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（[有化] 碳氢化合物） molecule. All hydrocarbons are made of, you guessed it, hydrogen（[化学] 氢） and carbon（碳）. But in benzene, the ratio（比率） of these elements is a little weird（怪异的）.

  Instead of having more hydrogen atoms than carbon atoms like most hydrocarbons, benzene has the same number of hydrogen and carbon atoms, 6 and 6. What strange molecular（ 分子的） structure could let these atoms fit together? Frustrated（失意的）, you turn your chair towards the warm fire and take a nap. As you sleep, visions（视） of atoms and molecules dance in your mind's eye. They turn into a series of snakes. Then suddenly, one of the snake's coils（线圈） around and bites its tail like the ancient symbol of the Urroboros.

  You've solved the chemical structure of benzene in a dream. Precisely（正好） as German chemist Auguste Kekule did in 1862, completely changing the future of organic（[有化] 有机的） chemistry in the process. And it all happened while he was asleep in a dream. It kind of makes you wonder, why do we dream? 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（ 认为） 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 particularly tough（坚韧的） question to answer. Going back to the Greek philosopher（哲学家） Plato and the Confucian scholar（学者(尤指文学方面)） Zushi, great minds have speculated（推测） 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（夜的） 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（ 幸存）. And we each spend about two hours dreaming every night. Over an 80-year lifetime（一生）, that's almost 60,000 hours, or the same as 10 years of waking life. Dreaming clearly must have some benefit, otherwise（另外） we wouldn't spend so much darn（织补） time doing it. Everyone dreams, even if we don't always remember them. You are more likely to remember your strangest dreams.

  Like, I had this one dream where Tom Hilston 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（月相） that the brain goes through during sleep.

  These phases are repeated in cycles throughout the night. In the first phase, we transition（转变） 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（描绘…的特性） by a particular pattern in your brain called delta（三角州） waves. After that, you start the REM, or rapid（快的） eye movement sleep stage. Your breathing gets faster, your eyes move all over.

  And during REM, is when dreaming happens. Throughout this phase, your brain is very active, almost as active as when you're awake. Almost all other animals, whales, wombats, wildebeest（羚羊(其中一种)）, 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（ 精神的） significance（意义）, and there are even dream interpretations（解释） in the Bible（《圣经》）. But there was nowhere where decoding（解码） dreams was more popular than in ancient Egypt. The Egyptians created these volumes（卷） of books, full of common dreams and their supposed meanings. Professional dream interpreters use these books to help people figure out what their dreams meant. So how does dream that my leg fell off? 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.

  Okay（好）. So I had this dream where I poured a jug of my own pee into Nile（尼罗河） River. Really? Really. Actually, that's great news too.