During her recent Q&A with Mammoth, Dr Nicola Barclay, Sleep Scientist and Psychologist, explains the different stages of sleep and what happens during each stage.
“So you’ve probably heard about two different stages of sleep and I’ll go into a bit more detail about what they are. So you’ve probably heard of non-REM sleep and you’ve probably heard of REM sleep. Now, what REM refers to is rapid eye movement and so there’s one sleep stage, REM sleep, where we do have darting rapid eye movement. I’m going to talk about that in a moment. I’m going to first focus on non-REM sleep, so you might think of this as one stage, but actually it’s not, it’s three stages. So we have three separate sub-stages of non-REM sleep.
First – stage one, this is kind of drowsiness, this isn’t really true sleep. This is like pre-sleep where our eyes start to kind of roll a little bit. We’re starting to feel a little bit sleepy, respiratory rate reduces, the cardiovascular system reduces, but we’re drowsy.
We then go from stage one into stage two sleep and this is the real onset of sleep. So stage two is still considered to be relatively light but what we see in stage two sleep is particularly EEG characteristics. So when we measure sleep in the lab, we have participants come to a lab and we hook them up with PSG, which is electrodes on the scalp to measure electrical activity in the brain. We also have sensors on various parts of the body to assess different patterns in muscular activity as well, which helps in the staging of sleep. But in stage two sleep, we see two particular characteristics. We see sleep spindles, which are really tight clusters of very fast brain activity. And we also see things called K complexes, which is a big hyperpolarization followed by a depolarization. And these two signatures have been thought to be associated with learning and memory. In some studies where participants have been presented with some material to learn, they then have a nap opportunity and we see a lot more sleep syndromes and K complexes in sleep after a learning period. So it’s not that these characteristics are doing something in the transfer of information from short term to long-term memory, but K complexes also have another function and this is called sensory gating.
So when we’re trying to get to sleep, we aren’t always aware of our surroundings. We’ve got our eyes closed, we’re not seeing, we don’t really hear what’s going on. When we’re fast asleep, we might miss a thunderstorm and not hear what’s going on in our surroundings. And that’s because the brain is filtering out extraneous information that it doesn’t need to attend to. And it’s thought that K complexes act as a gatekeeper of that information. So the K complex is kind of listening out in the environment and determining whether a sound is something that we need to be aware of or not. And if it’s not, then it doesn’t allow us to attend to it, then we can stay asleep. But there are some evolutionarily relevant sounds that we do attend to, such as babies crying. So parents always wake up to the sounds of their baby crying, it’s an evolutionary response. We need to and so the sensory gating system allows these sorts of sounds to be attended to and to arouse us from sleep. So that’s what’s happening in stage two.
We then move into stage three of non-REM sleep, which is sometimes called slow-wave sleep, sometimes called deep sleep. The reason why it’s called those things is because we see big slow wave activity on the brain activity from the EEG. And it’s during slow-wave sleep that we get a lot of our cellular restoration. So the slow-wave sleep is really for the repair of the brain and body. But during slow-wave sleep we also see the secretion of growth hormone. And in babies who sleep a lot, (supposedly, mine didn’t), but supposedly they’re supposed to sleep a lot, we see a much greater proportion of slow-wave sleep because they’re growing so much. They need that sleep for the cellular proliferation and for growth.
Following on from slow-wave sleep, we then move into rapid eye movement sleep and in rapid eye movement sleep, the brain activity we see is almost as active as the waking brain. So sometimes it’s called paradoxical sleep because actually, it looks quite active. And it’s during REM sleep that we see the darting eye movements. So we see rapid flickering across the eyes. And if you ever watch somebody sleeping, you’ll see these eye movements under the eyelids and we also experience most dreams. We do dream in other sleep stages as well, but we’re more likely to dream in REM sleep and the content of the dreams is more likely to be bizarre and kind of chaotic and colourful and vivid. And it’s thought that these eye movements may be us directing our attention during the dream to different kinds of scenes within the dream. So we’re kind of having these sacredic eye movements to kind of look around in our dreamscape. And it’s thought that dreaming and rapid eye movement sleep is really integral for emotional regulation in individuals that have major depressive disorder or other depressive symptomology.
We actually see abnormalities in REM sleep, so we have kind of predictions about the progression through the different sleep stages. So we typically go through stage one, stage two, stage three, and then rapid eye movement sleep, and that takes around 90 minutes. We then cycle back through some of the sleep stages, so from REM to stage three to stage two, to stage three and then REM. And over a typical course of the night, we may go through five whole sleep cycles. But the proportion of each of the sleep stages across those sleep cycles changes across the night.
When we first go to sleep, we’re more likely to get a lot of the rich slow wave sleep. And in the second half of the night we’re more likely to get more periods of rapid eye movement sleep. So we’re more likely to dream in the second half of the night.
But what we see with individuals with depression is that actually their REM sleep is shifted a little bit earlier. So rather than it taking around 90 minutes to get to that REM sleep, sometimes it may be pushed further where they experience that REM sleep around about 60 minutes and this is thought to be a marker of depression. We also see a greater density of rapid eye movements during REM sleep. So this is kind of how many rapid eye movements there are in individuals with depression. So it’s possible that these markers of depression which are coming out in REM sleep are something to do with that emotional regulation that were mentioned.”
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