As we drift off to sleep, our brains are still very active. In fact, sleep is critical for our brains to function properly.
During sleep, our brains consolidate and store memories from the day. This is why it’s important to get a good night’s sleep before exams or an important presentation. Sleep also helps to clear out toxins that have built up during the day. This includes a waste material called beta-amyloid, which is thought to contribute to Alzheimer’s disease.
So, what exactly happens in our brains while we sleep? Neuroscientist Penelope Lewis is trying to answer this question. She is a psychology professor at Cardiff University and previously directed the Sleep and Memory Lab at the University of Manchester in England.
Her book The Secret World of Sleep and TED Talk dives into the surprising science of what happens to our minds when we're at rest.
In an interview with NPR, Lewis highlights how sleep makes memory stronger. "When it comes to strengthening [memory], we know that the neural responses in your brain that are associated with things you've recently experienced are spontaneously replayed, or, we say, 'reactivated' while you're asleep. So that means, supposing you are learning to play the piano ... you're moving your fingers a lot. That's associated with responses in motor areas of your brain. ... Those areas will become active again while you're asleep, and that replay — or reactivation — is what we think is responsible for the strengthening. So it's kind of like your brain is rehearsing stuff without you knowing, while you're asleep."
This process of "reactivation" is important for consolidating new memories and skills, and it helps us to better remember things when we wake up. Dr. Lewis believes that sleep helps to "clean up" the brain, getting rid of unnecessary information and strengthening the connections between neurons. This helps us to focus on the things that are most important and to remember them more effectively. In other words, during slow-wave sleep, the brain is cleaning house, getting rid of the day's mental debris so that we can start fresh the next day. This is why a good night's sleep is essential for memory and learning. Sleep allows the brain to reorganize and consolidate information so that it can be accessed more easily in the future.
It's a bit of a mystery why we sleep, but one thing is for sure: sleep is essential for our health and well-being. When we sleep, our brains are hard at work processing information and consolidating memories. This is one of the ways that sleep makes us smarter and helps us to learn new information more effectively.
Sleep is clearly very important for our brains. And this is just the tip of the iceberg when it comes to what we don’t know about sleep. Lewis and her team are on the cutting edge of sleep research.
On what she hopes to learn about sleep, Lewis says: “My lab is particularly interested in how memories change while we’re asleep. People might think that memories, once they’re formed, they remain static and it’s just kept like a library book on a shelf, and then you can pull it out when you want to. But actually that’s not the case. Memories evolve constantly. They change, and a lot of that evolution seems to happen while you’re asleep.”
What exactly happens in our brains while we’re asleep? That’s one of the main questions Lewis is hoping to answer. And she’s not alone – sleep scientists all over the world are working to unlock the mysteries of this vital part of our lives.
You know, for a sleep scientist, I actually don't sleep very well. Any little chink of light in the room, and I am awake all night. And my eye-mask is just as important to me as my laptop. But really, I take sleep very seriously, and I am hoping this talk will be a sort of wake-up call to all of you to make some of you feel the same about it.
Now, what I am not going to do is preach to you about how you should get more sleep. We all know that. We all know we live in a sleep-deprived society. Instead, I am going to talk about something which I think is much more interesting. This is how we can manipulate the sleep that we do get in order to get the most out of it, in order to improve our quality of life. And I call this "the new science of sleep engineering.”
But let us start from the beginning. As humans, we spend roughly a third of our lives asleep, eight hours a day. That is more time than we spend doing anything else that is a huge amount of time. And just the pure fact of that time investment suggests that sleep must be doing something incredibly important. But what is this? Well, it turns out that sleep is all about the brain. Contrary to popular opinion, the brain does not just switch off when we go to sleep; instead, it goes through a series of highly specific, diverse types of activities. We can measure these by putting electrodes all over the scalp, like this. This, by the way, is a sleep scientist's idea of a selfie.
With these electrodes, we can measure the electrical activity of the brain. And during wake, it looks something like this - just a wiggly line, with time going from left to right. And what that tells us is the brain is active - good - and that the activity is not particularly synchronized, so things are not summing up in any particular way. But as we fall asleep, the pattern changes a little bit. It slows down a little bit, and the amplitude of those brainwaves gets a little bit higher, showing a bit more synchrony. And we also start to see occasional bursts of high-frequency activity that we call "sleep spindles." These spindles do not occur across the whole brain; they just occur in localized areas at any one time. I am going to come back to them several times during the talk, so try to remember what those look like.
Now, as we go deeper into sleep, the activity slows down still more, and we start to see these high-amplitude, slow oscillations that we call "slow waves." And this shows a high degree of synchrony in the firing across the cortex. So many neurons are all firing together, then pausing, then firing together. It is vastly different than the kind of activity that we see during wake. And if we go still deeper, we go into a sleep stage that I am sure you have all heard about - rapid eye movement sleep. This is famous for the way the eyes dart around under closed lids, and it actually looks very similar to the brain activity that we see during wake, probably because of the dreaming that's happening, not much cortical synchrony there.
So why do we do this? Why do our brains spend a third of our life going through these highly precise distinct types of activity in a cycle from one stage to another? Well, there are two main answers to this. One of them relates to sleep's role in maintaining a healthy brain, and the other to its role in learning and memory. And I am going to start by talking about the healthy brain.
Sleep plays a sort of housekeeping role: it cleans our brains, it helps us to remove toxins. And some of the most interesting studies of this have shown that the spaces between brain cells expand during that slow-wave sleep that I showed you by as much as about 60%. This allows cerebrospinal fluid, the fluid in the brain, to flush through and efficiently clear away toxins that build up during wake.
One of these toxins that is particularly interesting is something you might have heard about - beta amyloid. This is a protein that can build up not only during wake but actually across a lifetime. And buildups of beta amyloid are linked to a formation of plaques in the brain that are predictive of cognitive impairment, particularly problems with memory. If it gets bad, it is also linked to dementia and Alzheimer's Disease. Beta amyloid is also linked to cell death in the brain and a gradual degeneration of some parts of the cortex that can happen with aging, again, in dementia and Alzheimer's Disease.
So it's obvious it's important for us to flush this out of the brain if we can. Now, interestingly, as we age, our sleep patterns also change. Our sleep becomes more fragmented, and those high-amplitude, slow oscillations that I told you about gradually stretch and flatten out, and after the age of sixty-five or so, it is quite common not to get any more slow-wave sleep at all.
Furthermore, this gradual decline in slow-wave across the lifecycle has been shown to predict the extent to which the cortex actually atrophies and shrinks, so [for] some of the prefrontal regions of the cortex that shrinkage is predicted by the decrease in slow-wave sleep.
So wouldn't it be great if there was a way that we could maintain those slow-waves as we got older, and not have that decline? And this is where we come to sleep engineering. Very recent research has suggested a way that we can do this. If we play sounds to people - just click, simple click sounds - while they are in slow-wave sleep, and if we place those sounds near the peaks of those high-amplitude, slow oscillations, it turns out that can enhance them. Let me show you what this looks like.
So the clicks occurring just near the peaks boost the amplitude, and they've also been shown to improve memory the next day. That works very well in healthy young people. And I have gotten several quite sleep-deprived graduate students who are working hard on taking this to the older population. Right now, our results are very promising, so we are hoping that in not too many years we might be able to offer a sort of preventative treatment that could help people to maintain their slow-wave sleep as they get older, and possibly might slow down some of this decline - cortical and cognitive - that happens.
So, let me move on now to talk about learning and memory. In his famous book "One Hundred Years of Solitude," Gabriel Garcia Marquez wrote about a plague of insomnia that swept across a land. People just could not sleep. At first, they did not mind that at all. But eventually, negative symptoms started to manifest. And these symptoms were the fact that they lost their memories. They could not learn new things, and they started to forget what objects were. They had to cover things with notes, saying things like "This is a cow, it gives milk. Pull here."
Given that this book was published in 1967, when we knew almost nothing about sleep's role in memory, it is really quite remarkable that Garcia Marquez had insight into this. But subsequent research has shown that he was absolutely right. Sleep is important for forming new memories and also integrating those memories with what we know already, also strengthening memories.
Let me give you an example. I want you all to hold up your left hand, and we are numbering your fingers from one - pinky, to four - index finger, and now I want you to press your fingers on your thumb in this pattern: “4, 1, 3, 2, 4."
Okay, I think you all got it. This was the task that was used in the experiment I want to tell you about. People were asked to press "4-1-3-2-4 sequences" like this as quickly as they could for about two hours. They were pretty bored, but they also stopped getting faster, as the experimenters wanted, and when they got to that point, they were given a test. 30 seconds. Press as many 4-1-3-2-4 sequences as you can. And this all happened around about 10 AM, and people did just fine - they pressed about 21 sequences. And then, they were told to go away, come back 12 hours later, and do it again. And their performance did not change much.
Now, here is the interesting bit: they went away a second time, and they slept overnight, and they came back and did this a third time. And now their performance improved dramatically, about 20% improvement. Furthermore, the extent to which they improved was predicted by the sleep spindles - remember those high-frequency oscillations I told you about - that occurred over the bit of their brain, the motor cortex, that is particularly associated with hand movement. So, this is the kind of data that suggested sleep is doing something really important for memory. It is helping us to strengthen up memories.
But it turns out it does not just help us to strengthen memory. Sleep also helps us to integrate memories and to make connections between things that we might not otherwise have realized were connected. And this is critical for solving some kinds of problems. It is critical for creativity and forms of innovation, and I bet everyone here has experienced this: you have woken up in the morning with a solution to something that you had not been able to solve the day before. So, in fact, our history is peppered with examples of this.
A more scientific example comes from Friedrich Kekulé, who won the Nobel Prize for discovering the chemical structure of benzene is cyclical rather than acyclical, more linear. And again, this was inspired by a dream in which he saw a serpent biting its own tail.
Interestingly, this type of associative problem-solving is linked to REM sleep rather than spindles, or slow-wave, sleep. So, what you might be wondering is what is going on in sleep. How does sleep allow us to do these things? And the answer seems to be linked to the fact that memories are spontaneously replayed during sleep. So the neural activity associated with something that you've done replays spontaneously when you're asleep.
Let me explain to you an experiment which shows this. This is a maze which participants were asked to navigate around in a video game. And while they were doing this, their brain activity was measured, and unsurprisingly, the hippocampus - amongst other areas - which is associated with spatial memory and navigation, was active while they did this. Then, the same participants stayed in the scanner, and they were asked to sleep. And during slow-wave sleep, those high-amplitude, slow oscillations that I told you about, the same structure was active again. And the extent to which it was active actually predicted how much better they got at navigating around the town when they were tested again the next day.
Furthermore, the sleep spindles that I told you about are thought to be a marker of this type of reactivation. So not only is this experiment a nice example of how reactivation happens during sleep and is linked to strengthening memories but also that finger-tapping experiment - this explains why the spindles of the motor cortex were greater because, actually, that people were probably reactivating those memories as well.
So, what has this got to do with sleep engineering? Well, what's exciting about it is now that we know reactivation during sleep is important for strengthening memories, we've also learned how to manipulate it. So we don't have to sit and wait for reactivation to happen spontaneously, but instead, we can control it. Let me show you how this works.
If I show you a cat (meow) in this part of the screen and a dog (bark) in this part of the screen, and then tonight, when you are asleep and you enter slow-wave sleep, I am watching, and I play (meow). Then the next day when I test you, you'll be much more likely to remember where the cat was on the screen than the dog. And that is because that sound cue will have triggered reactivation of the memory and strengthened it. So this doesn't end with just simply strengthening memories. It has been shown that if we trigger some kinds of problems while people are asleep, then they can be better at solving those the next day as well.
So reactivation can also help with association and potentially with creativity and innovation as well. But as we all know, we do not want to remember everything always.
So I'm sure some of you have had a traumatic experience at some point in your life. You may have been mugged, you may have been in a car crash, maybe other things that happened to you that you'd really - you might not mind remembering the details, but you really don't want to be as upset about it every time you remember it as you were right after it happened. We were interested in this and how reactivation during sleep might help with it. It turns out that if we show people upsetting pictures or videos and then play them the associated sounds (gunshots, screaming) when they are in slow-wave sleep, then ask them the next day how upsetting that was, the things that have been replayed to them during slow-wave sleep will be less upsetting. So it looks as though triggering replay of memories can actually help to disassociate the emotional response from them as well.
We live in a time when we are hyper-aware of our bodies. We are all very aware of how important exercise is and how important diet is. We have gadgets to measure every calorie that we take in and every calorie that we expend. We even have gadgets to measure our sleep.
But I would like to finish here by suggesting that we could be taking this to the next level. Instead of just measuring our sleep, we could be using the information that we now have about sleep to manipulate it in order to enhance things like reducing our aging, improving aspects of our memory, enhancing our creativity, and also potentially controlling aspects of our emotional responses. So, as a sleep scientist, my hope is that in not too many years, when your boss walks into your office and gives you a very difficult problem to work on, you'll feel like the most appropriate response to make to show her you're taking this seriously is to pull out a pillow and tell her you'll sleep on it.