"Nightmare" is a song by American singer Halsey. It was released as a single on May 17, 2019, and was sent to top 40 radio on May 21, 2019, through Capitol Records. The song was written by Halsey along with its producers Benny Blanco, Cashmere Cat, and Happy Perez. The music video, directed by Hannah Lux Davis, accompanied its release the same day. The song was originally intended to be the lead single of their third studio album Manic but was ultimately cut and replaced with "Without Me". It was later added as a bonus track on the extended version of Halsey's fourth studio album, If I Can't Have Love, I Want Power (2021).

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