Re-Release "Why Do Brains Dream"

David Eagleman (0:04)

Why do brains dream? And what's going on with the bizarreness of dreams? Why doesn't your clock work in your dreams? And even though you spent a lot of your day working on cell phones and computers, why do those almost never make appearances in your dream content? There's so many cool questions that we're going to address today.

Is dream content the same across cultures and across time?

Are dreams experienced in black and white or in color? Why do I think about dreams as the strange love child between brain plasticity and the rotation of the planet? What is the relationship between schizophrenia and dreaming? And in the future, will we be able to read out the content of somebody's dream?

Welcome to Inner Cosmos with me, David Eagleman. I'm a neuroscientist and an author at Stanford. And in these episodes, we sail deeply into our three-pound universe to understand why and how our lives look the way they do.

And how we experience our life includes our nocturnal life.

We spend a third of our lives with our eyes closed, lying still and horizontal like a meat robot that's been switched off. But why?

In the last episode, we talked all about sleep, why brains do that, and why all animals do that, and what is happening in the brain during sleep. Today, we're going to zoom in on one of my favorite subjects, a very strange thing the brain does, which we call dreaming.

We spend a fraction of our sleep time locked in a different reality, swimming around in plots which aren't real, and they're often not even realistic, but which nonetheless convince us entirely. And today, we're going to see what that's all about.

Humans have been writing about dreams and pondering about them for a long time, and essentially we have evidence of this from the beginning of writing. We see detailed dream reports from ancient Egyptians, and Romans, and Chinese. The Bible is full of dreams. Every culture in the world has been fascinated by these strange nocturnal journeys that we go on. And as we'll see shortly, there's an interesting surprise, which is that dream content, in other words, the kind of things that people dream about, is essentially the same across cultures and across time.

And that serves as a clue that will allow us to unlock some mysteries about what otherwise feels like a deeply mysterious experience that we enjoy or suffer every night of our lives. So strap in for an episode full of surprises. So let's start with a poem that Lord Byron wrote in 1816 called The Dream. He wrote, Sleep hath its own world and a wide realm of wild reality, and dreams in their development have breath and tears and tortures and the touch of joy.

They leave a weight upon our waking thoughts. So dreams are indeed a wild reality. The thing we all find so amazing about dreaming is its bizarreness. You're seeing new things, you hold strange beliefs, and you fall for it all. Hook, line, and sinker. Whatever your brain serves up to you, you believe it entirely. So we'll talk about all this, but let's first zoom in on what's happening inside the brain.

Dreaming occurs during rapid eye movement or REM sleep, where your eyes are darting back and forth. There's nothing coming in the eyes, of course. They're shut, so all the activity is internally generated. For good housekeeping, I'll just note that REM sleep isn't necessarily equivalent to dream sleep, but that's when most dreaming occurs. Now, how do we know that? Well, it's because if you wait until someone enters REM sleep, their eyes are going back and forth, and then you shake them awake and you say, hey, what were you just experiencing? They'll tell you, whoa, I was just panicking because I was at work but realized I had forgotten to put on pants and I was hiding behind the plant pots and figuring out how to escape before the meeting started. But if you wake them up during the rest of the 90-minute sleep cycle, during the stages that we call slow wave sleep, and you say, hey, what were you just experiencing? They'll generally say, nothing, I wasn't there.

Now, for completeness, I'll mention that sometimes people report some form of mental activity during slow wave sleep. But when they do, this is usually just a thought or making a plan. But it lacks the visual vividness and the hallucinatory components of typical dreams. So dreaming of the sort we're interested in today happens during REM sleep. Now, although dreams seem so untethered and ethereal, there are specific things happening under the hood. Dreaming depends on the normal functioning of a particular network of brain areas. This is primarily in the limbic and paralimbic and association areas. Now, we know that from brain imaging, but also because damage in this network can produce temporary or permanent dream loss or impairment, like loss of visual dream imagery. Now, not all of those areas might sound familiar to you, but you've probably heard of the limbic system, which sits at the heart of your emotions. So presumably, this is why dreams are so overloaded with high emotion.