(NBC NEWS) -- After a concussion-causing blow it takes about 12 days for the average child to recover.
Now a new study shows that after a second or third concussion it takes kids much longer to bounce back.
"Kids who'd had a previous concussion had twice the duration of symptoms of those who hadn't had a previous concussion," says Dr. Matthew Eisenberg of Boston Children's Hospital.
The lead study author, Dr. Eisenberg analyzed nearly 300 young athletes who went to the emergency room with concussion.
Symptoms like headache, dizziness, nausea and difficulty concentrating lasted longer when concussions occurred within a year of each other.
"We think that there's a window of time after the head injury in which the brain is more vulnerable to a second head injury," Dr. Eisenberg says.
It may be that the brain hasn't fully recovered from the original trauma, but the lengthened recovery time could also be due to an entirely different reason.
The brain must rest to recover.
Doctors routinely tell concussed patients to stay away from school and sports: No texting, no computers, no video games.
With each additional concussion, doctors tell patients to rest even more.
But keeping kids contained like that can also lead to headaches and other symptoms that doctors could misinterpret.
"If symptoms that are produced from that process are misinterpreted, as being symptoms from the injury, then the answer is, well keep resting. And what happens? The symptoms keep going on," explains Dr. Jeffrey Kutcher of the University of Michigan Health System.
Dr. Eisenberg's study uncovered another interesting piece of information about concussions.
"Children who had lost consciousness actually recovered more quickly from their concussion," he notes.
It's unclear whether that may be the result of the particular patients in this study or another mystery in how the brain responds to injury.
Each child will respond differently to a concussion and experts say it's important to get a doctor's opinion on how to customize care for each patient.