A music box version (Track 013, "Home (Music Box)") plays when the protagonist sleeps in their room.
The melody is a variation of "Once Upon a Time" (the game's main theme), slowed down to evoke a sense of safety and nostalgia. Availability & Streaming Undertale OST: 012 - Home
The track is available on most major digital platforms, including: 012. Home (UNDERTALE Soundtrack) - Toby Fox A music box version (Track 013, "Home (Music