Maybe you were late for class, or flying, or you were in the middle of a long journey. Dreams can be fun, scary, or just plain strange—you never know what you’re going to get when you go to sleep. There are several things we do know about how we dream, but the actual purpose of dreams is still a mystery. It's one of the universal truths that we still haven't cracked. Here’s everything you need to know about the nightly movies in your head.

What are dreams?

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Dreams are the images, sounds, thoughts, voices, and sensations that you experience when you sleep. They’re often drawn from your real-life experiences but will most like have a fantasy quality to them. Everything that you dream — whether it’s people’s faces, sounds, or places — you have somehow experienced in your real, awake life. Nothing is invented. Sure, sometimes you might see something wild like a car with crab claws instead of tires, but maybe you happened to notice a nice Beemer in that shade of red you like in the parking lot of the seafood restaurant you went to for dinner. Your brain just took those memories and images and mashed them together for whatever reason.

When do we dream?

Dreaming occurs during the deepest stage of sleep called the rapid eye movement stage, or REM. The average person will go through an REM stage multiple times during a night and will have between four and six dreams in one sleeping session.

As the night goes on, dreams become longer. Your first dream might last only a few minutes, but in the later REM stages, it might last up to a half-hour. It’s been shown, though, that most people don’t remember up to 99 percent of their dreams.

Forgetting your dreams

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Nobody is quite sure why dreams are so easily forgotten. One theory suggests that when we sleep, production of a memory hormone called norepinephrine is greatly decreased. Without norepinephrine, it’s impossible to make new memories. Other researchers think that your brain simply isn’t paying attention. You remember only the crazy dreams because they’re interesting enough for your subconscious to listen to them.

If you want to remember more of your dreams, you have to train yourself to do so. Think about dreaming, and remembering, before you fall asleep. Eventually it’ll become ingrained in your mind. Keeping a dream journal is another way to help with dream retention. Many times, you forget your dreams within minutes of waking up. If you write down key aspects of your dream the second you open your eyes, you’ll train your brain to be able to remember in the future.

What happens when we dream?

During REM, a person's heart rate and breathing become inconsistent, their muscles are paralyzed (called sleep paralysis) and, of course, their eyes move rapidly, hence the name. While that might sound scary, it's essentially your body's defense mechanism: it’s designed so that when you’re moving around in your dream, you don’t actually jump off of your bed while soaring over the Grand Canyon. It might not hurt if you crash in dreamland, but waking up to a face plant certainly will.

While sleep paralysis is totally normal, some people wake up before it has completely worn off, which makes for a terrifying experience.

Why do we dream?

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There are plenty of theories about why people dream, but the true reason is still a mystery. It’s believed that dreams help with problem solving, recording memories, and processing emotions. Dreams have a way of laying out problems in a clearer format than in real life. Many psychologists, especially those who follow the teachings of Swiss psychiatrist Carl Jung, believe that our unconscious mind knows more than our conscious one, and dreams are the way the unconscious sends messages to the conscious mind. Jung called this process individuation.

There have been studies about the importance of dreaming in which researchers woke up some of the subjects just before they went into REM sleep. People who didn’t get to a dream state showed more tension, anxiety, depression, weight gain, lack of coordination, and difficulty concentrating versus the people who were allowed to experience their full dream cycle. Experts aren’t quite sure why we have to dream, but they do know it’s important.

Lucid dreams

For the most part, dreams are like movies. You can yell at the protagonist on the screen all you want, but they're never going to do what you say. But in some cases, you can realize that you’re in a dream and influence it directly. These are called lucid dreams. The influence can range from changing the story, speaking to other characters, or even altering the world entirely. Since it’s all in your head, there’s no limit to what you can do!