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Voiceover: Do you remember your last dream? Maybe it was really nice, maybe scary, maybe just a little weird. Maybe you don't even remember dreaming. Everybody does dream, [bearing] any brain damage and you dream every night during REM sleep. Sometimes you dream during non-REM sleep but those dreams aren't as vivid or memorable. You can usually tell that someone's dreaming because their eyes are moving rapidly underneath their closed lids. If you were to look at an EEG of their brain it would look almost like they're awake. Even though their body is completely non-responsive and mostly paralyzed. Most dreams last about five to 20 minutes at a time and they don't seem to be localized in any one part of the brain. If you remember your dreams when you wake up, you might remember that some pretty strange things happened even though they probably didn't seem that weird when you were in the dream. One reason that we don't realize how strange dreams are until we wake up is that during REM sleep, activity in our prefrontal cortex is decreased. That's the part of the brain responsible for logical thinking and planning, so if it's not very active then we wouldn't be aware of things in our dreams that defy logic like fooling around, or animals talking to you, or something. No one really knows why dreams occur but there are plenty of different theories. The most popularized one is Sigmund Freud's idea that dreams are unconscious thoughts and desires that need to be interpreted. There's very little science to really support this idea. Even within evolutionary psychology there are lot of other theories about why people dream. Some evolutionary psychologist think that it allows us to simulate threats so that we're better prepared for them in the real world. Some think that it helps us solve problems by thinking about them in an altered biochemical state and still other evolutionary psychologists think that dreaming is just a byproduct of our neural development and serve no real purpose at all. Even within one school of thought there are multiple theories about why dreams occur. You can imagine how many there are over all in all of psychology and all philosophy too. Just to give you an idea of some of the range of dream theories, I'll tell you about a couple others. Some psychologist think that the combination of conscious and unconscious elements that occurs during dreaming helps our brain maintain flexibility, enabling us to learn and be creative when we're awake. Still others think that dreaming helps our brain sort of clean up by sweeping away some thoughts and incorporating others into long term memory which is a process called consolidation. There definitely does seem to be some link between memory and sleep. People who learn something and then sleep tend to do better than people who learn and/or then deprived of sleep. The role of REM sleep and dreaming in particular is unclear. Another theory is that dreaming helps our brains repair and recuperate by preserving and developing neural pathways. One reason people think this might be true is that in infants who are constantly developing new neural networks and growing spend most of their time in the REM stage. As might be particularly helpful for them. Given the number of theories out there you should feel free to come up with your own ideas. Regardless of why dreams occur, we know that they do and it can be kind of fun, if a little scary to see what our brains come up with when we're not quite in control.