The concept behind "When the Party's Over" was inspired after Finneas O'Connell, Eilish's brother, had left his date's house "kind of for no reason" and was driving home alone late at night, simultaneously unhappy at the end of the relationship but also feeling a sense of safety through a lack of full investment in it.
[3] They consequently set out to "get the magic" of a live performance "into a space where it's still interesting to record," a process which O'Connell described as "really hard" due to the song's delicate nature.
[3] Eilish and O'Connell decided that the track would be "almost entirely" vocal, utilizing only sub-bass and acoustic piano as additional instruments; this made it "super challenging" to keep each measure interesting.
"[14] It discusses the end of a relationship, a common subject matter in the Eilish's previous work, with the singer wishing to be "more than a party of one;"[12][15] Suzy Exposito of Rolling Stone described it as one of the more sincere songs off her debut album When We All Fall Asleep, Where Do We Go?
(2019),[15] while Reed wrote that "each verse [grows] more mournful" as the song progresses[11] It opens with a sample of Eilish's debut single "Ocean Eyes" (2016), which Sean Ward of The Line of Best Fit argued demonstrated an "understanding of her own artistry.
Suzy Exposito of Rolling Stone wrote that the track is one of the moments off When We All Fall Asleep where "Eilish can't help but draw back the curtain [...] and let you in.
"[15] Similarly, Sputnikmusic reviewer SowingSeason called it one of the "still frame moments when she allows herself to be heard not as the cynical teenage pop star, but rather as Billie the seventeen-year-old girl who's going through all the same things you are;" they also commended her "mesmerizing whispers and hums.
"[12] Pitchfork editor Stacey Anderson commended the singer's vocal performance,[8] while Grant Rindner of Uproxx considered that the single proved "that she has one of the most arresting voices in pop.
"[17] Tanis Smither of Earmilk praised the song as "heart-wrenching," writing that it "somehow manages to maintain her brand of ballsy production and delicate, dreamy vocal.