No one knows for sure why you dream.

"That's the one part of sleep medicine we know the least about," says Charles Bae, a sleep medicine specialist at the Cleveland Clinic. "I think dreams help people process the multiple kinds of sensory input that come in through the day." Some people report experiencing eureka moments during dreams.

In her book about Lyndon Johnson, biographer Doris Kearns Goodwin wrote that LBJ dreamed he had a stroke and became paralyzed — and then a few months later chose not to run for president in 1968. "He had made a decision in his dream," says Myron Glucksman, author of Dreaming: An Opportunity for Change, and psychiatrist in New York City and Redding, Connecticut. "Dreams are like an internal diary. They're a nightly commentary on your life."

You dream throughout the night, not just during REM sleep.

Forget what you heard in college about dreams only occurring during REM sleep. You can remember stories from throughout the night, though not all are created equal. REM-sleep dreams, which are more common in the second half of the night, tend to seem more vivid and unrealistic. "If you dream you jumped out of a plane, and you saw rockets around you, that's almost certainly a REM sleep dream," says Jerry Siegel, professor of psychiatry and director of the Center for Sleep Research at UCLA. Dreams during the first three (of the four) stages of sleep may seem more mundane.

You remember a dream if you awake during it.

"The primary determinant of whether you'll remember a dream is being awakened during the dream," says Mark Mahowald, professor of neurology at the University of Minnesota Medical School and visiting professor of psychiatry and behavioral medicine at Stanford University. "If you don't wake up during the dream, the memory is gone. We're on a self-erasing tape while we're asleep."

Spicy foods may make you remember more dreams and nightmares.

"The meal makes it more likely you're going to wake up during sleep," says Mahowald. "The heavy meal has nothing to do with dream generation. It has to do with dream recall." In order to recall a dream, you have to be awake, at least for a few minutes. "Our brain isn't able to convert from short-term to long-term memory while we're asleep," says psychologist Lisa Medalie, a behavioral sleep specialist at the University of Chicago.

You may be able to change bad dreams.

Many therapists believe it's possible to "re-write" nightmares. People who suffer from post-traumatic stress disorder, for instance, can train themselves to recognize when they're in a dream. They tell themselves, "This is only a dream," says sleep researcher Ursula Voss, a psychology professor at the University of Frankfurt. Some of her patients came up with an idea that works: They make a bracelet that they wear to sleep. "If the bracelet is not in the dream, they know it's a dream," she says. They look out for "something that's bizarre" and then try to shift the direction of the dream.

Shelby Harris, director of behavioral sleep medicine at Montefiore Medical Center in New York City, uses "imagery rehearsal therapy" with her patients. She encourages them to clearly envision a new scenario when dreams take a bad turn. One patient kept dreaming she was surrounded by sharks and was starting to drown. "She just changed the sharks to dolphins," says Harris. "We wrote out a whole new storyline."