What's going on in our brains when we experience nostalgia?

A MARTÍNEZ, HOST:

Whether you're decking the halls or jingling the bells, holiday music is all around.

(SOUNDBITE OF SONG, "JINGLE BELL ROCK")

UNIDENTIFIED MUSICAL GROUP: (Singing) Jingle bell rock.

(SOUNDBITE OF SONG, "FELIZ NAVIDAD")

JOSE FELICIANO: (Singing in Spanish).

MARTÍNEZ: Maybe it's on your radio or at the grocery store. It can take you back in time. And that is nostalgia. Daniel Levitin is a musician and professor emeritus of psychology in neuroscience at McGill University in Montreal. He's done research on how feelings affect our bodies.

DANIEL LEVITIN: When we hear a song we haven't heard in a long time, it can trigger long-dormant memories, even ones we thought we'd forgotten. And the reason is that a lot of songs we hear are attached to a particular time and place, and so they get tagged with all the events and feelings and sights and sounds of that time and place.

MARTÍNEZ: The song, Daniel, that brings me the most joy during Christmas is Wham!'s "Last Christmas."

(SOUNDBITE OF SONG, "LAST CHRISTMAS")

WHAM: (Singing) Last Christmas I gave you my heart.

MARTÍNEZ: And here's a little backstory on that. So way back in the day when I heard that song in the '80s, I was really into this girl who told me that she liked me back. And I'm not kidding you, Daniel, just like the lyrics of the song...

(SOUNDBITE OF SONG, "LAST CHRISTMAS")

WHAM: (Singing) But the very next day, you gave it away.

MARTÍNEZ: ...I gave her my heart, and the very next day she gave it away to someone else. She was flirting with this other guy. So...

LEVITIN: (Laughter).

MARTÍNEZ: ...When I think back on that song, I get all kinds of feelings and emotion in addition to it being just a fabulous Christmas song.

LEVITIN: And that same song would mean something entirely different to someone else. When we talk about using music as medicine and having, you know, a prescription, you have your own autobiographical associations because it's unique to you, and the song is so personal. It can unlock all kinds of feelings.

MARTÍNEZ: Right. That song, "I'll Be Home For Christmas," Daniel, I've always felt that when I listened to it - I mean, I don't have a connection to it necessarily, but it sounds like such a sad song, and even though I have no connection to it, I feel sad hearing it.

(SOUNDBITE OF SONG, "I'LL BE HOME FOR CHRISTMAS (IF ONLY IN MY DREAMS)")

FRANK SINATRA: (Singing) You can plan on me.

LEVITIN: You know, I have a friend named Dillon O'Brian who's a great songwriter, singer and producer, and he says about songwriters, I'm going to make you cry and you're going to love me for it.

MARTÍNEZ: What's a song that makes you cry?

LEVITIN: Oh, "The Randall Knife" by Guy Clark, a song about burying his father.

(SOUNDBITE OF SONG, "THE RANDALL KNIFE")

GUY CLARK: My father had a Randall knife. My mother gave it to him.

LEVITIN: That always just gets me. And it's a beautiful sadness. The sadness that music evokes is safe in the sense that you know that this isn't really happening in the world. It's a way for us to try on emotions of terrible negative consequence without actually having something bad happen to us.

MARTÍNEZ: And that's why I would imagine that there are so many movies and songs that induce these kinds of feelings, especially around the holidays.

LEVITIN: Right. And sadness is actually cathartic. It's helpful. And sad music can release the hormone prolactin, which is the same soothing and tranquilizing hormone that's released when mothers are nursing their infants. So you get this kind of a warm feeling, almost like you've just had a hot bourbon, from listening to the sad music.

MARTÍNEZ: Daniel Levitin, author and musician in Los Angeles, also a professor emeritus of psychology and neuroscience at McGill University in Montreal. Daniel, thanks for weighing in on this.

LEVITIN: Thanks for having me. Transcript provided by NPR, Copyright NPR.