Just Woke Up That Way

"Spike:...why the hell are you suddenly a Fyarl demon? You just come over up all demon-y this morning?

Demon-Giles: As a matter of fact, I did. Thanks to Ethan Rayne."

- Buffy the Vampire Slayer

It's morning; time for you to wake-up. You're still groggy, but you manage to roll out of bed, same as always. Maybe you feel weird; maybe your bedroom seems out of place somehow, but you're still too tired to think about it. So you stumble to the bathroom, splash some water in your face, blink the sleep from your eyes, look at yourself in the mirror and...

GASP!

It's not your face in the mirror! At least, not the face you had last night. Maybe you've switched sexes; maybe you've become the one thing you hate most; maybe you've switched bodies with someone or turned into a giant insect, but whatever it is, you're not you anymore. You have been transformed without your knowledge or consent and it happened while you were sleeping. You don't know how or why it happened; you Just Woke Up That Way!

This is extremely common in stories about involuntary shapeshifting, particularly if the writers are trying to shock the audience. In situations where the source of the transformation is not rapidly explained, this overlaps with Ontological Mystery.

Film

 * Freaky Friday, after the characters switch bodies.
 * Big
 * Thirteen Going On Thirty

Literature

 * The canonical example is probably Gregor Samsa in Franz Kafka's The Metamorphosis, who wakes-up one morning transformed into a giant insect.
 * In Orlando: A Biography, the title character's Gender Bender takes place in just such a way, with the notable difference that she is not in the slightest perturbed by it.
 * The origin of George Shrinks in the original books. In the animated series he was born that way.
 * In the Animorphs book The Familiar, Jake wakes up as an adult.
 * In A Midsummer Nights Dream the clown Bottom wakes up with a donkey head. He takes it surprisingly well.

Live-Action Television

 * The episode Face of the Enemy on Star Trek the Next Generation opens with Counselor Troi getting out of bed to find that she's unaccountably become a Romulan.
 * Similarly, the Deep Space Nine episode Second Skin sees Major Kira wake up to find that she's a Cardassian.
 * One episode of Buffy the Vampire Slayer, A New Man, has this happen to Giles when he gets turned into a demon by Ethan Rayne.
 * On Are You Afraid of the Dark, the main character in The Tale of the Hunted rolls out of bed one morning as a husky wolf.
 * In The Prisoner episode "The Schizoid Man", Number Six wakes up to find himself with a new appearance. He is greeted as "Number Twelve" and given the assignment of impersonating Number Six (who has been replaced with an actual impersonator) as part of Number Two's latest head-gaming scheme.

Music

 * The King Missile song "Detachable Penis" starts with the lead singer realizing that his penis had disappeared.

Newspaper Comics

 * A storyarc in FoxTrot parodying The Metamorphosis features a sequence like this after Jason turns into a girl.

Webcomics

 * Misfile starts with Ash waking up with breasts that weren't there yesterday.

Western Animation

 * In Avatar: The Last Airbender, Zuko allowed that avatar's bison to go free and seems set up for a Heel Face Turn. His body suffers a dramatic fever in the process; after collapsing and spending days in bed, he wakes up, goes to the mirror, and discovers his scar gone, his hair shaven and covered in air-bending tattoos.
 * This is the premise behind the French animated series Martin Morning (Martin Matin), as Once an Episode, he wakes up in his bed in the morning to find himself turned into a fantastic and/or legendary being and is forced to go through his day like this, ie: One day he's a wizard, the other day he's a pharaoh, and another day he's a vampire.