20 years after writing 'The Happiest Baby on the Block,' Harvey Karp says crying babies and tired parents present healthcare and economic issues. He still believes he has a solution.

• The pediatrician Harvey Karp wrote "The Happiest Baby on the Block" and invented the Snoo.
• He was motivated by an anthropological mystery: Why do babies cry so much?

When Harvey Karp was in medical school, he thought he wanted to be a cardiologist. But he couldn't give up working with his youngest patients.

"I realized that it's so wonderful working with children and young families. They're so positive," Karp said.

For millions of families, Karp's career decision was a good move. Despite never raising an infant — his stepdaughter was 7 when she came into his life — Karp has become the "Godfather of sleep" for American parents. If you're familiar with the five S's — swaddle, side-stomach position, shush, swing, and suck — and the term "fourth trimester," you have Karp to thank.

Twenty years after he shared his work with the world in "The Happiest Baby on the Block," Karp, 70, said helping infants and their parents sleep better could prevent issues such as marital strife and accidents brought on by exhaustion.

"Helping people learn these things can save billions in healthcare costs and employer productivity costs related to tired parents and crying babies," Karp said.

An anthropological mystery

Early in his career, Karp became fascinated with why infants cry so much. Colic affects one in four American babies. That didn't make sense to Karp, especially when he learned that in some African tribes, babies cry for just minutes each day, not hours.

There was something else on his mind: "Most adults and adult doctors have recognized that most of us fall asleep on trains, planes, cars, hammocks, and to sounds of the ocean," he said.

Cultures around the world use a "shhh" sound to silence people. People who meditate use low, vibrational sounds and swaying, fluid movements. But no one had ever explained why these were such universal experiences. Karp theorized that mimicking the womb — with low sounds and movement — explained a bit about human sleep and how to help infants get more of it. The five S's were born.

Returning knowledge to the community

The five S's aren't new, Karp said. In past generations with larger families and more intergenerational living, people just passed on this knowledge. In fact, he even warned his publisher: Sell lots of books now because soon people won't need them.

"Once we get it back in the community, they're going to learn it from their friends," he said.

He was right and wrong about that. Last year, Merriam-Webster added "fourth trimester" to the dictionary, but "The Happiest Baby on the Block" is still a bestseller.

The strength of the five S's isn't only that they get babies to sleep but also that they make parents feel more confident and competent. According to Karp, the movie star Ashton Kutcher once told him, "now I love when a baby cries, because I can be the magician."

A modern twist on the 5 S's

In 2016, Karp launched the Snoo Smart Sleeper. The bassinet mimics the five S's. It helps soothe infants and gets parents an extra hour of sleep each night, Karp said. It was heralded as an expensive must-have baby item, but for Karp, the Snoo is a safety device.

"It helps parents prevent the tragedy and distress that happens when you don't use this thing," he said.

The rates of sudden unexpected infant death have been about the same since 2000. Data from the Centers for Disease Control and Prevention indicated that in 2020, 27% of cases were caused by accidental suffocation and strangulation in bed — which Karp said are more common when parents are tired and babies are unsettled. Karp is conducting studies on how the Snoo may reduce sudden infant death syndrome and postpartum depression, and improve brain development.

"Rhythmic stimuli are as important for little babies as calories for development and physical growth," he said.

Employers, including Under Armor, JPMorgan and the National Air Traffic Controllers Union, now offer Snoo rentals as an employee benefit. Karp hopes that with more research, insurance companies and state governments will continue to make the Snoo more accessible.

Karp hasn't forgotten about toddlerhood

Even if your child doesn't sleep during the fourth trimester, you'll make it through, Karp said. But a bigger challenge is looming: the toddler years.

"Between 8 months and 6 years of age, you've created a person," he said. Sleep, soothing, and communication become even more important. And Karp doesn't think the modern idea of talking tots through their emotions works.

"If you're pissed at me, words are meaningless," he said.

Instead of naming what kids are feeling, Karp said parents should use short phrases and repetition to mirror back a third of their child's emotions.

"You can't yank them out of the jungle of their feelings," he said. "You have to go in there and guide them out."