In this video I introduce the different stages of sleep and the brain waves associated with each. NREM Stages 1 and 2 are characterized by theta waves, while NREM Stage 3 slow-wave sleep is associated with delta waves. REM sleep features increased brain activity, heart rate, respiration, and sexual arousal, accompanied by rapid eye movements and body paralysis and is the stage of sleep associated with vivid dreaming. Finally I look at how we cycle through these different stages over the course of a night’s sleep.

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Video Transcript

Hi I’m Michael Corayer and this is Psych Exam Review. In this video we’re gonna look at the different stages of sleep. So you might think that you’re either awake or just asleep but within your sleep there’s actually different stages that you move through. So these were first identified in the 1950s by researchers including Nathaniel Kleitman, Eugene Aserinsky, and William DeMent. And the way that these researchers identified these different stages of sleep was by looking at EEG recordings of people who were sleeping.

So you might recall from the video on brain scanning techniques that the EEG or electroencephalogram is a device that has electrodes which attach to the scalp and these electrodes can detect electrical activity of the brain underneath and then this brain activity is graphed. This gives us these brain waves alright, these waves of electrical activity. Now when you’re awake the waves look something like this. Now you can do an image search and find actual EEG recordings to look at I’ve just drawn this by hand here but what we see is these sort of rapid low amplitude waves. Now within the waking state there are different waves that we see. There’s alpha waves which are associated with sort of relaxed meditative states. If you’re feeling maybe a little drowsy, and then there’s beta waves which are associated with states of vigilance. So if you’re threatened by something you have that fight-or-flight response you’ll probably see beta waves. But in general we see this sort of low amplitude rapid activity of the brain in the waking state. Now when you fall asleep things start to change. When you first fall asleep you move into what’s called stage 1 sleep and in stage 1 sleep your brain activity isn’t that different but it’s a little different right? So we see something sort of like this and these are called theta waves and they’re a little bit slower than the waking state and they’re a little bit more irregular. All right so what is stage one sleep like? Well you’ve probably experienced it if you’ve been asleep for just a few minutes and you suddenly wake up, you sort of have a feeling that you were asleep, maybe you had some imagery but you weren’t really dreaming right ? You had maybe some thoughts that were occurring you might have even experienced what’s called a “hypnic jerk” and this is a sudden muscle contraction that occurs. It’s not entirely clear why this happens and it might even wake you up but if you stay asleep you move from stage one after a few minutes into stage two.

So in stage 2 sleep we continue these same waves we had in stage 1 and these are called theta waves. So theta waves continue in stage 2 but there’s some characteristics that identify stage 2 from stage 1. So if you were looking at an EEG you could tell if somebody was in stage 2 because you’d see these sudden bursts of activity and you’d also see sudden high amplitude spikes. These bursts are called “sleep spindles” and these spikes are called “K complexes” and these are characteristic of stage 2 sleep. So we have theta waves with the addition of sleep spindles and K complexes and one theory of these sleep spindles and K complexes is they might have something to do with brain activity that’s designed to keep you asleep. When you first enter stage 2 you’ll stay there for about fifteen to twenty minutes but you’ll return to this stage repeatedly throughout the night and so over the course of an entire night you’ll spend about 50% of your total sleep time in stage two. Ok, after stage two we move to, you might guess, stage 3. This is where the brain activity really starts looking quite different from the waking state, you know? So in stage one and two we have these theta waves but starting in stage 3 we have these higher amplitude, slower waves. We have things that look something like this right? And these are called delta waves.

This stage of sleep is called slow-wave sleep because of these delta waves and this is the most different from the waking state. It’s the highest amplitude and the slowest waves that you’ll have and this explains why it’s so hard to wake up from stage 3 sleep. If you’ve ever been awakened while you are in this state you’ve probably experienced what’s called “sleep inertia” and this is this groggy feeling of like it takes you a few minutes to kind of get your bearings and figure out what’s going on. The reason for this is because your brain activity is so different; these delta waves are very different from your waking activity and so it takes a little bit of time to sort of get your brain back to its waking state. Whereas if you wake up from stage one, you know, if you fall asleep for just a minute or two and then you wake up you know you can sort of jump right back to what you were doing but when you’re in stage 3 it takes a while. Now you may see in some other texts reference to stage 4 sleep. There used to be a stage 4 but in 2007 it was consolidated into stage 3. It was also Delta waves also this slow wave sleep and it was decided that they they weren’t really differentiated enough. So now they’re considered the same stage it’s all stage 3.

Okay, so the three of these stages together are all called “non-REM sleep“. So all three of these here would be non-REM sleep and that might be an indication that the next stage that we’re getting to is going to be REM sleep. So what does this stand for? REM stands for “Rapid Eye Movement”. That’s because in this stage of sleep your eyes start moving around, whereas in the other stages, non-rapid eye movement sleep, you don’t see this eye movement. Okay, so now we have this interesting stage of REM sleep. So far things have probably made sense in what you might imagine about stages of sleep; the longer you’ve been asleep the sort of slower things get, your brain activity changes to become slower and it’s sort of a deepening of sleep throughout this process. That probably makes sense to you but then we get to REM and things are a little weird, and this is why REM is also called “paradoxical sleep“. So why is REM sleep paradoxical? Well, the first reason is that your brain activity looks a lot more like the waking state. You go from these slower waves that you had in stage 3 and then suddenly you start seeing this increase in brain activity and you get activity that looks very similar to the waking state. So we have an increase in brain activity, it becomes more rapid. The other things that we see are an increase in your heart rate and your blood pressure and your breathing also increases, and you even show signs of sexual arousal. So we have this sort of increase in all of this activation here and then we also have paralysis of the body so we have a decrease in body movement. You move around during the other stages of sleep; you can sort of toss and turn and move around in your bed but in REM you stop moving. Your body becomes paralyzed except your eyes start moving and you have this increase in your heart rate and your breathing and also the sexual arousal. So what’s going on here?
Well, what’s happening during REM, this is when you have vivid dreaming. If you wake people up from other stages of sleep they might report some imagery or some thoughts but it’s not very coherent, whereas you wake people up from REM sleep and this is when they report vivid dreams, stories that have some narratives to them and they have some objective; they were trying to escape from something that was chasing them, right? This is where we have the intense forms of dreaming. Now this makes sense with the body paralysis because when we have these vivid dreams we don’t want to act them out. So this is why we have this body paralysis. Let’s say you’re dreaming that you’re running from a predator, you don’t want those messages to actually get to your leg muscles and have you start running because you’ll run out of bed and right into the wall and that would probably be a bad idea. So this explains why we have this body paralysis, it keeps us from acting out our dreams.

Now an interesting side note here is, maybe you’ve had this experience, where you dream that you’re running, you’re running from a predator or something, and it feels weird right? It’s like I can’t really run, it feels like I’m running in slow motion or like through molasses or something. This is really common, a lot of people report experiencing this when they have you know running in a dream. So one theory of this is that what’s happening is that the paralysis occurs because the messages from the brain get blocked at the spinal cord so they don’t make it to your legs and have you actually start running. But normally when you’re running, if you’re actually running in real life, you know, you’re sending messages to your legs to move and you’re also getting feedback from those muscles telling your brain how things are going. When you’re dreaming, however, you don’t get the messages, they don’t get there and you don’t get any feedback from your leg muscles telling your brain how it’s going. As a result you have this sort of weird sensation. That’s one possible explanation for this this common occurrence that people report with this sort of slow running when they’re dreaming.

Okay, so this is the REM state and now I’d like to look at sort of the course of the night, the sort of passage from these different stages throughout the course of the night. So if you search for stages of sleep online you’ll certainly see charts like this. You may have it in your textbook and you’ll see something like this, we have sort of the waking state here and then we have you sort of go down to stage one and you spend a few minutes there then you go down to stage two spend a little longer there and then you go down to stage three down here spend some time there and then you sort of come back up to stage two for a little bit and then you go up to here, and then you have this same thing here and then down again.

Okay so you have some chart looking something like this and this is your sleep over the course of the entire night. And actually this, another sort of side note here is when you think about these early researchers doing these EEG recordings right, we sort of can simplify everything down into one little chart here and say okay here’s the different stages. But when they were doing these recordings I mean they’re using this EEG for eight hours at a time recording somebody sleeping and so over the course of a night the amount of paper that they would use for one person sleeping to record all that brain activity would be about a half a mile long. So you can imagine the researchers having to go through and look at these brainwaves and make sense of things and figure out the patterns that are occurring. It wasn’t, you know, we have these sort of simplified graphs that make it look like oh yeah you just look at it’s easy to see stage one stage two. But when they were doing this in you know they had half a mile of paper to look through for each person, you know? Just one night of sleep to try to figure out what the patterns were.

But anyway, back to this graph here. So we have this sort of deepening of sleep that’s what this is meant to represent as we move down here. We’re sort of deeper and deeper asleep. Stage 1 2 3 then we come back up to 2 for a minute then we go to these top parts and this is what’s a little bit confusing about these graphs and so I wanted to make sure this is clear. This is not actually you know awake and it’s not stage 1 and usually hopefully this is sort of labeled on these charts that this is meant to represent REM sleep here. Ok it’s not actually, you know, like between awake and stage 1 and you don’t pass through it on your first time entering the sleep state but that’s meant to indicate that it’s more similar to the waking state than the slow-wave sleep here.

Ok, so a couple things to notice about this chart. The first is that you have multiple REM sessions throughout the night and you go back and forth between these different stages of sleep. You cycle through them over and over. The first thing to notice, ok, so this is about an hour and a half here, this first cycle from first falling asleep until the end of your first REM cycle takes about an hour to an hour and a half. And as you go through the night you’ll notice that the REM periods get longer. So your first REM cycle this first part of REM might be 15 or 20 minutes. By the end of the night if you’ve been sleeping for seven or eight hours you may have 45 minutes to an hour of interrupted REM sleep. So we see REM increasing over the course of the night and you’ll notice that the slow-wave sleep does the opposite; it decreases. You have a longer slow-wave portion of sleep when you first are sleeping in this stage three sleep. And as the night progresses this slow-wave sleep gets shorter and shorter until eventually you stop going to slow-wave sleep. Sort of like you’ve had enough of it and you no longer need to go to that state.

Ok, this also explains if you’ve experienced that sleep inertia right? When you wake up from slow-wave sleep and you really groggy and out of it. If you’re sleeping long enough, if you’re sleeping for seven or eight hours a night, that won’t happen anymore because you won’t be going into slow-wave sleep anymore. You’ll either be waking up from stage two or from REM and so it’ll be easier to wake up feeling refreshed if you’ve actually slept for seven or eight hours. This also explains when you try to set your alarm and wake up early for something you sleep for like five hours or something that’s not enough you have a higher likelihood of waking up from stage three sleep and this is the sort of situation where you’re going to wake up, turn your alarm clock off and immediately fall back to sleep and you won’t even remember that you’ve done it. So then you, you know, wake up an hour later and realize you missed your morning class and you don’t even remember turning off the alarm. That’s why that happens it’s because you probably were woken up during this slow-wave sleep and you’re really out of it and you know didn’t get out of bed and give enough time for yourself to really wake up for that sleep inertia to go away.

Whereas when you wake up from REM you can sort of suddenly be wide awake and you’ve probably had that experience if you’ve had a dream and you suddenly wake up from the dream and you’re like immediately alert, like “oh that was a dream and now I feel wide awake” right? And that’s happening because REM is more similar to that waking state.

Ok, so hopefully this made some sense of these different stages of sleep and sort of the order that you move through them throughout the course of the night. I hope you found this helpful, if so, please like the video and subscribe to the channel for more. Thanks for watching!