While sleeping, people usually feel rested and refreshed, but for the patients of clinical neurologist Guy Leschziner, the moments when they do not wake up are generally anything but peaceful. You can tell she took her bike for a ride while she slept soundly after her landlady asked her where she was going. Another person moves in his sleep, plays out his dreams – and terrifies his partner. Still others suffer from insomnia, sleep deprivation and night anxiety.
Leschziner describes her cases and how they inform the growing world of sleep research in his recent book The Nocturnal Brain: Nightmares, Neuroscience and the Secret World Sleep. " These cases are interesting in two ways ̵
The Verge talked to Leschziner about what makes for a good night's sleep and what goes on in the brain when we dream, and how sleep disorders could open a window into other health issues.
This interview has been slightly condensed and edited for the sake of clarity.
You write a lot about abnormal sleep and the patients you encountered in your practice. First of all, I would like to ask the opposite: what does a normal good night sleep look like?
Sleep is not only an objective experience but also a subjective experience. From an objective point of view, a good night's sleep is considered to be a sufficiently consolidated sleep, divided into one or two tranches. If you look at the population, seven to eight hours are considered the optimal amount of sleep. However, the subjective sleep experience is slightly different for each person. There may be people who wake up several times a night, but feel refreshed and normal during the day. While other people who apparently and objectively sleep well, can feel extremely tired and not refreshed. So there is such a discrepancy between the objective physical manifestation of sleep and the subjective experience.
If you go to bed at about the same time, wake up at about the same time, and feel refreshed during the day, if you can fall asleep regularly at the same time You probably have enough sleep.
As a sleep researcher, how do you see the increasing use of sleep tracking devices?
I do not think it is wrong to persecute your sleep. But you have to ask, "Why am I following my sleep?" If you track your sleep because you know you are not getting enough sleep, you probably do not need a tracker to tell you that.
I think the big danger is that you are tracking your sleep because you have insomnia or because you feel very, very bad sleep, even though you spend a lot of time in bed. When people with some degree of anxiety about their sleep receive data that may not be accurate, it reinforces their prejudices about their sleep and can therefore lead to sleep-related anxiety and even make things like insomnia worse.
One of your patients, John, reported very vivid dreams and actually began to move while they slept. What is the connection between what they see and experience in their dreams and their physical movement in their sleep?
Usually we are completely paralyzed while we are dreaming. The only muscles that really maintain a certain amount of movement are the muscles that move our eyes – hence the term "rapid eye movement" – and also the muscles that allow us to breathe. Of course, if we stop breathing, evolutionarily, that would not be a good idea.
So there are mechanisms in the brain that produce this paralysis when REM sleep is turned on. But in people like John, this mechanism of paralysis goes awry. When he begins to dream, he is not paralyzed and therefore basically plays out the movements he makes in his dreams.
It's not surprising that dreams play a big role in your book on sleep – how do we understand why we're dreaming right now?
We know that the stage of sleep most associated with dreaming sleep in rapid eye movements [REM sleep] changes over the course of life. Before we are born, we probably spend about a third of every 24 hours in this sleep phase. But this begs the question: When we are in the womb, what are we dreaming of? What could be the function of the actual dream experience? The probability is that dreaming, or REM sleep in particular, has different functions during different stages of life.
When we are very early in development, either as a fetus or perhaps as a newborn, it is probably about developing certain pathways that are responsible for consciousness. Later in life, dreams seem to be particularly important for learning, but it also seems to be important for the regulation of emotions.
If you look at the brain like some kind of supercomputer, we take a whole load of information, and we interpret it after our experience of the world. [Our brains essentially] create a predictive model – we can fit the experience into this model and generate predictions from it. But we are constantly gathering information for each watch that affects our image of the world around us. This means that our model of the world around us has to be constantly evolving.
That may well be what dreams are – it is the refinement of our understanding of the world. I think that explains, for example, why we have been dreaming of certain events for a long time, but also of certain events that we experienced during this day. It is the interplay of all our experiences that have been reinterpreted in the context of this model that we have created for our external environment.
A recurring theme in your book is that some sleep disorders can potentially be a window to more serious health problems. Are you telling us more about it?
We begin to understand that sleep is important for regulating nearly all physiological processes in our body. There is a growing field of research to investigate whether sleep disorders, sleep deprivation or other conditions such as sleep apnea can lead to conditions such as dementia. We know that sleep disorders and sleep apnea are both associated with a cognitive decline, and there is increasing evidence that they are actually risk factors for dementia. We are not here yet, but this is a pretty exciting area. For, if we can prove that, for example, sleep apnea carries a high risk of dementia, this is a potentially modifiable risk factor.
The reason why [sleep] could be particularly important for dementia is that we know that there are a number of channels in our brain called the glyphatic system. The function of the glyphatic system is to remove toxins and metabolites from the brain. In deep sleep, these channels open by about 60 percent. The [idea] states that when you sleep, your brain flushes out these toxins more efficiently.
When we talk about channels that open about 60 percent, that's a pretty large percentage, but what's that? Size of these channels that we talk about?
They are microscopic. In fact, for years we did not even know they existed – they were discovered in the last decade or so.
Sounds like there is still much to explore.
What's so exciting about sleep medicine is that we do these things an average of eight hours a night, so we spend a third of our lives sleeping. And yet we hardly had any idea what it was about until the last few years. Things have developed dramatically in the last few decades. But there is still so much to learn.