Don't Wake the Sleeper

The Sleeper is asleep. And must remain so. Waking him up would be the worst idea ever, and for that reason there might even be some powerful guardians trying to keep you from waking him up.

The Sleeper might have a name and/or proper title. But if so, he is in many cases still referred to as "The Sleeper" or "The Dreamer" rather than by name or title. Different works have very different reasons why The Sleeper must remain asleep, but the four major types of reasons are:
 * The Sleeper is dangerous and will cause havoc if he wakes up. In effect, this is Sealed Evil in A Can, with a pillow instead of a canister.
 * The Sleeper is doing something very important in his dreams: Protecting something, defending the world from something, maybe even dreaming a world into existence.
 * You should wait until he is needed.
 * Waking up would be traumatic. Often about a sleepwalker.

If the Sleeper does wake up, a Dream Apocalypse might occur (even in the real world, if the sleeper in question is a Reality Warper).

Anime and Manga

 * The Kisin  in Soul Eater.
 * In one of Dragon Ball Insanity School chapters, Goku is sleeping and the rest of the main cast try to take a magazine from his clutch without waking him up. When this happens he's so angry that blast them all away with a kamehameha.

Comic Books

 * The fantasy adventure heroine Zethari once encountered a temple protecting a man who was eternally asleep, dreaming up the universe. She is hired to protect the temple from a villain who is trying to end the world by waking him up.
 * Near the end of Bone, Fone, Thorn, and Bartleby find that the valley they have to pass through is blocked by Roque Ja, sleeping. They've no idea whose side Roque Ja is on, and they know they can't beat him in a fight, so they sneak past without waking him.
 * In one Donald Duck comic, Donald and his nephews have to stop the villain from waking up a Chtulhu-like monster, as the world as we know it is controlled by the monster's dreams. Of course, it wakes up, and while it's awake, the Ducks' bodies mutilate horribly. Donald manages to make it fall asleep again by singing a lullaby for it.

Film

 * In Pans Labyrinth, this is the secret objective of one of the quests given. The protagonist is not told that there is a sleeper, or that horrible things will happen if she wake him up.

Literature

 * Ur-Example: Kumbakarna, from the ancient Indian epic Ramayana. He was a giant demon prince who was cursed by the god Indra to sleep for six months of each year, and was cursed with death if he was woken during that sleep. His brother, the demon-king Ravana, woke him so he could help turn the tide of a decisive battle. It was working for a while, but then guess what happened.
 * In Steven Erickson's Malazan series, the whole world is a sleeping goddess, Burn, whom it would be A REALLY bad idea to wake.
 * This is a discussed trope in Through the Looking Glass. Alice and another character (the White Knight?) talk about the Red King sleeping under a tree. The Knight supposes that he's dreaming their world. I'll let someone with a more recent memory of the book add it as an example, since I'm not sure I have the details correct.
 * In Jinx High by Mercedes Lackey, There's a Dangerous Sleeper under Tulsa, Oklahoma.
 * In Lord Dunsany's "The Gods of Pegana", the gods created the world, but created the gods, and from this task he now rests, lulled by the endless drumming of Skarl the Drummer (who is technically not a god, despite having been created along with the gods). If Skarl were to cease drumming even for an instant,  would wake up, and his awakening would destroy the world and the gods.
 * Jody Lynn Nye's "Waking in Dreamland" and its sequels are set in the world of dreams. All dreams contribute to the setting, but Seven Sleepers give it structure and coherence. When one of these Sleepers wakes up, there is a cataclysmic event called a Changeover in which some other dreamer's vision replaces the previous one. The Big Bad of the first novel wants to find out what happens if they all wake up at once.
 * The first Guardians of the Flame novel, The Sleeping Dragon by Joel Rosenberg has the titular creature, which guards the way back into the characters' reality.

Music

 * Tarsk Tavern has the song "Sleeper", based on the Ever Quest raid dungeon Sleeper's Tomb, urging the listener to abstain from waking him up.

Mythology

 * Later addendums to the Arthurian myth have Arthur and the knights of the Round Table, hidden in a cave and magically kept asleep, only to be awoken when he is truly needed. Waking him up early would be a very bad plan indeed (although in that case, one wonders why Merlin apparently included a giant gong in the cave.).
 * A running theme in the Cthulhu Mythos, with mad cults trying to wake up Great Cthulhu. Their success would be a very bad thing.
 * In the Tain Bo Cuailnge, Fergus notes that nobody dares to wake Cu Chulainn when he's asleep - the last man who tried found his forehead smashed all the way into the back of his skull.

Tabletop Games

 * The 1992 board game Don't Wake Daddy. We'll give you one guess as to how you lose... ;)
 * In the board game Dungeon Quest (original title: "Drakborgen"), the goal is to reach the Dragon's hoard and steal as much as possible from it. But if you steal so much that you wake up the dragon, you die.

Toys

 * Bionicle: Makuta Teridax used a virus to weaken Mata Nui to the point of causing the latter to sleep and cause the Great Cataclysm. A very long time later, Mata Nui is awakened, only to also cause him to slowly dying. Again he is put back to sleep and revitalized until the state of matters improves for him to be reawakened.

Video Games

 * World of Warcraft has the instance Sunken Temple, where the final boss is asleep and his underlings are fighting to keep you from waking him up. He was originally a guardian, his dreams protecting the world from an evil God - who managed to corrupt him and twist his dreams into nightmares.
 * Ever Quest had The Sleeper--or rather, his guardians--as one of the major bosses of the Scars Of Velious expansion. Waking him up results in destroying his dungeon permanently, robbing the community of an important source of loot. The Sleeper himself comes back in a later expansion.
 * The Legend of Zelda Links Awakening has the Wind Fish who is dreaming the entire island that the game takes place on, all the dungeon bosses try to stop you from waking it up and try to warn you about what you're doing. Ultimately, it turns out that.
 * The Milkman Conspiracy segment of Psychonauts revolves largely about the Driving Question of "Who is the Milkman?" and various forces preventing Raz from waking him up.
 * In Final Fantasy X, the summonable "aeons" are each the dream of a "fayth", who sleeps in that aeon's temple. There are also a whole bunch of fayth together on a holy mountain, dreaming something, but nobody knows what.
 * In Super Mario 64, you must walk slowly around sleeping Piranha Plant enemies otherwise they will wake and attack you.
 * A level in Super Scribblenauts had an objective to sneak past a sleeping dragon to get a key. Naturally, the path is filled with chandeliers and piles of junk that will shift and wake the dragon if you so much as touch them.
 * Gothic has the "Sleeper" as a deity worshipped by the cult that inhabits an entire camp in the prison colony. The members of the cult believe that if the Sleeper is woken up, he will free them from the colony.
 * The Adventures of Alice who Went Through the Looking-Glass and Came Back Though Not Much Changed, an Interactive Fiction game based on Lewis Carroll's Alice books, includes a version of the scene with the sleeping king who may be dreaming the world. If the player wakes him up, everything disappears and there is a Nonstandard Game Over.
 * Banjo-Tooie has two instances in the first world: a sleeping snake and a sleeping caveman, both of whom need to be approached quietly.
 * The Gauntlet-clone Demon Stalkers has an entire enemy type devoted to this: man-eating plants that don't move until you shoot one of them, at which point they all come alive at once.

Web Comics

 * In one Xkcd strip, Cueball accidentally wakes up the Sheeple from its 10,000 years long slumber, prompting everyone to run for their lives.

Web Original

 * The UA Newsletter has a series of articles by Alex D. Karaczun, supposedly about creating a setting and a plot, but the articles seem to mostly be an excuse to showcase Alex's invented setting about the sleeping god "Primion", whose dream is the entire setting of Rothon, and it's unclear if waking him up would cause a Dream Apocalypse or make Rothon a real place. (The articles: Part I, Part II, Part III.)

Western Animation

 * There's a Tweety and Sylvester cartoon in which Sylvester has to get past dozens of angry guard dogs to get to Tweety. At the end he tries to sneak in at night when they're all asleep, but then Tweety turns on the alarm clock.
 * Same gag is used in the Looney Tunes short "Roman Legion-Hare", this time with Bugs Bunny, Yosemite Sam and a cage full of lions.
 * Another Looney Tunes short, "A Pest in the House", involves a tired hotel guest asking manager Elmer Fudd for peace and quiet as he sleeps in his room, and threatening to punch Elmer in the nose otherwise. His slumber is constantly disturbed by the blundering of bellboy Daffy Duck, with the expected results.
 * The main plot for the Tom and Jerry short "Quiet Please!", in which Spike tires of Tom's racket in trying to catch Jerry, threatening violence if Tom wakes him up one more time. Tom immediately has to sabotage Jerry's vigorous attempts to wake Spike. A later short, "Royal Cat-Nap", replays this scenario with the Mouseketeers, Tom having to prevent them from waking the king he is guarding. They relent and help Tom get the king back to sleep when they realise the penalty is a beheading, however.
 * In Toy Story, Woody and Buzz try to sneak past Sid's dog Scud. Then Woody's pull string is caught and his voice box wakes Scud up.
 * A villain in Teen Titans, Plasmus, is a guy who has to be constantly asleep or he will become a mindless purple goo monster. (I really don't know how to word this)
 * The Chilly Willy short "The Legend of Rock-A-Bye Point" is half this and half Music Soothes the Savage Beast.
 * This trope figures into The Fox and The Hound, the scene where Todd is poking around Chief when Chief is asleep in his barrel-house.