you don't move as far down into the stages of sleep. So for the first couple of cycles, you might enter stage three, but then perhaps in the second cycle you're only into stage two, and then maybe the last cycles of the night you only get down to stage one before going back into REM sleep again. So essentially over the course of the night, the sleep cycles become faster and less deep in the sense that you don't go as deeply down into the stages. And that means that you get back to REM sleep more quickly and spend more time, relatively speaking, in REM sleep in the later part of the night compared to the earlier part of the night.

And this is a very consistent finding. Most people have a sleep pattern more or less like this, although, of course, there is variation. So each of these cycles will take, you know, somewhere from 90 minutes through maybe to 60 minutes or even less of the very. very end of the night. And so one might pass through four or five or so of these cycles over the course of the night of a full eight hours sleep. Now one interesting phenomena that's associated with this is a phenomenon called REM rebound. What this refers to is that if one is sleep deprived and then one is able to catch up on sleep at some point, not only will one sleep longer, because you can catch up on sleep that you've missed out, but also a higher proportion of that

sleep will actually be REM sleep. So it seems that the body catches up not only on just sort of sleep, but it disproportionately catches up on REM sleep as well. We really need that REM sleep. Part of this may also be that if you wake up early to an alarm clock, then it's likely that the stages of sleep that you are missing are disproportionately REM sleep because we typically engage in more REM sleep towards the end of the night. So if you're cutting that off, cutting an hour or an hour and a half or whatever of that off, you're most likely cutting off more REM than non-REM sleep. And that can also contribute to REM rebound if you're disproportionately missing out on REM sleep. Sleep studies have been done where the sleep regularly wake people up

when they enter REM sleep. And then they allow them to go back to sleep, right? And you can fall back to sleep fairly quickly. But the thing is you won't enter REM straight away, you'll have to go through the stages again, right? And so if you keep waking people up during REM sleep, you can prevent them from getting REM sleep or very much REM sleep, even though they may be sleeping for quite a large number of hours. And in these studies, it's found that, so people who are basically selectively deprived of REM sleep are very irritable and have trouble concentrating and things like that. So REM sleep seems especially important for, well, for some reason, again, we'll get to that in a moment, but it's definitely very important and something to bear in mind, particularly if one

wakes up to an alarm clock regularly, that you may be disproportionately missing out on REM sleep. All right, so that's a bit about the stages of sleep. Let's now talk about some of the aspects of the brain regulation and control of sleep. How is sleep controlled and regulated by the brain? How does the brain know, quote unquote, when to go to sleep? The anatomy here gets a little bit complicated, and obviously I won't be able to explain all of the detail in an audio podcast, but I'll just try to give you a basic idea of some of the key players here. One of the systems that's most important is something called the ascending reticular activating system, or arras. And so this is a set of nuclei, so nuclei is just essentially like a bunch of,

of neurons cell bodies that are clustered together. So we don't mean like an atomic nuclei. We mean like a bunch of cell bodies, specifically neurons. So the Aris is a bunch of nuclei in the found in the brains of vertebrates, which is responsible for regulating wakefulness and sleep-wake transitions. So it's a kind of a distributed cluster. It's not like it's in one precise location, right? But most of these nuclei are located in the ponds, thalamus, and a little bit of the of the forebrain. You may not be familiar with these anatomical regions, but essentially the ponds is located in the brain stem, so it's sort of the bulge at the very upper part of the spinal column and at the very base of the brain. And it's the kind of evolutionarily older part of the brain,

which deals with many metabolic and sort of basic functions. The thalamus is located kind of just a bit above that and has many, many different functions. And the forebrain is the evolutionarily sort of more modern part of the brain, which includes, among other things, the cortex. So the basic summary there is that many, although not all, but many of the regions that control sleep and the sleep wake cycle are located in the fairly evolutionarily old parts of the brain located sort of further down. So not really in the cortex, which is where the more higher level sort of cognitive function and sensory processing occurs. So the cortex is a kind of wrinkled outside part of the brain. It's kind of most of the regions we're talking about are kind

of below and or inside of that. Now, many of these neurons, the nuclei in the ascending reticular activating system, many of them release modulatory neurotransmitters, such as serotonin, neuropine, and histamine, which I believe we've talked about before, and these modulatory neurotransmitters project into across the cortex and throughout the brain, and essentially promote wakefulness, or many of them do. So the basic way we can think about this is that the nuclei in these activating systems project a modulatory effect throughout the brain, which kind of keeps it awake, promotes wakefulness, and kind of keeps the brain running. A modulatory neurotransmitter is one that helps to kind of regulate or modulate affects the

firing and the activity of other neurotransmitters. It's not involved directly in synaptic transmission in the same way as some neurotransmitters are, but it's involved in kind of modulating other, affecting other neurons. Damage to ascending reticular activating system typically causes coma an inability to be wakened. So we know that these regions play an important role in sort of keeping us awake. Now, during sleep, neurons in the ascending reticular activating system have a much lower firing rate than they normally do, which is consistent with the fact that they are not promoting wakefulness to the same extent. So in order for us to reach a sleep state, the modulatory signals of the ascending recticular

activating system have to be kind of dampened down, because that's what causes the brain ultimately to kind of enter the sleep states. An interesting aspect of this is that because one thing that we don't want to happen is for the brain to kind of be flipping in and out of sleep very rapidly, like a flickering screen, that's not really going to have any of the benefits of sleep, nor really any of the benefits of wakefulness. That's going to be a very sort of confused state. And so the brain has a system of mutual inhibition where effectively when the brain is in one state, say a wakefulness state, that promotes the inhibition of the other state. So it basically inhibits neurons that would promote sleep when we're awake. And vice versa, when we're asleep, that leads to an inhibition of neurons or nuclei that would promote wakefulness. Obviously, it does have to be a way for ultimately to transition between those two states.

Otherwise, you'll stay awake or asleep forever. But the point about the mutual inhibition is to ensure that once you're in a state, it's fairly stable and you stay there for some time. And that's achieved by this sort of mutual inhibition to make sure each state is fairly stable. Okay, so that's kind of how sleep is controlled directly by the, the brain. It's these particular populations of modulating neurons, mostly located, kind of in the brain stem or the thalamus, which are able to project these modulatory neurotransmitters throughout the cortex and the higher parts of the brain to promote weightfulness. But how is sleep regulated? That is, how do we control sort of when we want to get into the sleep state and then

when we want to come out of it again and go into the wake state? Well, this is actually quite interesting because, I mean, I think everyone knows that, well, as you're awake longer, you get progressively more tired, right? And eventually you've become sufficiently tired so that you can fall asleep. That factor or that aspect is called a homeostatic sleep drive. So basically, the homeostatic sleep drive is an internal drive to sleep, which gradually increases the longer you're awake. And it decreases while you're asleep. Not necessarily linearly, but the idea is that, well, the longer you're awake, the sleepy you get, and the longer you're asleep, the less sleepy you get, so to speak. So that's the homeostatic sleep drive. But that by itself is not

sufficient to regulate sleep. And there is another critical system or a set of forces which also helps demodulate and regulate sleep. And those are called circadian factors or circadian drive. And many people have probably heard of circadian rhythms. Socadian just means daily. And so So circadian rhythm is technically any set of processes which regulate bodily functions on roughly a daily time span. And there are actually many, many of these throughout the body. It's not just related to sleep. It's also related to production of hormones and digestion and the immune system and many other things as well. But in the case of sleep, which is what we're focusing on here, sleep is governed by a combination of homeostatic and circadian factors or drives, as they're called.

So it works like this. As I said, the homeostatic drive is sort of the simplest one. It increases while you're awake and decreases while you're asleep. The circadian one is linked to the 24-hour cycle. So the circadian drive for arousal, as it's often called, is highest sort of during the day and lowest during the evening and nighttime. So the drive for arousal typically increases around the morning, sort of late morning, and increases over the course of the day,

reaching a peak, sometime perhaps around midday or perhaps early to late afternoon depending that's going to vary between persons and then it stays fairly high until it rapidly decreases around kind of late evening so it's the combination of these two factors or if you like the interplay between these two factors which determines when we're actually awake and when we're actually asleep so the homeostatic drive to sleep pushes us to the sleep state remember we talked about those uh we talked about the sleep state and how the sleep in the wake states inhibit each other so it's a question of which one are we pushed into by these different drives right so the homeostatic sleep drive pushes us into the sleep state, whereas the circadian drive for arousal pushes us into

the wake state. Now, if we think about how this works over, over different times in the day, let's think about, say, around midday. So around midday, the homeostatic drive for sleep is fairly low. I mean, it depends when you wake up, I suppose, but you know, you've been awake for some number of hours, but you're still going to be awake for some number of more hours yet. So it's low to moderate at this point, whereas the circadian drive for arousal is kind of at its peak at that time during around midday. So you've got moderate sleep drive and high wake drive. So you're going to be awake at that time. As indeed most people are, you generally awake and don't feel particularly drowsy during the middle of the day. This is assuming if you're not, that you're not

sleep deprived. Now, it is a little bit more complicated because often there is a period of increased sleep drive during the kind of early mid-afternoon, and that corresponds to a time when some people take naps. I'm just going to kind of put that to the side at the moment because I just want to stick with a simpler circadian cycle. So let's put aside mid-afternoon drowsiness or like post-lunch drowsiness. And let's just talk about a simpler cycle. So we kind of have our fairly low to moderate sleep homeostatic sleep drive and high circadian arousal drive around midday. Let's fast forward and look at kind of mid to late evening. At this point, we have a fairly high homeostatic sleep drive because we've been awake nearly all day. So that's pushing us towards sleep. However, as long as the

circadian wake drive or arousal drive is still relatively high, we're still going to stay awake. And this persists until at some point later into the evening, at which point the circadian drive for arousal fairly rapidly collapses. And this corresponds to you feeling pretty tired. And as I'm sure most people know, at some point in the evening, you can feel, you know, I'm still feeling fine. And then fairly quickly, you know, within half an hour or so, you're feeling, you know, I'm actually pretty tired now. That, assuming there's sort of nothing else going on, of course there can be external stimuli which change this. But typically, what's happening there is that your circadian drive for arousal is rapidly

diminishing at the end of the day. And because your homeostatic sleep drive is quite high at that point, you're beginning to feel very, very sleepy. And so as your circadian drive for arousal decreases further, generally people will fall asleep, will go to sleep. And that's the point at which their circadian drive for arousal kind of reaches a low point and the homeostatic sleep drive reaches a maximum point. So now there's the maximal difference between the drive for sleep and the drive for arousal and sleep's winning out, so you fall asleep.

Now at this point, let's assume you stay asleep for around eight hours or so, seven or eight hours. What's happening while you're asleep is if you're sleeping during the night and to early morning, the circadian drive for arousal stays fairly low. It doesn't really change very much during the night. The homeostatic sleep drive is progressively decreased as you're sleeping, right? So you go from maximally sleepy to less and less as you sleep. So what happens, therefore, is by the time it reaches kind of late morning, which is when many people wake up, you know, 7 a.m. or 8 a.m. or, you know, whatever it is you wake up exactly.

By the time it reaches, it's coming up to that time. The homeostatic sleep drive is actually fairly low. But the circadian drive for arousal is also fairly low. So generally, you'll still stay asleep so long as you sort of still need some sleep. What can happen is if there is a disturbance in the, the earlier morning, such as a noise or light from the outside, for example, that can rouse you. And this can happen potentially before you've actually had quite enough sleep. But if you've had, say, suppose you need eight hours of sleep and you've had six and a half hours,

the homeostatic sleep drive is fairly low. The circadian drive for arousal is also fairly low. But if there's an external stimuli, that may cause you to awake sort of prematurely. But if that happens, it may be, and this happens to me quite a lot, quite difficult to fall back asleep because your homeostatic sleep drive is relatively low because you've you've had quite a lot of sleep but just not quite enough yet and this can be a potential difficulty right you've woken up too early but let's suppose that doesn't happen and you sort of sleep through and get enough sleep then you'll wake up at the point where homeostatic sleep drive has sort of reached its minimum and

been exhausted and the circadian drive for arousal then begins to rapidly increase you know usually around the late morning so that's what happens in the late morning you awake the circadian as the circadian drive for arousal is increasing and the homeostatic sleep drive has sort of reached its minimum. And then over the course of the morning and into the afternoon, the sleep drive for arousal rises fairly quickly and then stays fairly high for most of the day until the evening again, and the homeostatic sleep drive gradually increases over the course of the day. So it's these combination of factors between the homeostatic sleep drive and the circadian arousal drive that controls sleep and wakefulness. Now the circadian drive for arousal, as I said, is a daily cycle. So it's on pretty much

exactly a 24-hour cycle. Sleep studies that have put people in complete isolation with no clocks, no natural sunlight or anything like that, tend to find that people's circadian cycles naturally run on a slightly longer than 24-hour period. I can't remember exactly how long it was, like 10 minutes extra or something perhaps. So people become progressively desynchronized from the actual rising and setting of the sun. But that's only in artificial environments. Humans naturally live in sunlight and indeed it's not really healthy to live without any access to sunlight and fresh air for any length of time, partly for this reason. And so what happens is that exposure to the sun helps to reset and synchronize your circadian drive and the other circadian rhythms,

but including the circadian drive for arousal to the day and night cycle of the sun. Now that's not necessarily going to be the same for each person, right? Some people have circadian clocks such that they prefer to go to bed earlier and wake up. earlier, others prefer to go to bed later and wake up later. Part of that is cultural. Part of that is just whatever habits you've gotten into. But part of that is actually biological. There are studies on people that show that there are kind of built-in differences between people's sort of circadian clocks, which do vary. And so some people sort of will have a underlying predisposition to be early rises or later rises. But in either case, there is still going to be this

external input, which keeps the circadian clock synchronized with the rising and setting of the sun. And this actually happens through a variety of mechanisms, but one mechanism is these special light-sensitive retinal gangling in cells in the retina. These are light-sensitive, but they don't actually contribute to the formation of any images that you can perceive in your visual system. Rather, they send signals to a nucleus of neurons called the supra-chiasmic nucleus, which in turn projects to the various sleep-promoting and activating systems that we talked about before, mostly in the brainstem and the thalamus and hypothalamus. So it's actually directly through sunlight that's being perceived by special cells in your retina,

which is then projected through various brain systems, that your circadian cycles are actually synchronized with the sun. And this is one reason why it's important to get sunlight, in addition to needing it for vitamin D and other factors. you actually need to receive enough input of light to synchronize your circadian clocks. Now, there's another important regulatory system, which we should talk about, which is the hormone melatonin, which you may have heard about. This is a hormone produced in the pineal gland, and it binds to receptors in the supra-chaismic nucleus and acts as a signal of darkness.

What melatonin does, effectively, is that it reduces or inhibits the activity of the superchaismic nucleus and effectively then leads to relative deactivation or inhibition of the reticular activating system as well as other control systems and a relative activation of sleep-inducing systems. So the superkiosmic nucleus has these sort of two different inputs. You've got the sun, which is a signal of light, which ultimately through a series of connections via the super-geismic nucleus leads to promotion of weightfulness. and then you've got melatonin produced by the pineal gland, which serves as a sort of signal for sleepiness or a signal for darkness.

And so these act in concert, right? So that's why exposing oneself to light in the morning and during the day can help to sort of wake you up and kind of synchronize up your sleep cycle. And some people take, I do this as well, take melatonin orally, say an hour or so before bed to help promote sleepiness and sort of signal to your brain a series of darkness. Now, one thing you might be worrying about is how exactly is this 24-hour cycle maintained by the body? Because, as I said, it's synchronized by external sunlight.

But you don't need to have external sunlight. It won't stop if you don't have external sunlight. It just gets a bit off track, as I said, for people in those sleep studies. How does that work? Well, it's relatively recently that we've sort of uncovered this. And turns out these circadian rhythms are regulated by a particular set of neurons, as I mentioned, in the superk seismic nucleus, which is found in the hypothalamus, and projects to both the sleep-promoting and sleep-activating systems

that we talked about earlier. These cells in the supra-cysmic nucleus, in turn, contain a special genetic clock, which consists of a transcriptional and translational cycle, causing the outputs of the cell to vary on an approximately 24-hour cycle. So the genetic clock here, as I said, consists of a transcriptional-translational cycle. So basically what that means is that a gene is transcribed into a protein,

which in turn has a... effect on the transcription of itself, right? So it's a feedback mechanism, so it progressively increases and decreases the rate of transcription of itself. But that takes time, right? It takes time for the cycle to progress through all of the stages. And that's effectively a timekeeping mechanism, right? It sort of counts the number of ticks that it goes through. It doesn't literally count, but it allows for a mechanism that cycles the output of these special neurons in the hypothalamus on a roughly 24-hour cycle. And it's this genetic clock that allows for the intrinsic circadian drive to be maintained,

even in the absence of external signals. The external signals help to regulate it and keep it aligned with the external world, but they don't generate it in the first place. These genetic clocks are found in cells all over the body and regulate a wide range of circadian rhythms. As I mentioned before, circadian rhythms are very common in the body. However, the particular ones that are responsible for regulating sleep are found, as I said in the supra-chaismic nucleus of the hypothalamus.

And that projects then to the ascending reticular activating system, as well as other systems that we haven't talked about, which more directly control the sleep and wake the states of the brain. So as you can see, we understand quite a bit now about how sleep is regulated, how it's controlled, the different stages of sleep. One thing that we know very little about is the function of sleep. So we know kind of the how of sleep,

but we don't know very much about the why of sleep. So why is sleep necessary? And I want to come to that. But before we get there, I want to say a few other things about sleep, which is sort of relevant to understanding the functions of sleep. Because there are sort of two aspects that one can think about when thinking about the function of sleep. One is, well, what types of organisms sleep? Sort of where sleep sits in the story of evolution might give us a hint about what its function might be, depending on which organisms and what range of organisms exhibit sleep.

A second potential way of understanding the function of sleep is to look at what happens when we don't get enough sleep. So sleep deprivation. So these are the two things that I'm going to talk about now. Sleep in animals and sleep deprivation, the consequences of that. And then we'll sort of use those insights to talk a bit about the functions of sleep. Okay, so sleep in animals, this is something that people sort of ask about periodically is do bird sleep or do, I don't know, horse's sleep or whatever. Now, as it turns out, sleep has been observed in pretty much all animals that you can imagine. So it's been observed in, I think, all mammals, as well as birds, reptiles, amphibians, and some fish. Even insects and worms and other very simple animals exhibit some type of reduced activity, both bodily and in terms of the nervous system, if they even have one, which resembles sleep.

So it appears that sleep is close to, if not a universal requirement, for most animals and for all mammals. As far as I know, every single mammal has been observed to sleep. And as I said, most other animals, but not quite all animals. Some animals, it's hard to tell. So if we start at the very bottom, so to speak, unicellular organisms don't sleep exactly. I mean, they don't have a nervous system. But many of them do still have circadian rhythms. So remember I talked about that 24-hour genetic clock cycle.

So a single cell can still have something like that, right? Moving up to somewhat larger organisms. So, for example, jellyfish have been observed to, exhibit some type of sleep-like behavior, as well as the worm, the nematode sea elegans. So neither of these organisms even have a nervous system. And so it's not even clear that the function of sleep is essentially related to the functioning of the nervous system, because even organisms that don't have them exhibit some type of sleep. Although, it's also likely to be true that sleep for a jellyfish or a worm is very different than sleep for a human.

For example, they certainly wouldn't have REM and non-REM sleep in the same way that we do. Now, as I said, all mammals have been observed to sleep, at least as far as I know, every single mammal is known to sleep, but mammals do sleep vastly different amounts. So bats, for instance, sleep 18 to 20 hours per day, while others such as a giraffe only sleep three or four hours a day. You might wonder, well, what about aquatic mammals? So aquatic mammals like dolphins, whales and seals also sleep, as I said, all mammals sleep. They engage in something called unihemispheric sleep while swimming. So basically that means that one hemisphere of the brain, that the brain is divided into two sides or halves essentially, the left side and the right side. During unihemispheric sleep, one hemisphere of the brain remains fully functional while the other goes to sleep.

And they alternate over the course of, well, the day and the night, so that both hemispheres can be fully rested, while still allowing the animal to move about and breathe and so forth as it needs to. Now, this is very strong evidence that sleep performs a critical function, at least in mammals, because the fact that they had to evolve a special mechanism of unihemispheric sleep to maintain that, whilst living in an environment in which that's obviously difficult, seems to indicate that there's some important function that's served by sleep that they needed to maintain. Now, a big question is whether fish sleep. One of the problems is that in birds and mammals, sleep can be observed through eye closure and the presence of particular types of electrical activity in the brain. But unfortunately, fish don't have eye

and they don't have much of a brain either, so it's very hard to observe those external characteristics. However, some species of fish do show some behaviors of sleep, again, typically reduced activity, reduced responsiveness to stimuli, but certain fish don't, particularly those that always live in shoals or that need to swim continuously, are suspected to not ever need sleep. But we don't know for sure, because it's kind of hard to tell, as I said. reptiles have quiescent periods which are similar to mammalian sleep although reptiles are often quite sort of quiescent at the best of times because they are cold-blooded and therefore don't have as much energy as warm-blooded animals like mammals and birds but the EEG patterns in reptilian sleep differs

a fair bit from what's seen in mammals so it seems that sleep may be different for different types of organisms sleep in birds and mammals seems to be most similar and some have hypothesized that there maybe a connection between being warm-blooded and requiring a certain type of sleep, particularly birds also exhibit REM sleep, or at least something very similar to REM sleep, as mammals do. And so perhaps there's a connection between the need for REM sleep and warm-bloodedness, although I don't know too much about exactly what that might be. So to summarize, all mammals sleep, including aquatic mammals. Birds also sleep and show the most similar patterns of sleep to mammals. Reptiles also sleep, although the patterns are a bit different there. And some fish sleep,

as well as at least some more primitive organisms such as the sea elegans and some jellyfish. It therefore seems that sleep is a very old evolutionary state and is necessary for many organisms. It may not be necessary for every type of organism, as I said, some types of fish appear not to sleep, although it is quite difficult to tell. And certainly the case of aquatic mammals indicates that there appears to be a very important function that sleep performs that cannot be evolved away. This raises a question as to what are those things? functions. And how is it that different organisms can require such vastly different amounts of

sleep? You know, 20 hours in the bats compared to only four hours in the giraffes, even though evolutionarily speaking, they're still fairly closely related, both being mammals. This then leads us on to the second aspect of the sleep function that I mentioned, which is sleep deprivation. So what happens when you don't get enough sleep? So there's two sort of types or forms of sleep deprivation. Acute sleep deprivation is when you sleep less than usual or don't get enough sleep for a short period of time, so, you know, one or two days. Chronic sleep deprivation occurs over a much longer period of time. And the effects of these are not necessarily the same, although they share a lot in common.

Much of the study on the effects of sleep deprivation, as far as I can tell, has been on acute sleep deprivation, because obviously that's much easier to study in a laboratory setting. I don't know that there's actually been very much study on the effects of chronic sleep deprivation lasting for weeks or months or even years. So as pretty much everyone knows, staying up all night or getting much less sleep than one needs, can often make one feel irritable, so it has significant effects on mood, and often when you catch up on sleep, then your mood will return back to baseline or back to normal. Some of the most common symptoms reported as a result of acute sleep preparation include sleepiness, unsurprisingly, fatigue, confusion, tension, strong mood disturbance, all of which are recovered after one or two

nights of sleep. As many people know, when you are sleep deprived and then you are able to sleep for as long as you want, you will typically recoup that sleep. You'll sleep longer to make up for the sleep that you've missed, which is another sign that sleep appears to be forming an important function. Another thing that happens in response to sleep deprivation is something called micro-sleeps. And microsleeps are very dangerous, but also extremely common, and I think not very widely known about or appreciated in the public. So a micro-sleep is a brief period of sleep, usually lasting only a few seconds. Microsleeps happen mostly during fairly monotonous tasks like driving or reading a book or staring in a screen. A microsleep is kind of similar to a blackout, meaning that the person who

experiences them usually has no awareness that they are occurring, and that's what makes them so dangerous, because they often occur during monotonous tasks, such as driving, or perhaps performing a function that you need to monitor something in a computer or elsewhere, such as working with heavy machinery. So that can obviously lead to dangerous accidents or mistakes. which the person may have no awareness of the level of their impairment. Usually people who experience microsleeps think that they've been awake the whole time, or perhaps they think that they just lost focus for a brief period of time, whereas in fact they actually fell asleep.

And microsleeps appear to be a mechanism that the body uses to recoup even small amounts of sleep whenever it can. There are many other effects of sleep deprivation beyond effects on mood and microsleeps. So the American Heart Association recommends healthy sleeping habits for ideal, cardiac health because there are a large range of, by now, widely documented and well-established effects of sleep deprivation on blood pressure, cholesterol, diet, glucose levels, weight, smoking, and levels of physical activity. One example of this, and there are many mechanisms by which these are regulated, not all of which are completely understood, but a lack of sleep can cause an imbalance in many hormones that are critical for weight gain or maintaining a proper weight.

Remember, I said before, that there are a wide range of circadian regulatory systems in the body, which regulate things like the immune system and body temperature, hormone levels and so forth. And so these are, they're separate from, but still closely connected to sleep. And a disruption to sleep can lead to a disruption of these other circadian systems as well, which can lead to a disruption of metabolism connected to metabolic disorder, which is something that I want to talk about in a future episode, because there's a lot of very interesting phenomena associated with metabolism and nutrition and so forth. So that's a future series of episodes. But there is a connection with sleep there. So sleep deprivation

can lead to increases in ghrelin, which is a hunger hormone and decreases in the level of leptin, which is a hormone that signals fullness or satiety, resulting in increased hunger and desire for high caloric foods. Another factor is that people who don't get sufficient sleep also typically feel sleepy and fatigued during the day, which leads them to get less exercise. So there are a number of factors here which link sleep deprivation with general health and well-being, particularly with respect to diet, exercise, and cardiac health. So that's another important effect. There's also been a large amount of research on the effect of short-term total sleep deprivation, so that's no sleep at all, on various cognitive tasks and cognitive effects. There's a lot of research here, and

not all of it's entirely consistent, but one review that I looked at to try to get a sense of this, reviewed 70 articles containing 147 different cognitive tests. So these are tests of attention, processing speed of a cognitive task, working memory and reasoning skills, things like this. So basically these are the types of skills that are tested in like an IQ test, right? And the average effects kind of varied by the type of task, but if I were to put a rough number on it, I'd say that the effect was about half a standard deviation, meaning that experiencing total sleep deprivation for usually one nights or sometimes two nights leads to a half a standard deviation decrease in the performance on these different cognitive tasks. So a crude way to summarize that would be

that missing a whole night's sleep completely results in a loss of five to 10 IQ points, assuming that the standard deviation of the IQ spectrum is 15, which is sort of a standard one, right? So half a standard deviation of that is like seven or so. So, you know, we might say five to 10 IQ points depending. And there is a higher amount of variation between people, so some are less affected by acute sleep deprivation some more. But still, that is, I think, quite significant to knock off 5 to 10 IQ points from one night missing out on sleep. So pulling that all-nighter for the exam might not be the best idea, since you're probably going to have 5 to 10 IQ points less of performance. As I said, those studies have focused largely on short-term total sleep deprivation. It's less clear what the effects are

of chronic partial sleep deprivation. So if you get one hour less sleep than you need every night for six months, what effect does that have? As I said, it's likely that the body will partially compensate through that, perhaps through REM rebound or through micro-sleeps and other factors. But it's unknown exactly what long-term effect that's going to have and what effect that's going to have on cognition, obviously because it's difficult to study in control conditions. One additional piece of terminology that I'll mention here is something called sleep debt. So sleep debt or sleep deficit. is the, refers to the cumulative effect of not getting enough sleep. So it's not just not getting

out of sleep for like one night, but it's sort of measuring the accumulated effect of that over potentially very long periods of time. What is known is that sleep debt is something that can accumulate. So it's not just how much sleep you've got last night, but it's how much sleep you've gotten for some number of nights before. And studies have found that when people were progressively deprived of more and more sleep over a longer period of time, their performance worsened with no particular endpoint observed over the course of a study. But of course, the studies usually only last for a few days. So what is not known is exactly how much sleep debt you can accumulate, or what this sort of means physiologically. Is there some underlying mechanism that kind of corresponds

to or measures the amount of sleep debt that someone accumulates? Or is it sort of extinguished over some period of time? Could you theoretically accumulate years of sleep debt? It sort of seems a bit unimpausible, but I guess no one really knows because this hasn't been studied. It's very difficult, obviously, to study long-term sleep deprivation, what happens longer-term if we experience chronic sleep deprivation. We certainly know that acute sleep deprivation has many significant negative effects, as I said on cognition, on cardiovascular and general health, as well as on mood and feelings of fatigue, as well as the most basic feeling sleepy. One way that the amount of sleep debt someone is carrying can be measured is to use something

called the sleep-onset latency test. It's a very simple test. Basically, all it does is that you put subjects in a quiet dark room and ask them to lie down and relax. You don't ask them to fall to sleep. You just ask them, lie down, close your eyes, and relax. And then you just count the number of minutes it takes the person to fall to sleep. Or if they're still awake after 20 minutes, then you note that fact. And what's found is that slip onset latency decreases in fairly direct proportion to how much sleep deprivation you've experienced or how much sleep debt you're carrying. So if it takes you 15 to 20 minutes to fall asleep in an environment like that,

so your sleep onset latency is about 15 to 20 minutes, that's considered to indicate little to no sleep debt. Whereas if you have a sleep onset latency of zero to five minutes, that indicates severe sleep deprivation. Note that normally, as I mentioned before, sleep and wakefulness is regulated not just by the homeostatic sleep drive, but also by the circadian drive. So typically this test will be conducted at a time when the circadian drive for arousal is relatively high. So at some point, kind of in the mid-morning, but not maximally high, which would be more in the sort of around noon, but you theoretically can run the test at any time. So this is something you can actually use for yourself, right? If you're wondering if you're sleep deprived, you find a time,

you know, ideally in the later morning, although theoretically you can do it at any time, although ideally not right before you would normally fall asleep, because that's not going to be as informative, and just find a place that's quiet, lie down, and just relax for a few minutes. See how long it takes you to fall asleep. If you fall asleep very quickly and easily under those conditions, there's a good chance that you are sleep deprived. Let's now, build on what we've just talked about in terms of the sleeping animals and the effects of sleep deprivation and think a bit about the functions of sleep. So it's very clear from the fact that sleep is evolutionarily strongly conserved, certainly within mammals, but also birds, as we've seen, even in

aquatic mammals, and also because of the variety of effects of sleep deprivation and phenomena like REM rebound, where it seems the body's very keen to catch up on REM sleep, that there must be some very important functions of sleep. In fact, there's a famous sleep researcher who said something like, If sleep didn't have very important functions, then it would be one of evolution's biggest mistakes. Because after all, humans spend a third of their life sleeping on average, and it seems that that's an enormous waste if it weren't for some very good purpose. Unfortunately, although it seems that there is very strong evidence that sleep does perform some very important function, we don't really know what that function is.

And I think the most honest answer as to what is the function of sleep, is simply that we don't really know. there are a number of hypotheses, which I will mention a few of the main ones, but really we still don't know. So here are a couple of the main hypotheses that have been put forward. So one is preservation, right? So organisms are safer by staying out of harm's way, and sleep is a way to ensure that the organism stays out of harm's way and inactive during, you know, nighttime, at least for most animals that sleep during the night, you know, when it's harder to see and when they might be most vulnerable. These days, I don't think many sleep researchers think that this is a good explanation. This is something that was postulated, I'm not exactly sure when, some time ago.

And perhaps it may play a role in some animals. Perhaps it may explain something about the variation in sleep between different species, but it doesn't seem like that it explains the existence of sleep. As I said, it doesn't really explain why sleep is a homeostatic drive that sort of builds up over time that you can accumulate a debt for and then sort of repay later. That wouldn't really be explained. It would seem to be a solely circadian effect if it were just to, keep us out of harm's way during the night. It also doesn't really explain why we should lose

all conscious awareness instead of just entering a sort of a passive quiescent state, like relaxing. It also doesn't explain in sleep in animals that don't have natural predators. You know, so even animals like tigers, for example, or lions that don't have natural predators, humans, I suppose arguably, still sleep, or why sleep needs to be recovered following deprivation. So it doesn't really seem like a very good explanation for most forms of sleep in most, certainly mammals. Now, a more recent hypothesis is waste clearance. So the idea is that during sleep, metabolic waste products, such as immunoglobulins, protein fragments, or intact proteins that accumulate in certain disease states, such as beta amyloid, which accumulates in Alzheimer's disease, that these kind of waste products that accumulate are cleared from the intercellular fluid via lymph-like channels in the brain. So this area is still on active research.

there is some evidence that the volume of interstitial fluid increases during sleep and that there's maybe some sort of clearing out that's occurring, but it's still very preliminary, and not really entirely clear why this would require sleep per se. But I suppose the idea, as far as I understand it, is that the brain needs to kind of shut down, not completely, but in large part, diminish its activity for this kind of clearing out to occur. A third postulated function of sleep is memory consolidation. So although we can initially encode new stimuli very quickly, like within milliseconds, long-term maintenance of memories requires a much longer period of time. It requires minutes to days or even years to fully consolidate those memories. And it's thought that memory consolidation is

facilitated by sleep. In particular, there is evidence that declarative memory, so that's memory for explicitly articulable facts and things, may be facilitated during slow wave sleep, so that's non-REM sleep, by replaying of memories in the hippocampus of memories that have been initially encoded, you know, recently or during the day. So the idea is that the hippocampus is kind of replaying different bits of stimuli, and that helps to reinforce the connections and the synapses across the cortex, where the memories are actually stored. The hippocampus is a brain structure that's known to be involved in memory consolidation, but most memories are not stored in the hippocampus itself. They are stored throughout the cortex, throughout the outer part of the brain.

And it appears under this hypothesis that during slow wave sleep, that the hippocampus is reactivating and kind of replaying some of these memories to help consolidate and reinforce the memories. So it's not so much that you're learning as such during sleep, it's more you're consolidating existing memories and helping them to be retained for longer periods of time. And there is some evidence that memory improves and memory consolidation does occur after sleeping as opposed to after staying awake for the corresponding time. Although it is difficult to be sure, of course, because you know, you have to control for other factors such as what you were doing while you're awake. Now, the reason why this is thought to require sleep is essentially because if memory consolidation requires reproduction of stimuli or activity by the hippocampus, then you don't want new stimuli kind of getting in the way of that and interfering with it. So you want to significantly reduce or, if possible, eliminate any interference by new stimuli and just sort of focus for a period of time on consolidating what's already been, what you've already been exposed to, you know, that day or in

recent days. There is, in fact, evidence of this occurring in rodents where spatial and temporal patterns of neuronal firing in the hippocampus have been observed to occur during non-REM sleep following learning a novel environment, so like a maze. So basically, they measure a pattern of activity as the mouse is learning the maze in the hippocampus, and during sleep, particularly during non-REM sleep, they observe that pattern of activity, both sort of spatial and temporal patterns, reoccurring. And it's not known to my knowledge whether this is directly associated with consolidation memory, but it is certainly consistent with that hypothesis. A related hypothesis is that sleep may be involved in regularizing synaptic weights. So what this means is that the synaptic

connections between different neurons are changing all the time in a phenomenon called plasticity, as we learn. But many forms of plasticity, many rules that we have are rules that specify an increase in synaptic weights, basically increasing the connection strength between two or more neurons as they're involved in a similar activity, as they're firing together in response to some similar activity, the connection strengths between them increases. The problem with that is that if you only have a mechanism for increasing synaptic weights, then all of your synaptic weights are eventually going to increase an increase, and at least, I mean, eventually sort of all your neurons are active at the same time, and you have a seizure. So that kind of doesn't work, right? You need some regularization

method which pulls everything down. So certain synaptic weights are increased and then everything is pulled down uniformly so that some synaptic weights kind of win out over others. And it's thought that sleep may be a time where this kind of regularization occurs. It kind of weeds out unnecessary connections and pulls everything down, diminishes across the board the strength of synaptic connections. However, currently, to my knowledge, there isn't any experimental evidence for this. But it could be another mechanism by which memory consolidation occurs during sleep. And again, it's sort of easy to see why that would happen during sleep. It's something that you wouldn't necessarily want happening while you're awake, because it could interfere with perception or

formation of new memories. So for what it's worth, I think that there is strong evidence that sleep is essential for memory consolidation. However, it's certainly not clear whether that's the only function of sleep, nor is it clear exactly what the connection is between, say, REM and non-REM sleep and memory consolidation, or why different animals require very different amounts of sleep. So there are still many open questions here. Also, it seems that sleep probably evolved for different reasons than it is currently used for. So this is quite a common phenomenon in evolutionary biology where a trait or behavior or structure will originally be selected for to fulfill one function, but later be co-opted and modified somewhat for serving another function. So sleep may be

this sort of thing. It perhaps sleep initially evolved in non-mammalian invertebrate organisms for one purpose and has since been co-opted to use for, say, memory consolidation. Indeed, if indeed there are sleep-like states in organisms that don't even have a nervous system, it's pretty clear that we can't explain sleep in those organisms by appealing to the hippocampus or regularization of synaptic weights because these organisms don't have either of those. So I think it's likely that sleep evolved for some more generic reason, perhaps some waste clearance reason, which is applicable for a wide range of cells. and then because it was a time of relatively diminished input and responsiveness,

it was co-opted in organisms with much larger brains, particularly birds and mammals, to be a time for memory consolidation and regularization of synaptic weights. But that is speculative, and ultimately we don't really know if that's correct or not. Now let me finish out this episode by talking briefly about some slipping disorders and giving some advice about good sleep hygiene. Insomnia is a general term which refers to difficulty in falling asleep and or staying asleep. Insomnia is the most common sleep problem and many adults report occasional insomnia. In fact, I think the majority of the population reports occasional insomnia,

and about 10 to 15% report it as a chronic condition. There are a number of more specific sleep disorders, so I'll talk about three of them. The first is obstructive sleep apnea. This is quite a common condition as well, and it is when there are major pauses that occur in breathing during sleep, which disrupts the normal progression of the different stages of sleep, and often also cause other severe health problems. So the fundamental cause of apnea is when muscles around the patient's air pathway or airway

relax during sleep, which causes the airway to collapse and block the intake of oxygen. So as blood oxygen levels drop and the CO2 concentration rises, patients will come out of sleep in order to resume breathing. However, once we start breathing again, then we very quickly go back to sleep. So most individuals with obstructive sleep apnea are unaware of the disturbances in breathing while sleeping because they don't remember these brief periods of waking up. Typically, obstructive sleep apnea is observed and reported by partners or family members who observe snoring or they observe the partners stopping breathing, gasping or choking while they're asleep. Obese people are at a much higher risk of sleep apnea because of increased neck fat around, which can, you know, increase the risk of obstruction during sleep, although it's not,

obstructive sleep apnea is certainly not exclusive to the obese. One of the most common treatments is continuous positive airway pressure or CPAP treatment, which is a machine that provides positive pressure to the airway to keep it open during sleep, which is a very effective treatment. Narcolepsy, so this is something that most people have heard of, but I always suggest don't know very much about. So narcolepsy refers to a decreased ability to regulate sleep wake cycles. So symptoms include periods of excessive daytime sleepiness and involuntary sleep episodes.

So about 70% of those who have insomnia experience periodic episodes of sudden loss of muscle strength, known as cataplexy. The exact cause of narcolepsy is unknown, and it probably has multiple causative factors, as is often the case for these sorts of things. It is thought that the mechanism is related to the loss of orexin releasing neurons in the hypothalamus. So these are neurons which normally play a regulatory role in the slothel. sleep wake cycle. Nal epilepsy is more associated with excessive daytime sleepiness and just sort of falling asleep involuntarily, not necessarily falling asleep in a very strange or humorous position

or some of the more comical ways that this is often portrayed in the media. Finally, sleepwalking. So this is another sleep disorder that's often portrayed in the media. In this case, somewhat accurately, it seems actually, in the sense that sleepwalking is a phenomena where you sort of combine aspects of sleep and wakefulness, which is a little bit strange since we just talked about how different they are. It tends to occur during slow wave sleep, so in non-REM sleep. And one of the reasons for that is, as I said, because during REM sleep, typically there's a loss of muscle tone

which removes the ability to control skeletal muscles, so one typically can't walk around during REM sleep. During episodes of sleepwalking, the person will perform activities that are usually only performed during full state of consciousness, but while still being in a state of diminished responsiveness to stimuli and not really having any awareness of what they're doing. So often the activities are quite benign, like talking or sitting up in bed, walking around the room, eating food or even cleaning.

But sometimes people have been known to engage in hazardous activities, like driving cars. I don't know how common that is, but it is something that apparently has happened sometimes that you can actually drive a car while asleep. I don't really know how that works. Again, obviously, it's important to realize that sleep is not a time when there is no processing by the brain. It's just diminished responsiveness to stimuli and diminished alertness. It's not really clear that there's any particular harm to waking a sleepwalker, although they'll typically be disoriented if awakened while sleepwalking, as it occurs during the deepest stage of sleep.

So as I said, slow wave sleep, so that's sort of stage three. And that's the deepest stage of sleep when you often feel quite groggy when woken up. The goals of sleepwalking is unknown, but it is known that it doesn't involve acting out dreams, as dreams mostly occur during REM sleep, which is not when sleepwalking typically occurs. So what the cause might be is really quite unclear. All right. So let's conclude with a brief discussion of sleep hygiene. Because I've been talking about sleep and the effects of sleep deprivation, I think it only

makes sense to say a few words on how to get better sleep, or at least things that you can do to help increase the chances of better sleep, because none of these are guarantees, of course. Sleep hygiene refers to a set of behaviors, and I guess even mindset, I suppose, which are likely to contribute to better sleep. and to reduce time spent in bed trying to fall to sleep. One important thing is establishing a regular sleep schedule. So basically that means going to bed around the same time each day and waking up around the same time each day as well,

or at least not varying that dramatically from day to day. One of the reasons for this is because of the circadian drive-for-arousal, which we talked about, because this is regulated on a 24-hour time span, the body has a great deal of difficulty in regulating the sleep-work cycle if you're constantly disrupting that by waking up dramatically early, or dramatically later. It's generally best to have a regular approximate bedtime and a regular approximate waking up time. Now, one thing that's often recommended, although I think the evidence

for this is not as strong, is that it's not a good idea to engage in vigorous physical exercise, or for that matter, highly stimulating mental activities too close to bedtime, maybe in an hour or two hours before bed. Generally, it's best to engage in sort of quieter, less stimulating activities in the lead-up to bed. This is, of course, easier said than done. in today's lifestyle, but that can just help us to avoid overstimulation. Another factor that appears to help is only using bed for sleep. So if you can't fall asleep within, and I've heard something like 30 to 40 minutes or so, well, I suppose people will say different things.

It's a good idea to get out of bed and do something else for a while and then try again later. Also, it's generally not a good idea to use the bed for working or reading or eating or other things during the day. part of that is because it can help to sort of associate your mind and body with the bed as a place where you do wakeful things, whereas it can be helpful, it can be more helpful to sort of have an association with bed and sleep-sleepiness. Although I don't know how strong the evidence for that is, but it is a common recommendation, which I think does make an amount of sense. One thing for which there is a lot of evidence is avoiding alcohol, as well as any stimulating drugs such as nicotine, caffeine and other stimulants in the hours leading up to bedtime. they can cause significant disruption to the circadian cycle. And another factor that's very important is sleeping in a relatively comfortable,

relatively dark and relatively quiet place. Obviously, there's variation in terms of how much light or noise or comfort affects people, either while falling asleep or while asleep. But it's pretty universal that humans do best, sleep best in relatively dark and quiet environments. If it's difficult to achieve that, then a face mask or, earphones to cancel that noise or play some music or some white noise to drown out disrupting sounds might be helpful. So none of those things are guaranteed to improve sleep quality. And as I said, the evidence quality for those varies a little bit. But I think overall

there is reason to give those a try if you are having trouble with sleep. Certainly they help me. They don't magically fix insomnia, but they, you know, like other forms of hygiene, like dental hygiene and bodily hygiene and having a good diet and exercise, sleep hygiene is an important aspect of staying healthy. And I guess the final aspect is just get enough sleep. The recommendation is seven to nine hours for most adults. There will be some people who need less. So there are some people who only need six hours of sleep and some who need more. Some people need maybe up to 10 hours of sleep. Anything really more or less than six to 10 hours is probably pathological and is something that you should talk to your doctor about. Adolescents need more

sleep than that and then children progressively more. So the amount of, so newborns will get from 14 to 17 hours of sleep, right? And that progressively diminishes right through childhood, down through maybe eight to 10 hours during teenage years and seven to nine hours by adulthood. And as I said, some people will only require six and some might require 10. But it's extremely unusual for anyone to require less than six and also more than 10. And one thing that you can try, if you're unsure, if you're, you get less than maybe you get six or seven hours of sleep and want to know whether you're sleep deprived, you can try the sleep-onset latency test on yourself to see if you're sleep-deprived. Another way is simply just do you typically feel really tired and drowsy during the day,

particularly during the afternoon. If you're generally well rested, you shouldn't feel, you maybe feel like it's normal to feel a little bit more tired, a little bit sleepy maybe after lunch, but not too tired or sleepy. Unless of course you typically take an afternoon nap, then that's a bit different, right? But the basic idea is you should be generally fairly alert throughout most of the day, most of the time, if you're well rested. And if you regularly struggle to stay awake during the afternoon, it's likely that you are sleep deprived. So anyway, I hope that those pieces of advice might be useful for you. And if you have significant issues with that, I suggest talking to your doctor. Many people never talk about sleep with their

doctor, even though it's a very important factor and a very important reason why they can connect many other health problems. So that concludes what I wanted to talk about today. Hopefully you found that interesting. We talked about the stages of sleep, how the brain control. sleep, the regulation of sleep through the homeostatic sleep drive and the circadian drive for arousal and the brain systems that control those. And then I talked about the effects of sleep deprivation on cardiovascular health, on mood, and on cognition. We talked about some theories of the function of sleep, including the preservation hypothesis, the waste clearance hypothesis, and memory consolidation hypothesis. And we talked a bit about sleep disorders, including sleep apnea, narcolepsy,

and sleepwalking. If you enjoyed this episode, consider some supporting the podcast by leaving a positive review of the aggregator of your choice. You can also support the podcast financially through Patreon. You can also email me. My email address is Fods12 at gml.com. That's FODS12 at gmail.com. I am very keen to hear questions, suggestions or other feedback. Thanks once again for listening and I'll talk to you next time.