Dreaming the Truth

The character goes to bed and dreams. Often intensely symbolically. Wakes up either having pieced together all the clues he had before but hadn't seen the significance of, or having made a decision that troubled him.

Can also occur in delirum and hallucinations.

These are not prophetic or psychic dreams, or having it communicated to him; he had all the information in hand, he just hadn't put it together. Though some dreamers assume they are dreaming the truth for prophetic or communicating dreams. Seldom if ever Recurring Dreams, except where they overlap with Bad Dreams, especially if the character is suffering Trauma-Induced Amnesia.

Anime & Manga

 * At the end of Monster, Tenma has a dream (or not) of Johan revealing to him what it was about his childhood that plagued him the most.
 * Happened to Lupin III once; Fujiko left Lupin a few cryptic negatives with strange lines and symbols that belong to Pycal/Piker, a menacing magician. Lupin had already figured out his fire-shooting and levitation tricks but is completely clueless about how he becomes invulnerable. Later, however, Lupin accidentally falls out the window and the resulting blow knocks him into some sort of dream where the negatives are projected over each other, revealing

Literature

 * In Madeleine L'Engle's The Other Side of the Sun, Stella (an Englishwoman) goes to live with her husband's father in the Deep South shortly after the American Civil War. After a dream involving fireflies and her husband metamorphizing into a (black) man she had met there, she wakes to the realization that her husband and this man are half-brothers.
 * Used in Twilight after Bella learns Edward is a vampire. Even though Dream-Edward is attempting to lead her to her doom, Bella realizes she doesn't care if Edward's a vampire as she's more upset that Dream-Edward is about to be attacked. The same dream with a slight twist is used in the sequel New Moon so she can realize
 * In Graham McNeill's Warhammer 40,000 Horus Heresy novel Fulgrim, Fulgrim hears voices nagging at him every night. He convinces himself that it's his subconscious.
 * In Graham McNeill's Fulgrim, Serena d'Angelus forgot that she was a murderer until her Bad Dreams made her discover the bodies. (Leading to her suicide.)
 * In Graham McNeill's Warhammer 40,000 Ultramarines novel The Killing Ground, the Lord of the Unfleshed remembers deliberately putting himself somewhere where fumes would make him dream deeply, and remember his past.
 * In James Swallow's Warhammer 40,000 Blood Angels novel Deus Sanguinius, Arkio had dreams in which his glorious triumph was tainted by something evil. He prays for guidance as to what it means.
 * Inverted in The Second Chronicles of Amber. In Sign of Chaos, Merlin muses (I don't have the exact text on me, so I'm paraphrasing) "Someone who had been through all the truly bizarre crap I had over the past few days should have had a revelatory dream, waking up with new insights as to how to deal with their problems. Me? I woke up in the middle of the night and realized that my feet hurt.
 * Buttercup has dreams in The Princess Bride which upset her enough to make her want to call off her wedding to Prince Humperdinck. The film gives her just the one, but in the book she has bad dreams for a good week.
 * In Felidae, Francis the cat has a highly disturbing dream that symbolises the motive behind the murders he's investigating. He still has to do some more conventional investigation to work out what the dream means, though.
 * In John C. Wright's Count to a Trillion, Menelaus, owing to his mental modifications, can consciously think while dreaming—which still leads to this.
 * In Barbara Hambly's Ran Away, Benjamin January dreams of his dead wife asking where Sabid is—which causes him to consider whether Sabid might actually be in New Orleans, making trouble again for the same man he attacked years ago.
 * In Thief of Time Jeremy clockson has this happen while working on the glass clock. He wakes up to find his sheets and wall covered in diagrams and notes.
 * In Michael Flynn's Up Jim River, Donovan, in his drugged sleep, realizes a few things (on top of getting his soul pieced back together).

Live-Action Film

 * Woody Allen's Match Point
 * Vertigo
 * In The Princess Bride, Buttercup declares she will not marry Humperdinck after dreaming of the Ancient Booer, who calls her garbage for setting aside true love in favor of becoming Queen.
 * The 2004 remake of The Manchurian Candidate makes use of this trope.
 * The Incredibles originally was to include a Dream Sequence where Helen's worries about Bob took shape. This was one of the first things cut from the plot partly because it was too lazy. It's discussed on the DVD extras, though.

Live-Action TV

 * Twin Peaks—Agent Cooper has an iconic dream early on in which the victim whispers the name of her killer. He immediately forgets it, however, and it's a while before he is able to remember.
 * In an episode of House, as I recall. The not-so-good doctor had a dream which gave him the key to resolving his latest case (it also involved him wearing a catheter which ruptured, but that had more to do with his own issues at the time).
 * They've used it again. Although in this case, you can't be sure if he just hadn't put the facts together or if he was actively in denial.
 * In The Sopranos, Tony had a dream where he was forced to face that he knew one of his underlings was wearing a wire.
 * In the same series, Dr. Melfi has a dream which leads her to realize that she can, if she wants to, have Tony punish her rapist - though she chooses not to.
 * In Battlestar Galactica whilst near death, Laura Roslin hallucinates and dredges up a forgotten memory of seeing . Though she met both later, she didn't until that moment connect the two in her head.
 * This happened in an episode of Medium. A Texas Ranger was troubled by dreams that had only recently started to become clear... and then the things he was dreaming started happening (a pair of EMTs killed and dumped where he dreamed they would be, and so on). It later turned out that he truly wasn't having prophetic dreams; he'd spent some time comatose in a hospital, and while in that state had overheard someone making plans that later presented themselves as dreams as his mind made sense of the information.
 * The Doctor Who 2-parter "Human Nature/The Family of Blood" has the temporarily-human/amnesiac Doctor/John Smith dreaming about his Time Lord adventures and recording them in his Journal of Impossible Things.
 * In the Series Five episode "Amy's Choice", the Eleventh Doctor's feelings are revealed when psychic pollen gets stuck in the TARDIS time rotor, heats up, and induces a dream state for Amy, Rory, and the Doctor. The pollen latches on to the Doctor's massive amounts of darkness and reveals the difficult truths about Eleven's character to Amy and Rory in two dreams.
 * The Pushing Daisies episode "Bitches" had Emerson to discern a clue he hadn't noticed before which allowed him to deduce that the dog the case revolved around was still alive. This in the same episode in which he told off Ned for treating a dream as anything other than random images.

Video Games

 * In Jays Journey, Gaia has a dream where everyone in the dream (save Gaia herself) repeats something said earlier in the game, which lets Gaia see how selfish she's been, and why Carol's been acting so cold to the man she previously loved. And to get an opportunity to repeat all the game's Running Gags in quick succession.

Web Comics

 * In Westward, an Intrepid Reporter who was once captured and tortured by communists starts having strange dreams about the experience many years later that don't match his conscious memories. Eventually he confirms that his dreams reflect reality, and this leads him to stumble upon a mystery of potentially cosmic proportions.
 * In Sinfest, Slick dreams some marvelous truths -- which he forgets, like dreams, after being awake a minute.

Western Animation

 * Happened a few times on The Simpsons. Twin Peaks was even explicitly parodied at one point during one of these.

Real Life
"Hogamus Higamus Men are Polygamous ''Higamus Hogamus Women Monogamous"
 * The practice of "sleeping on a problem" is common practice in any field where mental roadblocks are commonplace. Often, forcing yourself to sleep will at least help you be more refreshed and ready to tackle the problem later; also, when you're asleep, your subconscious mind is free to make wilder leaps of logic without being bothered by the part of your brain that determines whether or not a particular idea makes sense.
 * The German chemist Friedrich August Kekule was trying to figure out the structure of the compound benzene. He had a daydream about a snake biting its own tail. When he woke up, he realized that the benzene molecule was in the shape of a ring, and this was later confirmed. The Other Wiki's take on it, and Mr. Kekule's own description of the experience.
 * Subverted by Otto Loewi, a German pharmacologist who earned the Nobel prize for his discovery of of the neurotransmitter acetylcholine. One night in the spring of 1921, he awoke with the sudden realization that he knew how the the brain's electrical signals traveled along synapses. He hurriedly wrote down his revelation and went back to sleep, only to discover in the morning that, to his horror, he could not read what he had written down.
 * Fortunately, he had the same dream the following night. Taking no chances this time, he got up and went right to the lab (where his experiments confirmed his idea).
 * There is a legend that Dmitri Mendeleev had seen the periodic table of the elements in dream.
 * A famous anecdote from the middle 20th century plays on this trope. While it appeared in many places and was attributed to many individuals (including prominent psychologist William James), the earliest known version appeared in the Cleveland Plain Dealer newspaper in November 1939. According to the article “Thanksgiving Nightmare” by Claire MacMurray, a Mrs. Amos Pinchot had a dream in which she had achieved a profound insight into the ultimate of truth of life and distilled it down into a poem of ineffable beauty and wisdom.  At the end of the dream she half-woke and scribbled down the poem so as not to lose it.  In the morning she had forgotten all but the broadest outline of the dream, and eagerly read what she'd written in the night: