0:00:06.643,0:00:08.412
In the third millenium BCE,

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Mesopotamian kings recorded and[br]interpreted their dreams on wax tablets.

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A thousand years later,

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Ancient Egyptians wrote a dream book

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listing over a hundred common dreams[br]and their meanings.

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And in the years since,

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we haven't paused in our quest[br]to understand why we dream.

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So, after a great deal[br]of scientific research,

0:00:28.121,0:00:29.787
technological advancement,

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and persistence,

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we still don't have any definite answers,[br]but we have some interesting theories.

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We dream to fulfill our wishes.

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In the early 1900s,

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Sigmund Freud proposed that while all[br]of our dreams, including our nightmares,

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are a collection of images[br]from our daily conscious lives,

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they also have symbolic meanings,

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which relate to the fulfillment[br]of our subconscious wishes.

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Freud theorized that everything [br]we remember when we wake up from a dream

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is a symbolic representation

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of our unconscious primitive thoughts,[br]urges, and desires.

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Freud believed that by analyzing[br]those remembered elements,

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the unconscious content would be revealed[br]to our conscious mind,

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and psychological issues stemming[br]from its repression

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could be addressed and resolved.

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We dream to remember.

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To increase performance [br]on certain mental tasks,

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sleep is good,

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but dreaming while sleeping is better.

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In 2010, researchers found

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that subjects were much better [br]at getting through a complex 3-D maze

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if they had napped and dreamed[br]of the maze prior to their second attempt.

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In fact, they were up to [br]ten times better at it

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than those who only thought of the maze[br]while awake between attempts,

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and those who napped but did not dream[br]about the maze.

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Researchers theorize that certain[br]memory processes

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can happen only when we are asleep,

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and our dreams are a signal[br]that these processes are taking place.

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We dream to forget.

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There are about 10,000 trillion [br]neural connections

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within the architecture of your brain.

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They are created by everything you think[br]and everything you do.

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A 1983 neurobiological theory of dreaming,[br]called reverse learning,

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holds that while sleeping,[br]and mainly during REM sleep cycles,

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your neocortex reviews [br]these neural connections

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and dumps the unnecessary ones.

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Without this unlearning process,

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which results in your dreams,

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your brain could be overrun [br]by useless connections

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and parasitic thoughts could disrupt[br]the necessary thinking

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you need to do while you're awake.

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We dream to keep our brains working.

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The continual activation theory proposes[br]that your dreams result

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from your brain's need to constantly[br]consolidate and create long-term memories

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in order to function properly.

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So when external input falls[br]below a certain level,

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like when you're asleep,

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your brain automatically triggers

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the generation of data [br]from its memory storages,

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which appear to you in the form of[br]the thoughts and feelings

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you experience in your dreams.

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In other words,

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your dreams might be[br]a random screen saver your brain turns on

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so it doesn't completely shut down.

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We dream to rehearse.

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Dreams involving dangerous and threatening[br]situations are very common,

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and the primitive instinct [br]rehearsal theory

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holds that the content of a dream[br]is significant to its purpose.

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Whether it's an anxiety-filled night of[br]being chased through the woods by a bear

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or fighting off a ninja in a dark alley,

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these dreams allow you to practice[br]your fight or flight instincts

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and keep them sharp and dependable[br]in case you'll need them in real life.

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But it doesn't always have [br]to be unpleasant.

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For instance, dreams [br]about your attractive neighbor

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could actually give your reproductive[br]instinct some practice, too.

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We dream to heal.

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Stress neurotransmitters in the brain[br]are much less active

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during the REM stage of sleep,

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even during dreams [br]of traumatic experiences,

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leading some researchers to theorize

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that one purpose of dreaming is to take[br]the edge off painful experiences

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to allow for psychological healing.

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Reviewing traumatic events[br]in your dreams with less mental stress

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may grant you a clearer perspective

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and enhanced ability to process them[br]in psychologically healthy ways.

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People with certain mood disorders[br]and PTSD often have difficulty sleeping,

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leading some scientists to believe[br]that lack of dreaming

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may be a contributing factor[br]to their illnesses.

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We dream to solve problems.

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Unconstrained by reality[br]and the rules of conventional logic,

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in your dreams, your mind can create[br]limitless scenarios

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to help you grasp problems

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and formulate solutions[br]that you may not consider while awake.

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John Steinbeck called it [br]the committee of sleep,

0:04:51.527,0:04:53.024
and researcher has demonstrated

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the effectiveness of dreaming [br]on problem solving.

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It's also how renowned chemist [br]August Kekule

0:04:58.958,0:05:01.879
discovered the structure [br]of the benzene molecule,

0:05:01.879,0:05:05.180
and it's the reason that sometimes [br]the best solution for a problem

0:05:05.180,0:05:06.920
is to sleep on it.

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And those are just a few of the more[br]prominent theories.

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As technology increases our capability[br]for understanding the brain,

0:05:13.856,0:05:15.497
it's possible that one day

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we will discover [br]the definitive reason for them.

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But until that time arrives,[br]we'll just have to keep on dreaming.