A dream is a collection of images and ideas that occur involuntarily during certain periods of repose. When you first drift off, your heart rate slows, your temperature drops, and your brain is busy processing the day’s events. During this initial sleep stage, dreams are made up of flashes of thoughts and images from your waking life: what you ate for lunch, a phone call you made during the day, the movie you watched before bed. You rarely remember these dreams unless you wake up during them.

After about 90 minutes, you fall into the rapid eye movement (REM) stage of sleep, where vivid, often surreal dreams occur. The amygdala, the area of the brain responsible for processing emotions, and the hippocampus, the seat of memory, are both active, which is why REM dreams have a story-like quality and are the ones you tend to remember the next day and recount to friends. If you get six to eight hours of sleep, you experience four to five REM periods of various lengths, all of which are dream filled (though you probably won’t remember most of them).

What Purpose Do Dreams Serve?
The topic is still hotly contested, but the leading position holds that dreams “help us process new, emotionally important information and add it to our conceptual memory system,” says Rosalind Cartwright, a psychologist and the founder of the Sleep Disorder Service and Research Center at Rush University Medical Center, in Chicago. Once the information is in your memory, it influences your waking behavior and decisions. For instance, research has revealed that dreams can:

- Help you understand new experiences. REM dreams link new events to old ones, putting them in context. For example, if you’re feeling anxious about your job, you may dream about another anxious time, like when you were taking a test in college. 

Indeed, when scientists do brain scans on subjects during REM sleep, they find that the visual center of the brain, the dominant area that processes all the new information people encounter while awake, is shut down. The visual memory center, though―the part of the brain that stores images from the past, like what your childhood bedroom looked like―is in overdrive. This indicates that all the images we “see” during our dreams are being pulled from our memories, says Linden, who is also the author of The Accidental Mind: How Brain Evolution Has Given Us Love, Memory, Dreams, and God.

- Prepare you for change. Dreams can be a rehearsal for new challenges. When a person in love dreams about weddings or an athlete dreams about competitions, this helps the dreamer mentally prepare for the future. Says Cartwright, “Your brain is taking this ‘emotionally hot’ material and helping you process it so that you can better deal with it when you’re awake.”

- Help you cope with trauma or loss. Cartwright studied people going through divorces and found that those who were the most depressed in their waking lives had the flattest, least emotional dreams, while those who were managing well had highly expressive, furious dreams, complete with scenes of throwing objects at their soon-to-be exes. 

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Does Everyone Dream?
All humans dream. (And as most mammals and birds experience REM sleep, it’s presumed that they do, too.) When people say, “I don’t dream,” they’re really telling you that they don’t remember their dreams. Remembering is easier if you wake up in the middle of a dream or almost immediately afterward. Consequently, light sleepers, who are apt to wake up frequently during REM sleep, generally have better recall than their sound-asleep bedmates. Remembering is also easier when you awaken naturally, like on the weekend or during a vacation. The jolt of an alarm clock, on the other hand, is liable to make your thoughts jump abruptly from a fantasy dream to a nagging to-do list.

Why Are Certain Dreams So Common?
Falling from a cliff. Being chased. Flying. These themes persist across cultures and generations. “There’s folklore from almost every civilization showing that we all dream about these things,” says Cartwright. The predominant anxiety-dream themes express the fear of feeling humiliated, losing one’s beauty, not being desired, or not having the capabilities to succeed at something.

What Are Nightmares and Recurring Dreams?
Intense, frightening dreams are common and normal, says Deirdre Barrett, a clinical assistant professor of psychology at Harvard Medical School and the editor in chief of the scientific journal Dreaming, as long as you don’t have them every night. Nightmares are often brought on by a real-life event, such as moving to a new place, or a trauma, such as being the victim of a crime. These dreams, too, can be instructive. 

Children tend to have more nightmares than adults do. “They haven’t developed the psychological tools to deal with emotions, so they’re more likely to feel overpowered by them,” says Cartwright. “They have a lot of nightmares about animals and monsters. This could be symbolic of all the big things they don’t yet understand.”

Why Are Dreams So Weird?
There’s a biological reason. The prefrontal cortex of the brain, responsible for logic and reasoning, is inactive during sleep, thus allowing all sorts of crazy images to evolve uncensored. And as your dreams are linking new memories to old ones, those associations often turn out to be a little kooky. 

Hormonal changes, like those during pregnancy or the postpartum period, can also make your dreams crazy, as can some antihistamines and most antidepressants. (Both can be sedating, altering sleep patterns, and antidepressants change the brain’s chemistry). A fever can also affect sleep and dreams, as can a stomachache, which is why some people believe spicy foods or eating right before bed causes wild dreams.

Can Dreams Be Predictive?
Perhaps. According to legend, Abraham Lincoln told his wife that he dreamed he had been assassinated just days before he was killed. But no one really knows whether dreams can act as crystal balls, because it’s nearly impossible to study the phenomenon. Researchers would need to track thousands of people’s dream journals for many years to come to any conclusions. “My guess is that the majority of ‘predictive’ dreams are purely coincidental,” says Delaney. “Without evidence, it’s impossible to say for certain. But I’d be willing to leave the door open to the possibility.”

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