Young children who snore could be at greater risk of becoming hyperactive later than those who sleep quietly, U.S. researchers reported on Friday.

Their study, published in the journal Sleep, strengthens earlier conclusions linking Sleep Disorders and hyperactivity and also seems to confirm that it is the snoring that comes first.

In 2002, a University of Michigan team reported that among 229 children studied, those who snored regularly were twice as likely to later be hyperactive or have attention issues than non-snorers.

For boys under the age of 8, the rate was four times higher.

Now, after four years of follow-up, the team has confirmed its finding and found even more cases of hyperactivity. They found that young children who snored were four times as likely to become hyperactive four years later.

Dr. Ronald Chervin and colleagues said their findings held even after they took into account which children already had been identified as hyperactive during the first study.

The boys who were under the age of 8 at the beginning of the study and who had the worst sleep-breathing problems were nine times more likely to have developed new hyperactivity four years later than boys of the same age who hadn’t had such sleep problems, the researchers said.

“To our knowledge, this new study is the first long-term, prospective research to show that regular snoring and other clues to the possible presence of sleep apnea predict future development of inattention and hyperactivity,” Chervin said in a statement.

“These findings strengthen the hypothesis that untreated sleep-breathing problems in childhood can contribute to the development of hyperactivity.”

Chervin noted that a study like this one, starting with a group and following them over time, reveals more than one in which people are asked to recall events from the past.

“Sleep problems in both children and adults are often undiagnosed, even though they can have a major impact on health, behavior and quality of life,” he said.