Naughty by Night

"''You know her by day But you don't know Karen by night"

- Jill Sobule, "Karen by Night"

Nina is a librarian. During the day, she wears long skirts, a loose blouse done all the way up to her neck, her hair in a bun and sensible shoes.

Get Nina home to her significant other, allow her to have a glass of something alcoholic and well... the blouse is on the floor, the skirt is in the hall and the hair is all over the place. Some other stuff is NSFW even to describe...

A Sub-Trope of Living a Double Life.

Compare Seemingly-Wholesome Fifties Girl.

Anime and Manga

 * A Hentai Manga take on this: Were Slut (unfortunate name created from whole cloth by Eros Comix, the US distributor) is about a shy college student named Kimiko who takes a magic beauty pill to make her attractive enough to win her childhood friend (read: she takes off her glasses). It works, but, due to the magic pill being past its expiration date, it keeps kicking in whenever she gets aroused. After the first time, though, it also kills all her inhibitions.
 * Ditto for the manga/anime Sprite: Between Two Worlds,wherein Manami, a severely repressed and meek college student turns into Nami, an outrageously uninhibited flirt whenever she's put into any situation that is too much for her to handle. The catalyst is her cousin Toru, who convinced Manami to be more confident around him when they were young by convincing her she was really Nami.
 * In the Ecchi manga My Balls, the main character's primary love interest Minayo is a sweet, kindly young woman who works at a daycare center. When she gets drunk, however, she goes into what the protagonist describes as "full-blown nympho mode". Unfortunately, said protagonist can't have sex for a month or the demon sealed inside his balls (hence the name of the manga) will be released and cause The End of the World as We Know It. Minayo happens to be an alcoholic, and Hilarity Ensues.
 * Paranoia Agent has an episode that plays with this. Harumi Chono is a quiet, conservative, mild-mannered school teacher by day, but at night her other personality takes over--Maria, a popular prostitute. When Harumi gets engaged, she starts to desperately get rid of Maria before her wedding day, which causes Maria to fight back.
 * Code Geass's Nina Einstein and the infamous table scene. Somebody actually walked in on this, but fortunately she was blind already.
 * And thank god she's (probably) too young to properly...interpret any sounds she might have heard.
 * Then again, said girl is Wise Beyond Their Years...
 * Paprika: Straight-laced Chiba becomes a lot more flirty and extroverted when she turns into.
 * In a way Nyamo-sensei of Azumanga Daioh. While she's generally straight-laced and down-to-earth in contrast to Yukari, the one who you'd expect to have a wild social life, Yukari makes a comment about her Nyamo "not being alone" in a previous summer, and Relax-O-Vision or not, the hilarious incident where she gets drunk and gives The Talk implies she's fairly experienced.
 * In the manga Crimson Spell the protagonist is under a strange curse that causes him to turn into a Demon that can only be vanquished by being raped into submission. Conveniently he never has any memory of this the next day.

Comic Books
""God, Woody, you didn't tell him about 'Cats', did you? It's not every girl who loses her virginity in a Broadway show... I'll bet I can get us some tickets. Meeoww.""
 * While it never came up in the comics, writer Mark Waid's character notes for the Fantastic Four speculate Susan Storm was like this as a young woman to cope with the stress of having to raise her little brother.
 * Aurora was like this in Alpha Flight's early years, exaggerated by her Split Personality. One persona was Jeannie-Marie: strict, stuffy schoolmarm teaching at the same girls' school she was raised in. And then there was Aurora: sultry, adventure-loving, free-spirited partier.
 * In Quantum and Woody, Amy Fishbein appears to be a prim and proper white-collar professional, but she starts dropping Anvilicious innuendos and undressing herself as soon as she's alone with Woody.

Film

 * The movie Belle de Jour is about a seemingly average housewife who's a prostitute in secret. But here this trope is inverted: as she is a housewife, she can actually do this job only by day instead of by night. (This is where the name of the film comes from.)
 * The red-headed librarian from Tomcats looks like a plain, uptight young lady. She (and her grandma!) are also into S&M.
 * This is a source of some confusion in the film Desperately Seeking Susan.
 * Somewhat subverted in the film Blind Date where Bruce Willis' sister-in-law tells him not to let the date get drunk as she goes wild. This information is relayed to him via his brother who adds lots of leering to the information. Cue the man getting his date,(Kim Basinger), drunk, to find that she goes wild in a drunk-driving, crash the car, ruin the evening way.

Literature

 * The Torchwood novel SkyPoint takes this to extremes; a very prim and proper little old lady, who is actually an S&M prostitute.
 * Almost every Cairheinan noblewoman in the Wheel of Time is heavily implied to fit this trope. Though during some festivals they will run around almost nude on the streets, especially during the Feast of Light (during which normal prohibitions against inter-class kissing (and other things) are lifted), leading to a large number of commoners having bragging contests about who made out with the most important noblewoman.
 * ONLY during the Feast of Lights, after which any such incident is ignored by all parties. With Cairhien being partially based on Versailles France, this roughly corresponds to Mardi Gras.
 * A relatively tame version occurs in Good Omens. Sweet little old lady and part-time medium Madame Tracy also offers appointments for "strict discipline and intimate massage every evening except Thursdays", and there are hints that she goes a little further than that on Thurdays.
 * In Saving Zoe, Teresa is a pretty preppy girl who acts and sings. By night, she hangs out with drug dealers and porno producers. The same could be said about Zoe, who seems like a perfect bubbly girl but smokes drugs and has sex when her parents don't know. This tendency leads to Zoe's death.

Live Action Television

 * On the Buses long-suffering Olive would love to be Naughty By Night, but her miserable husband Arthur would rather sleep. Actually, he'd probably rather do his taxes...
 * Effy from Skins: innocent schoolgirl by day, Fille Fatale by night.
 * Sheila from the US version of Shameless. By day she's a sweet agoraphobic and germaphobic housewife. But at night she is very very into pegging filthy alcoholic Frank.

Real Life

 * Ever hear the phrase "a lady on the streets and a freak in the sheets?"
 * In Mexico it is said that the perfect wife is "a saint at church, a lady at the table and a whore in bed"
 * It used to be "Paris in the bedroom, Boston in the parlor, implying Boston the classy one instead of Paris. Both cities have reputations for being on the up and up while simultaneously being WretchedHives.
 * Boston is supposed to uptight, prudish, and hypocritical, like the rest of America. Paris is European and casual. As Marlene Dietrich once put it, "In America sex is an obsession. In Europe it is a fact of life."
 * "Boston Brahmins" were prim, proper, and probably born knowing the Blue Laws, attributed (accurately or not) to their Puritan ancestors. There's also the expression "banned in Boston", implying something was especially racy. A Dead Horse Trope now, by and large, thanks in part to a huge influx of Irish and Italian immigrants, two very Catholic ethnicities that absolutely horrified the genteel "real" Bostonians (all those kids - what must have they been up to?).
 * A mangled variation in the Carolyn Mark song Honest Woman goes: "they want a whore in the kitchen, a wife in the bedroom, and a saint to live on in their minds."
 * Roman satirist Martial wrote "If you enjoy being a prude so much, be one all day long. but at night, I want you to be my high-priced hooker."
 * This was supposedly true for Messalina, Emperor Claudius's wife, who if Suetonius is to be believed, left the palace at night to work as a prostitute.
 * According to this Cracked article, on average women have more sexual partners than men.

Theatre

 * The play Miss Julie fits this with regard to personality, but replace "librarian" with "aristocrat".
 * Fantine in Les Misérables is fired from her factory job when she is discovered to have a child, and her fellow workers claim that she is a prostitute. However, she had only ever had one lover, who had abandoned her, and it is only after she is fired that she has to become a prostitute.
 * The song "Miss Byrd" from Closer Than Ever is about a secretary exulting in how no one who sees her would think that she's anything but "that dull Miss Byrd," yet she has secret trysts with her superintendent.
 * Parodied in The Music Man: The town gossips manage to convince the Professor that Marian the Librarian is Naughty by Night primarily because she reads "racy" books by "Chaucer...Rabelais...Balzac" and was kind enough to befriend the town outcast. She's not, but he falls in love with her anyway.

Video Games
"Question 18: Honor student by day... stripper by night!"
 * Leisure Suit Larry 7 got this one, cranked up to 20, with Hot Librarian Victorian Principles.
 * Nekoko from Yume Miru Kusuri.
 * You Don't Know Jack Movies parodied this one for one of the Question 18 segues:

Western Animation

 * In Futurama, a prim bureaucrat is utterly turned on after hours by Fry's horrifyingly slobby lifestyle.