The day after New Year’s, John Reichert of Boulder, Colo., had a heated argument with his 14-year-old son, James. “I’ve failed you as a father,” he told the boy despairingly.

During the long months of lockdowns and shuttered schools, Mr. Reichert, like many parents, overlooked the vastly increasing time that his son was spending on video games and social media. Now, James, who used to focus his free time on mountain biking and playing basketball, devotes nearly all of his leisure hours — about 40 a week — to Xbox and his phone. During their argument, he pleaded with his father not to restrict access, calling his phone “his whole life.”

“That was the tipping point. His whole life?” said Mr. Reichert, a technical administrator in the local sheriff’s office. “I’m not losing my son to this.”

Nearly a year into the coronavirus pandemic, parents across the country — and the world — are watching their children slide down an increasingly slippery path into an all-consuming digital life. When the outbreak hit, many parents relaxed restrictions on screens as a stopgap way to keep frustrated, restless children entertained and engaged. But, often, remaining limits have vaporized as computers, tablets and phones became the centerpiece of school and social life, and weeks of stay-at-home rules bled into nearly a year.

The situation is alarming parents, and scientists too.

“There will be a period of epic withdrawal,” said Keith Humphreys, a professor of psychology at Stanford University, an addiction expert and a former senior adviser to President Barack Obama on drug policy. It will, he said, require young people to “sustain attention in normal interactions without getting a reward hit every few seconds.”

Scientists say that children’s brains, well through adolescence, are considered “plastic,” meaning they can adapt and shift to changing circumstances. That could help younger people again find satisfaction in an offline world but it becomes harder the longer they immerse in rapid-fire digital stimulation.

Dr. Jenny Radesky, a pediatrician who studies children’s use of mobile technology at the University of Michigan, said she did countless media interviews early in the pandemic, telling parents not to feel guilty about allowing more screen time, given the stark challenges of lockdowns. Now, she said, she’d have given different advice if she had known how long children would end up stuck at home.

“I probably would have encouraged families to turn off Wi-Fi except during school hours so kids don’t feel tempted every moment, night and day,” she said, adding, “The longer they’ve been doing a habituated behavior, the harder it’s going to be to break the habit.”

The concern is not just over the habits of teens and tweens. Legions of children under 10 are giving countless hours to games like Fortnite, and apps like TikTok and Snapchat. An app called Roblox, particularly popular among children ages 9 to 12 in the United States, averaged 31.1 million users a day during the first nine months of this year, an increase of 82 percent over the year prior.

Over all, children’s screen time had doubled by May as compared with the same period in the year prior, according to Qustodio, a company that tracks usage on tens of thousands of devices used by children, ages 4 to 15, worldwide. The data showed that usage increased as time passed: In the United States, for instance, children spent, on average, 97 minutes a day on YouTube in March and April, up from 57 minutes in February, and nearly double the use a year prior — with similar trends found in Britain and Spain. The company calls the month-by-month increase “The Covid Effect.”

Children turn to screens because they say they have no alternative activities or entertainment — this is where they hang out with friends and go to school — all while the technology platforms profit by seducing loyalty through tactics like rewards of virtual money or “limited edition” perks for keeping up daily “streaks” of use.

“This has been a gift to them — we’ve given them a captive audience: our children,” said Dr. Dimitri Christakis, director of the Center for Child Health, Behavior and Development at Seattle Children’s Research Institute. The cost will be borne by families, Dr. Christakis said, because increased online use is associated with anxiety, depression, obesity and aggression — “and addiction to the medium itself.”

Crucially, the research shows only associations, which means that heavy internet use does not necessarily cause these problems. What concerns researchers, at a minimum, is that the use of devices is a poor substitute for activities known to be central to health, social and physical development, including physical play and other interactions that help children learn how to confront challenging social situations.

Yet parents express a kind of hopelessness with their options. Keeping to pre-pandemic rules seems not just impractical, it can feel downright mean to keep children from a major source of socializing.

“So I take it away and they do what? A puzzle? Learn to sew? Knit? I don’t know what the expectations are,” said Paraskevi Briasouli, a corporate writer who is raising four children — ages 8, 6, 3 and 1 — with her husband in a two-bedroom Manhattan apartment. Device time has replaced sports on weekday afternoons and soared 70 percent on weekends, she said.

Before the pandemic, Ms. Briasouli’s 8-year-old, Jesse, sometimes used his father’s old iPad Pro. During the pandemic, he got an iPad mini and so did his 6-year-old sister.

“And we got a Nintendo Switch because everybody got a Switch,” Ms. Briasouli said. Some days, she said, she watches her son sit with three devices, alternating play among them.

The boy’s father, Jesse Taylor, said his own concerns about the heavy technology use were being offset by some optimism that his children were becoming able digital natives.

“These are the tools of their lives,” he said. “Everything they will do, they will do through one of these electronic devices, socialization included.”

Julia Gregor, an investigator for a public defender’s office in Seattle with two sons, ages 12 and 10, said the older boy had grown increasingly despondent during the pandemic. She and her husband bought him an Xbox for his birthday and an iPhone for Christmas, accelerating the timetable she’d originally had for those gadgets.

She also relaxed a rule against first-person shooter games. “I kind of gave up on that, too,” she said. When her older boy plays Xbox, “he laughs and has some social interaction with his buddies,” she said. She’d hoped he would use his new phone to text and talk to friends. But, she said, “he mostly uses it for games.”

Recent neuroimaging research suggests heavy use of certain video games may cause brain changes linked to addictive behaviors. One of the study’s authors, Christian Montag, a professor of molecular physiology at Ulm University, also co-authored a recent overview of digital use during the Covid-19 pandemic, published last month in Addictive Behavior Reports. It reported that German teens are playing video games with much greater frequency than before lockdown and concluded “that overuse of digital technologies represents a likely phenomenon and outcome of the Covid-19 pandemic.”

Dr. Humphreys, from Stanford, said he believed that adults and children alike could, with disciplined time away from devices, learn to disconnect. But doing so has become complicated by the fact that the devices now are at once vessels for school, social life, gaming and other activities central to life.

Dr. Humphreys called this concept “bundling,” and said it created particular challenges because so many different kinds of rewards were mingled together that it could be hard to separate the good from the bad.

For instance, Dr. Humphreys said, people who smoke and drink at a bar, where they meet with friends, may find it harder to quit smoking because there is the extra reinforcement of alcohol and friendship mixed into the experience. Similarly, he said, children now associate their devices with multiple forms of pleasure, and so, disconnecting them during the pandemic has been like “trying to preach abstinence in a bar.”

Dr. Radesky said that the mingling of all of these functions not only gives children a chance to multitask, it also allows young people to “escape” from any uncomfortable moment they may face. If they are doing schoolwork that bores them, she said, they can easily move into a “pleasure cocoon” by switching to watching YouTube, chatting with friends, playing a game.

And parents might not know that is going on. In research published in July in Pediatrics, Dr. Radesky and her co-authors used tracking software to show that children were accessing “tons of apps we classify for adults,” — like horror apps with scary characters, first-person shooter games and other media — and that parents did not know their children were doing so.

A dynamic playing out in many families was on display during an interview with the Reichert family. Fourteen-year-old James is an only child who started high school this fall and said that because of Covid-19 and distance learning, he didn’t have many chances to meet new people. Instead, he hangs out online with his old friends.

“The only way to talk to them, besides going to their house, is through my Xbox,” he said. “We play on there every night.”

He said the games felt so compelling, particularly when they offered achievement incentives. “If you play a lot and do well, you can try to max out your rank — that takes up quite a bit of time,” he said. “But sometimes we just play for fun.”

The family dog died on New Year’s Eve and James said that playing games with his friends helped him to not think about the loss. This concerned his mother, Kathleen Reichert, who felt that her son was escaping the emotions of real life.

“What are you going to do when you’re married and stressed? Tell your wife that you need to play Xbox?” she said to her son during the interview.

As a new semester started, the parents put new rules into effect: no Xbox or phone during the weekdays for at least a few weeks, and their use will have to be earned for the weekends, through chores. Ms. Reichert feels wrenched by the whole thing.

Before the pandemic, James had so many options, she said, adding: Now, “it makes me feel badly when I try to restrict him. It’s his only socialization.”