- Imagine this, it's winter 1862, and you're a chemistry professor in Belgium.

You're working on one of the most pressing problems in chemistry, the structure of benzene.

Benzene is a smelly, and highly flammable, hydrocarbon molecule.

All hydrocarbons are made of, you guessed it, hydrogen and carbon.

But in benzene the ratio of these elements is a little weird.

Instead of having more hydrogen atoms than carbon atoms like most hydrocarbons, benzene has the same number of hydrogen and carbon atoms, six and six.

What strange molecular structure could let these atoms fit together?

Frustrated you turn your chair towards the warm fire and take a nap.

As you sleep visions of atoms and molecules dance in your mind's eye.

They turn into a series of snakes.

Then suddenly one of the snakes coils around and bites its tail like the ancient symbol of the ouroboros.

You've solved the chemical structure of benzene in a dream, precisely as German chemist, August Kekule did in 1862, completely changing the future of organic chemistry in the process.

(Joe snores) (Joe snorts) And it all happened while he was asleep in a dream.

It kind of makes you wonder, why do we dream?

(upbeat music) Hey, smart people, Joe here.

Kekule's dream discovery of benzene's molecular shape is one of history's most famous dreams.

But for most of us dreaming is about more than making chemistry discoveries.

Humans have been trying to figure out why we dream for thousands of years, probably as far back as we've been asking questions.

And since it's an experience we only have while we're asleep, it's a particularly tough question to answer.

Going back to the Greek philosopher Plato and the Confucian scholar Zhu Xi, great minds have speculated about the function and meaning of dreams.

But it's only been in the last few decades that scientific experiments have started to show us what benefits our nocturnal narratives could have.

That's right, benefits with an S. Scientists think dreams may have many functions that influence our success, our smarts, and even our survival.

And we each spend about two hours dreaming every night.

Over an 80 year lifetime, that's almost 60,000 hours or the same as 10 years of waking life.

Dreaming clearly must have some benefit.

Otherwise we wouldn't spend so much darn time doing it.

And everyone dreams, even if we don't always remember them.

You know you are more likely to remember your strangest dreams.

Like I had this one dream where Tom Hiddleston and I were rowing a boat across the ocean.

And then we got hit by a big storm and we were eating sandwiches for some reason.

That was weird.

I really dreamed that, that's real.

Almost half of us remember at least one dream a week and women are more likely to remember their dreams daily compared to men.

There are a number of phases that the brain goes through during sleep.

These phases are repeated in cycles throughout the night.

In the first phase we transition from wakefulness into sleep as you begin to relax and your breathing slows.

As your body temperature drops and your breathing slows down even more, you enter light sleep.

After that, you enter the deep sleep phase, characterized by a particular pattern in your brain called delta waves.

After that you start the REM, or Rapid Eye Movement sleep stage, your breathing gets faster, your eyes move all over, and during REM is when dreaming happens.

And throughout this phase your brain is very active, almost as active as when you're awake.

Almost all other animals, whales, wombats, wildebeest, sleep, and many also experience REM sleep.

So scientists think that many of these animals also dream, including of course your cat or dog.

The way we think about dreams has changed a lot throughout history.

In most cultures around the world, dreaming has held spiritual significance and there are even dream interpretations in the Bible.

But there was nowhere where decoding dreams was more popular than in ancient Egypt.

The Egyptians created these volumes of books, full of common dreams and their supposed meanings.

Professional dream interpreters used these books to help people figure out what their dreams meant.

So I had this dream that my leg fell off?

Ooh, that means dead people are judging you.

Yikes.

I dreamed that I died violently.

Oh, that's great.

Really?

Really, it means you'll live a long life.

Uh, okay.

So I had this dream where I poured a jug of my own pee into the Nile River.

Really?

Really.

Well, actually that's great news too.

It means you'll have a plentiful harvest.

Right, obviously.

The belief that dreams held hidden messages to be interpreted or decoded remained the dominant way to look at dreams through the first part of the 20th century.

In 1900, Sigmund Freud published this influential book, Interpretation of Dreams.

And in it, he claimed that dream interpretation could be used to understand unconscious desires.

It all started when he had a dream, a dream so famous it has a name and a Wikipedia page.

It's called Irma's injection.

"A large hall, numerous guests whom we were receiving.

Among them was Irma."

It was about a former patient of Freud's that he felt he wasn't able to completely heal because she refused his treatment.

"I at once took her to one side as though to answer her letter and to reproach her for not having accepted my solution yet."

This dream sparked Freud's theory that our wishes that aren't fulfilled while we're awake are expressed in our dreams.

Because some of those wishes might be kind of embarrassing, Freud thought our minds deliberately confused the dreams to hide their true meaning.

And Carl Jung expanded Freud's theories.

See, Jung considered archetypal symbols that often appeared in dreams, like a wise old sage or a trickster to be universal among humans.

Jung thought that dreams were interpretable if we could crack the code of these universal symbols, the language of dreams.

According to Jung, dreams had two functions.

They compensated for things that the dreamer ignored or repressed, and they looked forward to give the dreamer hints about what might happen in the future.

Lots of people still look for meaning in their dreams, especially to do with the future.

In one study, people were more likely to say having a dream about a plane crash the day before a flight would make them cancel their trip then if they were given a government warning about a high risk of a terrorist attack.

But modern science has moved away from Freud and Jung and viewing dreams as buried messages to decode or interpret.

Today, researchers are asking what functions and benefits our brains themselves might get from dreaming?

At first, scientists believed that the strange mishmash of pictures and stories and events that we experienced while dreaming were just side effects of basic biological processes in our brains, a sort of neurological noise that we experience as we sleep.

Scientists thought that other parts of the brain tried to make sense of this noise by threading this random slideshow into a story.

Often a very weird story.

This is called the activation-synthesis theory of dreaming, but experiments showed that dreams are not actually random.

Some things are more likely to appear in our dreams than others.

So scientists began to wonder, maybe dreams aren't random noise.

Maybe our brains need to dream to be healthy.

Now what we dream about often has to do with what we do while we're awake.

Especially if we're learning something new.

In one study about a third of participants who played Alpine Racer 2, an arcade downhill skiing simulator, had dreams about the game.

I mean, consider this, the things that happen to you every day only happen once.

In our short-term memory these experiences are fragile.

They can easily disappear.

Our daily experiences might only make it into our long-term memory if they're replayed several times, you guessed it, in our dreams.

The patterns of brain activity just after dreaming look a lot like when our brains store and retrieve episodic memories, memories of things that actually happened to us.

So dreams might be a sort of memory replay of our experiences with an extra layer of weirdness on top just to make it fun, I guess.

There was one study to test this where scientists asked people to play seven hours of Tetris across three days.

And hang on, where do I sign up for these tests?

This I would totally, I would do that for free.

(Tetris music) Oh, yes.

Doing a science, doing big science.

This is sleep science folks, hold on, busy right now.

Anyway, after some serious Tetris time, participants reported seeing images of tetrominos when falling asleep.

They seemed to be replaying the game to store their new skills in long-term memory.

It's not all fun and games though.

Dreams can turn into nightmares.

Now, there's no universally accepted definition of what a nightmare is but they're commonly considered some distressing or terrifying dream, one that goes so far as to wake you up.

About 1 in 40 dreams is a nightmare.

So why do our brains replay our worst fears and memories?

Well, the threat-simulation theory suggests that dreams let us practice dangerous events and situations.

That's why some people relive traumatic experiences in dreams.

The brain is trying to condition us to survive threatening experiences by practicing in a safe environment, inside our sleeping brain.

Back when life threatening situations were a part (tiger growls) of our species everyday life, (cartoon Joe screams) simulating threats could have helped us survive.

But in the modern world, reliving awful situations and chronic nightmares can be debilitating.

But we also practice social situations in dreaming.

That's the social simulation theory of dreams.

Scientists noticed that our dreams are heavy on social situations, you know, a fight with a close friend, a date with a crush, a clash with a coworker, not wearing pants to school.

Since being social is so important to our species, practicing these situations would have been an evolutionary advantage.

The strange experience of dreaming can be used for problem solving too.

The novelist John Steinbeck once said, "It is a common experience that a problem difficult at night is resolved in the morning after the committee of sleep has worked on it."

When college students were given a homework problem to focus on each night before bed, a quarter of them dreamed the answer within a week.

Because dreams aren't limited by logic or physics, that means they're a great place to problem solve and come up with creative and sometimes weird solutions.

It's especially helpful when the solutions to problems need a very different approach compared to conventional wisdom.

And this may be why we owe so many great pieces of art to dreams, the Beatles iconic song Yesterday, Salvador Dali's Melting Clocks, Mary Shelley's novel Frankenstein, and countless others are inspired by dreams.

So can you dream your way to a Nobel Prize or a Grammy?

Well, maybe.

Scientists are experimenting with dream incubation as a way to prime the brain before sleep to be more creative.

So, because dreaming is so common and because we spend so much of our lives doing it, it's almost certainly useful for one or many reasons.

But why did dreaming evolve to begin with?

Maybe thanks to the rotation of our planet?

That's right.

Scientists think the origin of dreaming just might have to do with the sheer amount of time that humans and all animals spend in the dark.

So our ability to use our peepers and see the world around us is obviously an extremely important evolutionary advantage.

Because it's so important, the part of the brain responsible for sight, called the visual cortex, takes up a big chunk of our brains.

But here's the thing.

Our brains can also be rewired pretty easily.

If you were blindfolded, your brain would begin to change within an hour of not using your sight.

The neurons of the visual cortex start being taken over for other tasks.

The lengthy darkness of nighttime would have meant that the visual cortex of our human ancestors was at a high risk of being taken over by other functions while we slept.

If we didn't use it, we could lose it.

Dreams and their highly visual nature may have evolved in mammals to keep those vulnerable brain areas active at night and keep the brain from rewiring itself in unfortunate ways.

So, which is it?

Is it problem solving, and practicing, seeing?

Or is it problem solving and some social and a little bit of seeing?

Or is it problem solving and just social and not practicing?

Or what I mean to say is scientists don't know if one or many of these ideas will prove to be correct.

And so research continues.

Ah, science, always figuring stuff out but never quite figuring stuff out all the way.

It's even possible that dreams may have evolved for one function but stayed around so long because dreaming ended up helping us in a bunch of other different ways.

Trying to peer into the workings of the sleeping brain is one of the most challenging problems in psychology and neuroscience, but we're building a fuller and fuller picture.

The science of dreaming, one dream at a time.

(Joe snores) (Joe snorts) You wouldn't believe the weird dream I just had.

Stay curious.

Egyptians were there and, I don't know, Tom Hiddleston?