Good Night is the 101st track in the Undertale Soundtrack and the 13th track in the Undertale Demo OST, where it is called "End." It is a short, slow and calm piece that takes its melody from Once Upon a Time, the primary theme of the game.
This track is only used at the neutral ending of the Undertale Demo and is unused in the final version of Undertale. The OST version is slightly faster.