Question: What key is the musical piece "One More Night written in?
About the article "One More Night (Maroon 5 song)", section "Composition and lyrics".

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The dominant key in a given musical composition is the key whose tonic is a perfect fifth above (or a perfect fourth below) the tonic of the main key of the piece.
Put another way, the key whose tonic is the dominant scale degree in the main key.
If, for example, a piece is written in the key of C major, then the key of C is the tonic key.
The key of G major is the dominant key since it is based on the dominant note for the key of C major.
With a key signature of one sharp, G major features one more sharp than C major.
In sonata form in major keys, the second subject group is usually in the dominant key.
Even with the widest roaming modulations in the development, the dominant key exerts influence and eventually forces a return to the tonic key.  
The "Humoreske" consists of seven sections (not originally indicated as such by the composer except for the last one, ""), to be played attacca after each other.
Although the piece is nominally written in B-flat major, most of the piece is set in B-flat major's relative minor key, G minor.
The musical texture and emotional tone, though, varies widely and differs greatly between the sections: 
The song "One Day More", from the sung-through musical "Les Misérables", based on Victor Hugo's novel of the same name, was written by Claude-Michel Schönberg, with original French lyrics by Alain Boublil and Jean-Marc Natel, and an English-language adaptation by Herbert Kretzmer.
It is composed in the key of A major and is set to the tempo of Moderato at 86 bpm.
One Day More is a choral piece featuring many solos from the main characters of the stage show, all of whom sing in a counterpoint style known as dramatic quodlibet, featuring parts by the ensemble.
The song borrows motifs and themes from several songs from the first act.
Each character sings his/her part to a different melody at the same time (counterpoint), before joining for the final chorus:  Valjean picks up the melody of "Who Am I?"
without any changes (A major)  Marius, Cosette and Éponine sing to the melody of "I Dreamed a Dream" with Éponine's taking the bridge ("But the tigers come at night", sung by Éponine as "One more day all on my own") (A major, modulating to F# minor)  Enjolras repeats the bridge melody of "I Dreamed a Dream" but in a major key, with Marius singing a minor countermelody.
(C major)  Javert sings to the already often-used theme from "Valjean Arrested, Valjean Forgiven" or "Fantine's Arrest", only slower and in a major key.
(A major)  The Thénardiers sing to a slightly changed melody from "Master of the House" (A major)  The revolutionaries repeat the bridge melody of "I Dreamed a Dream" in a major key with a counter melody that is only instrumental in Fantine's solo.
(A major)  At the end of the song, everyone sings the melody of "Who Am I?"
(C major)  
Certain musical instruments are sometimes said to play in a certain key, or have their music written in a certain key.
Instruments that do not play in the key of C are known as transposing instruments.
The most common kind of clarinet, for example, is said to play in the key of B-flat.
This means that a scale written in C major in sheet music will actually sound as a B-flat major scale when played on the B-flat clarinet; that is, notes sound a whole tone lower than written.
Likewise, the horn, normally in the key of F, sounds notes a perfect fifth lower than written.
Similarly, some instruments may be said to be "built" in a certain key.
For example, a brass instrument built in B-flat will play a fundamental note of B-flat, and will be able to play notes in the harmonic series starting on B-flat without using valves, fingerholes, or slides or otherwise altering the length of the vibrating column of air.
An instrument built in a certain key will often, but not always, have its music written in the same key (see trombone for an exception).
However, some instruments, such as the diatonic harmonica and the harp, are in fact designed to play in only one key at a time: accidentals are difficult or impossible to play.
The highland bagpipes are built in B-flat major, although the music is written in D major with implied accidentals.
In Western musical composition, the key of a song has important ramifications for its composition: As noted earlier, certain instruments are said to be designed for a certain key, as playing in that key can be physically easier or harder.
Thus the choice of key can be an important one when composing for an orchestra, as one must take these elements into consideration.
In the life of the professional clarinettist, for example, it is common to carry two instruments tuned a semitone apart (B-flat and A) to cope with the needs of composers: Mozart's well-known clarinet concerto is in A Major.
To play it on a B-flat instrument would be difficult, and to rewrite all the orchestral parts to allow the piece to be played in B-flat major would be an enormous effort.
Even so, it is not unheard of for a piece published in B-flat to include notes a semitone (or more) below the range of the common B-flat clarinet.
The piece must then be played on a more exotic instrument, or transposed by hand (or at sight) for the slightly larger 'A' clarinet.
There are clarinets with an extended range, with a longer bore and additional keys.
Besides this though, the timbre of almost any instrument is not exactly the same for all notes played on that instrument.
For this reason a song that might be in the key of C might sound or "feel" somewhat different (besides being in a different pitch) to an observer if it is transposed to the key of A.
In addition, since many composers often utilized the piano while composing, the key chosen can possibly have an effect over the composing.
This is because the physical fingering is different for each key, which may lend itself to choosing to play and thus eventually write certain notes or chord progressions compared to others, or this may be done on purpose to make the fingering more efficient if the final piece is intended for piano.
In music that does not use equal temperament, chords played in different keys are qualitatively different.  
A key signature is not the same as a key; key signatures are merely notational devices.
They are convenient principally for diatonic or tonal music.
The key signature defines the diatonic scale that a piece of music uses without the need for accidentals.
Most scales require that some notes be consistently sharped or flatted.
For example, the only sharp in the G major scale is F sharp, so the key signature associated with the G major key is the one-sharp key signature.
However, it is only a notational convenience; a piece with a one-sharp key signature is not necessarily in the key of G major, and likewise, a piece in G major may not always be written with a one-sharp key signature; this is particularly true in music of the Baroque and earlier, when the concept of key had not been developed, and in minor keys, which typically require accidentals even with a matching key signature.
In any case, more extensive pieces often change key ("modulate") during contrasting sections, and only sometimes is this change indicated with a change of key signature; if not, the passage in the second key will not have a matching key signature.
The Toccata and Fugue in D minor, BWV 538 by Bach has no key signature, leading it to be called the "Dorian", but it is still in D minor; the Bs that occur in the piece are written with accidentals.