My grandsons don’t get much screen time. By that I mean they are almost never exposed to television, laptops, tablets, smartphones and whatever is the latest thing keeping our youth awash in entertainment.

I thought my son and daughter-in-law might be too strict on this. Was it right that the boys — 5, 3 and 1 — knew nothing of the hit film “Frozen” other than what their friends told them at school? My children had watched TV while growing up without any noticeable harm.

But my kids, I realized, had been outdoorsy types who did not spend much time with the tube. As a new book reminded me, research is showing that screen time is a much bigger problem now, with consequences not only for learning but also having healthy lives. The book’s recommendations of a 45-minute daily limit on passive watching and a regularly scheduled homework period — more than twice that number for high school students — make sense to me.

“Technology has . . . become an ever-present influence on our lives,” say Stephanie Donaldson-Pressman, Rebecca Jackson and Robert M. Pressman, authors of “The Learning Habit: A Groundbreaking Approach to Homework and Parenting That Helps Our Children Succeed in School and Life.”

When you exceed 45 minutes of what the book calls “daily media time,” the negative effects on educational, emotional and social development begin to kick in. Ninety minutes of screen time a day can lower a child’s achievement nearly one grade level, according to their book.

The authors are a bit hysterical about teen culture, but reasonable people can disagree on that. The book’s major weakness is its reliance on the authors’ very large online survey. It yielded 50,000 responses, but, as they acknowledge, such a flood of volunteered, self-reported data can be skewed. The book benefits from several smaller, more scientific research results.

We have known for decades from solid data, such as the University of Michigan time diaries, that high school students on average spend less than an hour a day on homework and more than twice as much time with TV and video games. Those of us who remember the ill effects of not doing our homework know that poor time management has something to do with our learning slump.

So how do you get teenagers to study more and spend less time with their gadgets? The authors offer a clever approach I have never seen seriously discussed in our long national homework debate. Each family should pick a reasonable amount of homework time, perhaps 10 minutes for every grade — so second graders do 20 minutes a day and 12th graders do 120 minutes. Once the student has worked the allotted time, he or she should stop, even if the assignments are not done.

The authors provide research and case studies that convince even me that if teachers have assigned more than what fits into the reasonable period a family has adopted, they have probably included some useless busy work that is best not done at all.

Once the homework is done, under the Learning Habit system, students are free to play with their devices, catch up on “The Walking Dead” or shoot some baskets. Learning should not be a nightly battle, the authors say, but a dependable part of the day. Good time management, once internalized, makes college and life easier.

My grandsons are getting a gradual introduction to screen-heavy popular culture. They got to watch “Frozen” twice during a vacation last month. Their parents, and others, will find “The Learning Habit” a helpful addition to their shelves.