[1] The song was written by Halsey along with its producers Benny Blanco, Cashmere Cat, and Happy Perez.

In May 2019, Halsey posted a link on her social media, directing users to the website "nightma.re" to sign up for email alerts and prompting them to share what their worst nightmare is.

[14] Directed by Hannah Lux Davis, the video features appearances by Cara Delevingne, Debbie Harry,[15] Ryan Destiny, and Suki Waterhouse.

Billboard magazine said the song is musically "unorthodox" containing "stylistic curveballs" and "killer lines" that "exalt autonomy, sexual freedom and self-acceptance".

[17] The line "I've been polite, but I won't be caught dead/ Letting a man tell me what I should do in my bed" was noted as a "barbed assault on patriarchal expectation" by Billboard and "a necessary gesture of rage and empowerment" by Rolling Stone.

"Nightmare (Reprise)" is a song by Halsey included on the Walmart bonus track edition of her fourth studio album If I Can't Have Love, I Want Power (2021).