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In the track Lovato looks for a hero, someone to lean on, singing, "Sing to me/ I know you're there/ 'Cause baby you're my sanity". During an interview with songwriter and record producer Emanuel Kiriakou she said: "Nightingale is definitely a personal song. It’s about one of my friends, actually, that passed away when we were thirteen. I feel like, sometimes I literally, I know when he’s in the room with me. And, I just have a really strong connection to him. I feel like he’s my nightingale". In other words, she feels like her friend is her guardian angel. The song is entirely dedicated to her friend Trenton Stout, who hanged himself at the age of thirteen.
On May 9, 2013 in a track-by-track Billboard review, writer Jason Lipshutz stated: "This is the type of ballad that Lovato makes look like a lay-up: the theatrical notes are in place, but so is the patience, maturity and attitude that establish the singer from her pop peers. "Nightingale" soars when Lovato is joined by a chorus of backup vocalists in its final passage; if she can perfect this song live, that finale could become enlightening." 
She performed the song as a part of her brief piano-driven set during her interview with GRAMMY.com in October 2013. Lovato performed the song live on Good Morning America on June 6, 2014 as a part of their Summer Concert Series. The song was also a part of Lovato's setlist for The The Neon Lights Tour and the Demi World Tour.
Demi released a music video on December 24, 2014 as an early Christmas present for her fans.