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Tag Archives: Slow-wave sleep
Around 2,200 years ago, Zhuangzi wondered whether he had dreamt he was a butterfly, or he was the butterfly dreaming he was Zhuangzi. About 1,800 years later, Descartes wondered whether he was dreaming even though he was under the impression that he was fully awake. What are dreams and what are the main philosophical worries associated with dreams and dreaming? In the next however many blog posts, I want to write about dreams and dreaming in a bid to shed some light on an area of enquiry that is often taken for granted, as well as hopefully saying something interesting about them in the process. This first post will be covering some of the preliminary science of sleep & dreaming as it’s almost impossible to really get to grips with some of the issues of dreaming (nowadays, anyway) without a basic understanding of how the brain works during sleep. If you’re already familiar with the following, then it’ll (hopefully!) serve as a refresher course, if you aren’t then I hope you don’t find it too taxing & enjoy finding out about one aspect of brain function. Onwards!
The Science Of Sleep
Scientific interest in dreaming has only really took off in the past 60 years ago, since the discovery of REM sleep in 1953 by Nathaniel Kleitman & Eugene Aserinsky (as is often noted in the literature, this was the same year Francis Crick & James Watson discovered the structure of the DNA molecule). In order to understand why the discovery of REM sleep is important for dream research, we’ll first need to understand the basic science of sleep as a whole.
We humans spend approximately 1/3 of our life sleeping and as you will already be aware, we are not the only animals to do so. In fact, the phenomena of sleep is almost universal across the entire animal kingdom (all mammals, all birds and some cold-blooded vertebrates sleep), but giving a universal definition of sleep is difficult because, given the huge variation & diversification across different species, there aren’t many traits we all have in common. Scientists use different aspects of sleep to identify which animals do sleep, or exhibit some behaviours associated with sleep (think of it as analogous to a family resemblance. Your family may not share one unique trait -such as a short nose, for example- but there may be several traits members of your family have some of, but you don’t all have the same ones -some might have the short nose, others the long chin, etc). For simplicity, we’ll stick with human sleep behaviour. Before I outline the various stages of sleep, I’ll need to quickly fill you in (assuming you don’t already know) about brain waves.
You probably know that there is quite a bit of electrical activity going on in the brain. One way of measuring this electrical activity is through the use of an EEG (ElectroEnthephalogram) machine. You’ll have probably seen these in the movies. Small metal disks are put on the subjects scalp which are wired to a machine which draws squiggly lines (in the older machines, paper would continually be fed through whilst a pen moved up and down depending on the electrical activity the disks picked up, more modern machines will do this digitally (click here for an example of an EEG readout)). Measuring this electrical activity is a measurement of brain waves. Awake as you are now, your brain waves will be very frequent (so, a measurement of how often they occur is a measurement of frequency), up to 15 every second, meaning that the neuronal firings will be in rapid succession. However, they are low voltage waves (which is measured by the amplitude -this is a measurement of the distance between the top & bottom of the wave). After an hour or so of being asleep your brain waves will be high amplitude but low frequency (this is stage 4 sleep, which I’ll get to in a bit), so the waves are high voltage but occurring less often.
Stages 1-4: NREM
Got all that? Good. Now, there are five stages of sleep which can be divided into two groups; REM & NREM sleep. There are four stages of NREM sleep and then the one stage of REM sleep. You will go through the four stages of NREM sleep (from stage 1 through to 4) and back again (from stage 4 back to stage 1) before you hit REM sleep.
When you’re getting sleepy, your brain moves from what is called beta activity (brain wave activity associated with being awake with a frequency between 15-30 Hz -Hertz, or Hz, is the standard unit of measurement for amplitude) to alpha activity (8-12 Hz). Anyone familiar with the Greek alphabet might note that this seems counterintuitive (beta is the second letter of the alphabet, alpha is the first), so you might not be too pleased to know that this odd use of the Greek alphabet just gets worse. When theta (eighth letter of the alphabet!) activity starts to show (registering a frequency of 3.5-7.5 Hz), this is an indication that the person has entered stage 1 of NREM sleep, though this stage is transitional between being awake and being asleep. At stage 2, sleep spindles will start to appear (short bursts of 12-16 Hz waves), and it is at this point that the person will become harder to wake. As you move through to stage 3 & 4 you start to get much more delta (fourth letter of the alphabet) activity (high amplitude waves occurring at less than 3.5 Hz). Aptly then, stages 3 & 4 are called slow-wave sleep. Stage 4 is reached in less than an hour but can last up to half an hour.
During stage 4 sleep (which is the deepest part of sleep), several things can happen (perhaps you’ve experienced them at some point during your life). Sleepwalking, which can range from just getting out of bed then climbing back in again to getting into one’s car (apparently, sleepwalkers don’t try to drive their cars). This is not thought to be the acting out of one’s dreams (for reasons we’ll get to later on). While sleepwalkers are difficult to wake up, contrary to popular belief it is completely safe to wake them; it won’t cause them any harm, though for obvious reasons they might be a bit confused! Sleeptalking is more likely to occur during NREM but not always. Night terrors or enursesis (bed wetting) can also occur. Night terrors are not to be confused with nightmares. Typically, night terrors are not preceded by bad dreams (or at any rate, bad dreams are rarely reported upon waking -again, we’ll come to why dreaming in general isn’t taken to occur in NREM sleep later), they are just the result of people (usually children) quickly waking from stage 4 sleep, screaming and frightened. The best treatment for night terrors is no treatment at all (it is not an indication of anything bad -the same advice goes for sleepwalking).
Stage 5: REM
Returning to our sleeping subject, the EEG machine will pick up the fact that after stage 4 they will start to regress, that is that they’ll start to move back through the NREM stages all the way to stage 1. However, their heartbeat will become irregular and their breathing will become shallow, occasionally gasping. If one were using an EOG (electrooculgram) machine (it monitors eye movement), you would notice the person’s eyes were darting rapidly around (up and down, side to side). Interestingly, the EEG readout will look like someone who is fully awake (high frequency, low amplitude waves). In fact, they are in REM sleep. The first episode of REM sleep will last around 20/30 minutes (after which they will repeat the NREM cycle). As each sleep cycle is completed, the period of REM sleep increases and the deeper stages of NREM tend to disappear. During an average 8 hour sleep, you will go through REM sleep four to five times (newborns spend about 50% of their sleep in REM, above the age of 5 this goes down to 20-25% then down again to 18% or less in old age -stages 3 & 4 can completely disappear in old age too). During REM sleep the person is completely paralysed, save for the heart, diaphragm, eye muscles & smooth muscles (e.g. muscles of the intestines & blood vessels).The brain is largely independent of its sensory & motor channels at this point, however discharging neurons that originate in the brain stem extend into the areas controlling eye movement and motor activites. This means that although the body is paralysed, the brain is registering activity from the areas of the brain associated with walking around and seeing the world, despite the fact it’s doing neither.
Thus Ends The Science
Well, for the most part.
So here ends Part 1, I hope you found it interesting! In the upcoming parts I’ll be discussing the difference between NREM & REM dreams/thoughts & some of the philosophical problems of gathering evidence for dreams. Hope you enjoyed this post and will stick around for more!
Feel free to leave questions & comments, I’ll answer them as best I can.