All over the world, in every culture and every language, mothers sing lullabies to calm their babies. Now new research shows this kind of soothing music helps smaller, sicker babies become healthier, too.

Many infants are born too early, too small or too sick. And sleep doesn't always come so easily for these littlest babies. They often end up in neonatal intensive care units, where they can be hooked up to probes and wires and subjected to painful, but necessary, poking and prodding from doctors and nurses.

Neonatology researcher Manoj Kumar from the University of Alberta has been in many neonatal intensive care units. Often he sees the nurses play music next to the incubators holding these sick infants.

"It's very informal. Usually, the choice of the music is by the parents," says Kumar. "Usually these are slow, sort of smooth lullaby type of music."

But Kumar says no one knew for sure whether this music really benefits babies, and he wanted to know more about the effects.  So he reviewed all the studies he could find about lullabies and neonates. He found only nine done over the past 20 years which were rigorous enough to prove anything. He recently collated their data in an article published in the Archives of Disease in Childhood.

Some of the studies found music was useful for babies having painful procedures, such as a skin prick to collect blood or a circumcision.

"Usually when babies are subjected to painful procedures, their heart rates go up and stay up for a fairly long period of time, and they come down as the stress level comes down," Kumar says.

"So what they noticed was there was more stability in their heart rates. There was more stability in terms of other physiological parameters as the babies who were not subjected to music while they were going through painful procedures."

But Kumar says babies stopped reacting to pain more quickly when music was played, and they calmed themselves more efficiently.

The most interesting thing Kumar says he found was that playing music for babies who were undernourished helped them eat better.

"If they were subjected to a system where while they were sucking on their bottles, they were able to activate lullaby music, their feeding habits improved," he says. "They were able to finish their bottles much faster."

And Kumar says the faster and better these babies ate, the sooner they were able to go home with their families.

Kumar says he'd like to do some original studies on music and infants.  He says he wants to see if music alone is best or if the music is more effective when combined with the sounds of a heart beat - the kind of rhythm babies might have heard when they were inside their mothers' wombs.