Today, scientists still aren't exactly sure why we have dreams, but a number of theories have emerged. One idea is that we dream to "practice" responding to problems. This is called "threat simulation theory" and might explain why our dreams so often involve stressful events such as forgetting your homework. Scientists believe that your brain might be preparing for how it would respond if you really do forget your homework. These scientists point out that humans aren't the only animals who dream; if you've ever seen a dog moving its legs while it's sleeping, it's probably dreaming about running.
Another theory says that dreams play an important role in keeping people asleep. Scientists have long known that sleep produces a number of benefits for people's moods and long-term health. Dreams might allow people to get the sleep they need, theorizes South African scientist Mark Solms. He compares dreams to Saturday morning cartoons; your dreams keep your mind entertained so that the brain can do important work in the background — just like how cartoons keep small children entertained so that their parents are free to complete work around the house.

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