Kids need to cut back on mental exertion, as well as physical exercise, when they’re recovering from a concussion, a new study shows.

Nearly 50 percent of the kids and young adults who didn’t reduce their mental strain took 100 days or more to fully recover, according to the study published in Pediatrics on Monday. Among those who cut back the most, almost all had recovered by 100 days, most within a couple of months.

While specialists had long suspected that cutting back on mental activities would shorten recovery time, this is the first study to back that recommendation up with data. The study could be especially important for treating children, whose developing brains are particularly vulnerable to concussion damage.

For many kids it will be enough to cut back for three to five days, said study co-author Dr. William Meehan, director of the Sports Concussion Clinic at Boston Children’s Hospital. “Then you can gradually reintroduce them to cognitive activity. They should do as much as they can without exacerbating their symptoms.”

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For the new study, Meehan and his colleagues followed 335 patients, aged 8 to 23, at the sports concussion clinic at Boston Children’s Hospital.

At each visit to the concussion clinic, the patients, whose average age was 15, were asked about their symptoms and also to describe how much they’d worked their brains since the last visit. The five possible choices ranged from complete cognitive rest to no cutback at all.

The new findings fall in line with animal research and imaging studies that have taught us what happens during a concussion, said David Hovda, a professor of neurosurgery and director of the Brain Injury Research Center at the University of California, Los Angeles.

When the brain is jolted hard, it experiences a sort of mini-seizure, Hovda said. “All the cells fire and the brain needs an enormous amount of fuel to equilibrate,” he explained. “The brain is then exhausted so it shuts down and becomes very quiet.”

That quiet is what the brain needs to repair itself, Hovda said. “If you activate the brain during the time it’s trying to shut itself down, it will activate, but that will make recovery much more prolonged.”

People need to realize that “it can take days to weeks for processes in the brain to mop up the mess from a concussion,” agreed Dr. Douglas Smith, a professor of neurosurgery and director of the Center for Brain Injury and Repair at the University of Pennsylvania Medical Center.

Just how much kids needs to reduce their mental activity will depend on the number and severity of symptoms, said Dr. Robert Cantu, a professor of neurology and neurosurgery at the Boston University School of Medicine and co-director of the Center for the Study of Chronic Traumatic Encephalopathy.

“You do everything based on what doesn’t aggravate the situation,” Cantu said. “If kids can do 15 to 20 minutes on the computer without aggravating their symptoms, let them. But if headaches get worse after only 5 or 10 minutes, then you need to shut this down completely. Everything needs to be tailored to the individual.”

For some kids the slightest mental exertion can worsen symptoms. “There are definitely individuals with symptoms that are so bad that the best thing to do is to take a period of months off from school,” Cantu said. “That is a small minority, however.”