Sleep and the Circadian Rhythm. We're in 6B here of the psychology section, and we're down around this, well, more like this area. And here's an overview of what the lesson will cover. I will also show a summary slide at the end. We'll start off here with sleep cycles. Show Transcript
So, sleep is divided into both cycles and also stages. And each cycle lasts around 90 to 110 minutes, and that's in adults. So if we're looking at infant cycles, they are very short, around 50 minutes. For little babies, it gets longer over childhood. And so in a typical night, an adult will probably go through about 4 to 5 cycles, closer to 4.
4 is a little more typical. And you might be doing the math and saying, but wait, isn't 4 cycles about six hours? Don't people sleep more than six hours? And the answer is yes, but it's common to wake up slightly between cycles usually right after REM sleep, and also cycles can get longer over the course of the night.
Within the frame of a cycle, we see 4 stages of sleep. So around 4 cycles and then within the cycles, around 4 stages. We cycle through these stages and some stages more frequently than others. We don't necessarily experience every stage in each cycle, which I'm gonna go over in a little bit more detail when we look at a sample graph of what a night of sleep might look like.
Only 4 stages, question mark, really? So a bunch of sources, including a lot of MCAT prep sources actually that I've come across, continue to use an outdated numbering system when they talk about stages of sleep. But definitely learn the current numbers, you'll expect on the MCAT test to see updated numbers, updated ways of phrasing, terming.
And so what you want to know is that well, that we have 4 stages. The old system had 5 stages. But in 2007, stages 3 and 4 were combined into a single stage. And you'll also notice that these stages sometimes are called N1, N2, N3, instead of stage 1, stage 2, stage 3. The N stands for non-REM, and REM stage, down here, it's technically the fourth stage, but it's usually not referenced in terms of a number.
It can be, you should know that REM is the fourth stage, the last stage within a cycle. Next topic here is sleep stage sequencing. And we're looking at basically how do we move through these four stages within a cycle, or within multiple cycles in a night? And it's not as straightforward as you might think.
We've already established that there are four stages, 1, 2, 3, and REM. But stage 2, for instance, actually usually occurs twice per cycle. Lets look here at the first stage or the first cycle of a night of sleep and how the stages are sequenced within it. Because it's actually that first cycle of a night that's going to be the most predictable and the most standardized.
As the night goes on, sleep cycles have more variability. And that's both from person to person and also just from day to day. This is the order for the first and usually also the second cycle of a night. Could be the third, but definitely the first cycle of the night looks like this. We have stage 1, 2, 3, and then back to stage 2 before entering REM sleep. Subsequent cycles don't always contain the stage 3.
So here's an example of what a subsequent cycle might look like. And you'll notice it's the exact same pattern as the first, so that's one potential sequencing. But let's say, maybe in your fourth cycle of the night, you might not actually dip into stage 3, which means that you would just go from 1, to 2, into REM.
In a little bit here, I'm gonna talk about what actually happens in each of these stages. But first, since we're talking about the sequencing of stages, let's look at how this might appear in a graph form over the course of a night. But cycle 1 here, we start awake, go to N1, dip down to 2, into 3, up to 2, into REM.
Cycle 2 looks similar, but there is less time spent down here in stage 3, and that's a trend over the course of a night. The other trend you'll notice is that the REM stages get longer with each cycle. By this fourth cycle in this particular diagram, anyway, there is no stage 3 sleep. Another thing we see is that this person woke up towards the end of the night.
And it's actually more common to wake up after an REM stage. It would be more probable then for the awake time to happen here. Let's look at what's actually happening in these different stages. Stage 1 sleep is very light sleeping. It only lasts about 5-10 minutes. If awoken during the stage, you might not even realize that you had been asleep.
And I don't know if you've had this experience, I've had it many times where I'm maybe sitting next to somebody watching a movie. And say, hey, you just missed that part of the movie, you were asleep. Other person says, no, I was resting, I didn't actually fall asleep, but you know they were snoring. So, from their perception, they are not aware of having drifted into sleep.
It's just one of the characteristics of stage 1 sleep. But somebody looking from the outside can definitely tell they had fallen asleep. And the dominant brain wave during stage 1 sleep is the theta wave. You'll also see some alpha activity, you'll see some eye rolling which actually does show up on an EEG. The electrodes that are placed kind of near the temple area will show some activity.
There are a few occurrences that you might see during stage 1 sleep. Sometimes this stage contains what are called hypnic jerks, and these are involuntary twitches that startle you awake. Most people have experienced these. Sometimes it's accompanied by kind of a light dreaming or hallucination. But that's not always the case.
Hypnic jerks are more frequent among people who have irregular sleep patterns, also people involved in extreme physical activity. And there are some theorists who say that there might be an evolutionary advantage for these jerks, like maybe they primed the body to be able to catch itself from falling off a high ledge or a tree or a cliff. Most scientist though just think that it's a byproduct.
It's a byproduct of the changes on the nervous systems that occur when the body moves from awake into stage 1 sleeping. A similar phenomenon is what's called hypnagogic sensations, and these are life like hallucinations. And they often involve floating or falling, and hypnagogic actually does mean being on the border of awake and asleep.
Stage 2 sleep, most of our sleeping time is actually spent in this N2 stage. Here we have the heart rate slowing down, and the dominant brain wave is theta. If you were looking at an EEG, there wouldn't be that much of a change from stage 1 in terms of the brain waves. There might be a bit of a reduction of alpha waves. The thing you would see in an EEG that would distinguish N2 from N1 are sleep spindles and K-complexes.
This is what they look like on an EEG. So a sleep spindle, you can remember S or squiggle. For K-complex, I think of like, well, the k, the hard c sound, kind of like for a cliff, something very sharp, drop off. Sleep spindles are the readout caused by electrical activity that actually comes from the thalamus.
The thalamus is doing some important work here. It's filtering sensory input, that helps us remain asleep. And you always wanna remember that the thalamus is one of the sensory filtering devices in the brain in general. The traffic director, it's often called, whether awake or asleep. And then the other thing it's doing is helping to consolidate memories.
K-complexes, they're actually generated in the cortex. So there's cortical activity, and kind of like the sleep spindle, it's also a marker of the brain's attempts to keep us asleep. The electrical activity in this case is generated by a suppressing energy of the cortex. The cortex helps us with awareness, focus, alertness, and so during sleep, we don't want a lot of that.
And actually it requires extra work of the brain to suppress that. And then also the K-complexes are demonstrating activity that are related to memory consolidation, just like with the sleep spindles. Stage 3 sleep is also sometimes called slow-wave sleep, actually probably more frequently, it's referred to as that. It's the deepest sleep, most of our N3 slow-wave sleep occurs during the first half of the night.
And it's the most important stage for repair and for recuperation. A lot of people assume that REM Is the most important stage of sleep. And that's because, I think, really, just because it's the stage that we've heard about. However, that's not the case, and it actually makes sense that the body, the brain would front load the most important stages of sleep earlier in the night.
And so, that way, in cases of sleep deprivation, we're hopefully at least still getting this more important type of sleep, this deeper sleep. Stage 3 sleep decreases drastically with age. And we'll talk more about that in a future slide when I look at the different ages and how sleep duration changes over the life course. This is when human growth hormone is released.
And the dominant brain wave here is delta. And finally we have REM sleep, and this stands for rapid eye movement. Most muscles during this stage goes slack, and they're actually even pretty much paralyzed during REM sleep. But the eyes move and there's an increased blood flow to the genitals. And that's true among males, as well as females, infants, kids, adults, all ages and sexes.
The brainwaves here actually look a lot like brainwaves when somebody's awake. And for that reason, it's sometimes called paradoxical sleep. And that's referencing the paradox between, on the one hand, having paralyzed muscles, but a very awake looking brain. Most, but not all, dreaming happens during this stage. So at different stages of sleep, we can have these kind of like fleeting realistic type dreams.
And a lot of times, it's something like just dreaming that someone knocks on the door or dreaming that the phone rings. The dreams aren't long. It's REM sleep where we have the long dreams and the dreams that are more imaginative, that have plots. We spend more time in REM during the second half of the night, and I'm gonna be talking about sleep debt in the lesson on sleep disorders.
We often have what's called REM rebound. REM rebound means that we have a greater proportion of REM sleep than we normally would. And that's because when we miss out on sleep, as I'd said before, the body prioritizes the slow wave sleep. So if you're gonna sleep in ten hours on the weekend, you're gonna have more REM sleep.
Also certain drugs can reduce REM sleep. Most famous of which is alcohol, so when people go through, let's say, alcohol withdrawal, they've been drinking every day, stop drinking. They experience more REM sleep, which can also ignite more nightmares, more sort of crazy dreams. That's all part of REM rebound.
Looking at some of the benefits of sleep then, well, memory consolidation occurs at pretty much almost every stage of sleep, N2, N3, REM. Sleep assists mood, energy, creativity, facilitates perceptual abilities, helps aid in good judgement making. It regulates the immune system, cortisol levels, and it regulates appetite. You might have seen some of the studies, there have been a lot published pretty recently, that have been conducted on the relationship between sleep deprivation and increased appetite.
Another benefit is that during sleep, glial cells shrink in size. And so this actually gives the brain more room, more space to dump waste. They've also done many studies that have shown sleeping around 7 to 8 hours a night on average has been correlated with the highest life expectancies. Again, this is correlational, it's not causal. Looking at sleep needs across the life course, the first thing to say is that everyone differs.
People differ, there's a range of what's healthy. What I'm showing here are numbers that come from the National Sleep Foundation recommendations of sleep that would be appropriate for the majority of people at these different ages. Looking over here, infant needs are very high. Infants can sleep up to 17 hours, even 18 hours.
The youngest infants are sleeping the most, newborns. And even by age two, you're still looking at an individual who's spending half of their life asleep. Preschoolers are sleeping pretty much almost half as well. By the time you get into school age kids, you're looking at 9 to 11 hours of sleep. Teens, a little less than that, and by early adulthood, sleep needs stabilize at around 8 hours.
The only difference over the course of aging is that the upper end of the suggested range goes down. So the range is narrower. There's a misconception that older people, older like over 60, 70, that they don't need as much sleep, that there's a drastic reduction. And this is not true.
What is true is that older adults tend to get a lot less sleep. But their sleep needs are actually really very similar. REM sleep by age decreases over the life course. Infants are dreaming about half of their sleeping time. The level decreases throughout childhood. By the early 20s, it stabilizes.
And there isn't really a difference between a 25 year old and a 75 year old in terms of how much of their time is spent in REM sleep. It's around 20%. N3 sleep, or slow-wave sleep, newborns actually don't experience low-wave sleep at all when they're first born, but they rapidly develop it. They have it early on.
By 12 months, they're spending a lot of their sleeping time in this slow-wave state. It begins declining markedly at puberty. It stabilizes in the mid to late 20s, but people in their early 20s actually still have a lot of slow-wave sleep. And one last factoid, this one's related to sex differences.
They found that the average man over 30 spends less time in slow-wave sleep than the average woman over 30. And there aren't sex differences up until the age of 30. So it's probably related to hormones, most likely. It could be other factors, lifestyle factors. Last topic of this lesson is the Circadian rhythm, and I'm just gonna point out some of the big picture ideas.
Humans and many other lifeforms have what are referred to as internal clocks. They are set at around 24 hours, and that's give or take a pretty small margin. This daily rhythm, or clock, is located in the suprachiasmatic nucleus in the hypothalamus. And these brain structures integrate sensory input mostly from the retina. Usually daylight, but the Circadian rhythm does not depend on light.
As you would imagine, people who are blind still have certain Circadian rhythms, people who are confined to darkness still have Circadian rhythms. And contrary to early research which said that people's natural clocks are 25 hours, it turns out that's not the case. Those findings were based on research that had some methodological flaws. What we know is that humans have evolved to match, well, the Earth's rotation, this 24 hour cycle.
The Circadian rhythm is mediated by, we've got melatonin, cortisol, and then we've got neurotransmitters signaling. And those signals are happening at each stage of the process. Some of the more notable ones are epinephrine and we've got serotonin. Those are probably the two most important ones to know about. Artificial lights can disrupt this clock.
And usually the degree or the direction of the disruption is in the direction of extending the clock up to 25 hours or more. So as you'd guess, people who work night shifts are particularly vulnerable to these disruptions. The Circadian rhythm can be reset by exposure to daylight. And that's why they say if you have jet lag, it can be an effective strategy to get morning sun when you first wake up.
And that actually also happens to be a strategy for people who are suffering from seasonal affective disorder, although that's a topic for another lesson. Teenagers have different Circadian rhythms than adults, but still the same duration. But their alertness peaks a little later, about two hours later than for adults. And it's one reason that teenagers tend to be night owls, so to speak. And also to sleep in late and it's actually kind of unfortunate that the high school schedule is 8 to 3.
A 10 to 5 schedule would probably be more conducive to peak learning for kids, maybe even a little later.