The strongest evidence for avoiding excessive screen times involves the "opportunity cost" — the valuable learning opportunities children miss out on during the hours they spend on digital devices.

In order to develop cognitive, language, motor and social-emotional skills, young children need to experience the world hands-on — playing with toys, exploring outside, experimenting with different materials, and having back-and-forth interactions with nurturing caregivers, said Ameenuddin. When they are watching digital media, they lose that time to grow and learn.

This is particularly true for babies and toddlers, because there isn't much evidence that they can learn through screens.

For preschoolers, there's more evidence that educational shows like "Sesame Street" can help improve literacy and social development, but only in limited amounts. Heavy media use in the early years has been linked to a greater risk of obesity because these children often miss out on physical activity and outdoor time. They're also more like to see advertisements for sugary foods and drinks.

Children who are watching screens also have fewer valuable interactions with caregivers and hear fewer words during the course of their days, which is linked to cognitive, language and social delays. Some studies have found evidence linking excessive screen time with behavioral issues such as ADHD, though the research did not show that one was actually caused by the other.

A bigger question is whether the screen time is changing the wiring of babies' and young children's brains. A small MRI study of preschoolers found that children who watched more than the recommended one hour a day had lower development in the brain's white matter that supports language and early literacy skills. But Ameenuddin says the evidence isn't clear yet that screens themselves are affecting brain development.

Is screen time harmful for babies?

Babies should be playing and exploring the world, not watching screens, experts advise.

In the first three years of life, more than 1 million neural connections are formed every second, and key to this development are the "serve and return" interactions between children and their caregivers, according to Harvard's Center for the Developing Child. Babies babble and make faces and gestures, and the people who love them respond in kind. Without these important interactions, the brain's architecture can't form the way it should.

These sorts of interactions don't happen through screens.

A recent Japanese study found that the more time a baby spent watching screens at age 1, the more likely they were to have developmental delays in communication and problem-solving at ages 2-4 — particularly when they watched more than four hours a day.

But Nesi, the psychiatry professor, said there's no need to shield a baby's eyes when in a room with a television on. "There's a lot of fear messaging around this, and there's no evidence to suggest that your baby catching a glance of a screen every once in a while could do harm."

How can I make the most of screen time?

"There is a lot of incredible, cool stuff for kids to watch and do on screens," said Jill Murphy, chief content officer at Common Sense Media, which offers quality ratings and media reviews for children. In general, Murphy says it's safer to stick with branded content from a production company that's intended for young children, which often have child development staff or advisors.

YouTube Kids requires more parental guidance, she said, and parents need to evaluate videos in advance. If they can't, they should create a profile with a child's selected interests and a set number of videos coming into the feed.

"Anything violent is a hard no for young kids, even if it's play slapping or hitting each other with a stick," said Murphy. "They're very quick to mimic that behavior."

Researchers recommend age-appropriate programming that actively involves children by asking them questions, helps them make meaningful connections to their everyday lives, and includes "socially meaningful" characters they can get to know rather than a disembodied voice.

Murphy says parents should designate screen-free zones and times, and set clear limits around when screen time will end. And whenever possible, stick with high-quality educational content without commercials, like the kind found on PBS Kids, which has been found to lead to better behavioral outcomes and language skills.

Set boundaries, avoid screens around bedtime, and whenever possible, watch alongside your child.

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