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What is it about a lullaby that helps kids fall asleep?

JUANA SUMMERS, HOST:

OK, parents and caretakers of young kids. Listen up. This one's for you. It's been a long day. You're tired. You just want to relax. But you have a kid to put to bed, and they're not sleeping. For our Weekly Dose of Wonder series, NPR's Selena Simmons-Duffin reports on her secret weapon for making sleep come faster - free idea for you to steal.

SELENA SIMMONS-DUFFIN, BYLINE: Pretty much every night, I turn on the sound machine...

(SOUNDBITE OF SOUND MACHINE DRONING)

SIMMONS-DUFFIN: ...And climb up into my 8-year-old's top bunk to lie down with her.

(Yawning).

Sometimes she wants to talk or just snuggle, but a lot of the time...

Do you want a song?

UNIDENTIFIED CHILD: Yeah - sleep, sleep, sleepyhead.

SIMMONS-DUFFIN: OK. (Singing) Sleep, sleep...

This is a favorite lullaby from the music together class she took when she was younger. Just about 90 seconds later...

(Humming).

...Ninety seconds, and she is out. Honestly, when it works like this, it makes me feel like I have a superpower or I'm casting a spell. You will fall asleep. Listen to my voice. It does fill me with wonder, but it also makes me curious to understand what's happening and why. So I called Professor Tiffany Field of the medical school at the University of Miami.

TIFFANY FIELD: When you look at lullabies, they're all slow and rhythmical.

SIMMONS-DUFFIN: That can help calm children's thoughts, she says, so they can lull themselves to sleep. She did a study of toddlers and preschoolers taking naps in the university nursery. The teachers played classical music at the beginning of naptime.

FIELD: With the toddlers, there was a 35% faster sleep onset. With the preschoolers, there was a 19% faster sleep onset. So, of course, the teachers loved that.

SIMMONS-DUFFIN: Many of the studies on this are done with preterm infants in the NICU, including one which compared infants who heard Mozart to infants who heard their mothers' lullabies plus a control group that didn't hear any music.

FIELD: And what they found was that the mothers' lullabies were more soothing to the infants. They slept better, but they also showed a lot of effects of decreased heart rate and respiration, better feeding, which probably explained why they had fewer days in the neonatal intensive care unit. And their mothers' anxiety was reduced.

SIMMONS-DUFFIN: Now, I love to sing, but that is not a requirement, says Field. You can sing badly, or if you really don't want to sing, a back rub can have similar effects. But there is just something about lullabies, says Sam Mehr of the University of Auckland, who directs the Music Lab. His team did a study where they played songs for infants in an unfamiliar language. Some of the songs were lullabies, and some weren't.

SAM MEHR: When they're listening to these lullabies, even though they're totally unfamiliar and, you know, not in the language the baby understands, they relax more. So there's something in, like, the kind of DNA of a lullaby that helps to calm infants.

SIMMONS-DUFFIN: He says in a lot of their research, they turn to lullabies because they're just everywhere all over the world. Hirut Kassa is from Ethiopia and a mom of two, including a 1-year-old son. This is what she sings to him.

HIRUT KASSA: (Singing in non-English language). That's the way they sleep.

SIMMONS-DUFFIN: She says it works like magic for her, too. Selena Simmons-Duffin, NPR News.

(SOUNDBITE OF BRIAN CRAIN'S "BRAHMS' LULLABY")

ARI SHAPIRO, HOST:

If you're driving right now, please don't fall asleep at the wheel. Transcript provided by NPR, Copyright NPR.

Selena Simmons-Duffin reports on health policy for NPR.