Now, follow-up data from some of the same children who took part in that study gives those findings a new, long-term dimension. The latest results are published in the journal Sleep.

Children in the original study who snored regularly were about four times more likely than those who did not to have developed new hyperactivity by the time the U-M team contacted their families four years later. In other words, snoring early in life predicted new or worsened behavior problems four years later.

Similar behavior was seen among children who had had other symptoms of obstructive sleep apnea, in which repeated pauses in breathing disrupt sleep and can reduce blood oxygen levels. For example, children with daytime sleepiness in the original study also were more likely to have developed hyperactivity four years later.

Large Body of Evidence

The findings held true even after the researchers took into account which children already had been identified as hyperactive during the first study, and which ones were taking prescription behavior medicines during the follow-up survey.

In fact, inattention and hyperactivity at follow-up usually were predicted better by snoring and other sleep-apnea symptoms four years earlier than by those same symptoms at follow-up, notes lead author and U-M sleep researcher Ronald D. Chervin, M.D., M.S.

Dr. Chervin and other sleep and breathing researchers have built up a large body of evidence on this issue in recent years. The sleep-behavior link rests on the concept that snoring, sleep apnea and other breathing problems during sleep diminish the quality of sleep, repeatedly reduce oxygen levels, and affect daytime behavior.

One group of boys under the age of 8 who had the worst sleep-breathing problems during the first study were approximately nine times more likely to have developed new hyperactivity four years later than were boys of the same age who had not had such sleep problems.

The results are from a prospective study of 229 children who are now between the ages of 6 and 17. The children were drawn from the group of 866 2- to 13-year-olds whose parents originally were surveyed in the late 1990s in the waiting rooms of several community-based pediatrics clinics.

The parents agreed to allow the researchers to mail them a follow-up survey four years later; 229 returned it. The follow-up group was statistically comparable to the initial group.

Support for Causal Relationship

Both at the baseline and at follow-up, the parents completed standardized questionnaires that measure a child's behavior and sleep characteristics. Children were encouraged to help their parents complete the questionnaires.

The initial study, published in March 2002 in the journal Pediatrics, found that kids who snored regularly were twice as likely as non-snorers to have hyperactivity or attention issues at the same time. Among boys under the age of 8, the rate was four times.

"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," says Dr. Chervin, who serves as director of the U-M Health System's Michael S. Aldrich Sleep Disorders Laboratory and associate professor of neurology at the U-M Medical School.

"These findings strengthen the hypothesis that untreated sleep-breathing problems in childhood can contribute to the development of hyperactivity," he added.

Definitive Proof Elusive

Data from small groups of children who received treatment for their sleep-breathing problems -- usually by removal of the tonsils and adenoids -- have indicated that behavior may improve as sleep improves. Larger studies of pre- and post-treatment sleep and behavior patterns are now underway; Dr. Chervin and his colleagues currently are analyzing data from one performed at U-M.

But definitive proof that breathing problems during sleep affect daytime behavior is still elusive, the U-M authors write. And a long-term randomized controlled trial, the gold standard of medical research, might never be done because it would require researchers to withhold treatment that has become an accepted standard of care.

The new paper is the first to show that sleep problems come before hyperactive behavior, and that one predicts the other -- which may help bolster the sleep-behavior theory.

"In research, a prospective study that follows a group over time and assesses them at two or more time points carries more weight than one that looks for risk factors and possible outcomes all at the same time," says Dr. Chervin. "That's what we set out to achieve, and we believe we've shown a clear predictive link."

Quality of Sleep and Behavior Attributes

The study combined two different validated survey instruments: one that asked parents about their children's behavior patterns, and one asking about snoring, sleepiness and characteristics that may indicate sleep-disordered breathing problems.

The sleep portion of the survey asked about frequency and severity of snoring, as well as the tendency to struggle to breathe or stop breathing temporarily during the night, to breathe through the mouth during the day, to wake up feeling unrefreshed, or to have a hard time waking up.

All these can mean a child has sleep-disordered breathing, which can affect the quality of sleep. Scores for snoring frequency and severity, and for sleepiness and sleep-disordered breathing, were tallied for each child to allow each tendency to be considered on its own.

The behavior portion of the survey asked about attention to tasks and schoolwork, distraction, forgetfulness, fidgeting, inappropriate action and excessive talking, as well as other symptoms of attention-deficit/hyperactivity disorder. The children were assigned scores based on the total number of attributes their parents said applied to them, and how often.

Sleep Problems Often Undiagnosed

Dr. Chervin and his colleagues say that a further prospective study is needed to confirm their results and compensate for some of their study's limitations. The research focused on a mild to moderate level of hyperactivity, and it's not certain whether the findings would apply to children with full Attention Deficit/Hyperactivity Disorder.

The authors also note that more precise measures of sleep-breathing problems, and of sleep quality, might reveal even more.

Until such studies can be done, Dr. Chervin says, parents should pay attention to their children's sleep -- and their own.

"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 says.

"Getting enough sleep, adopting good sleep habits, and seeking medical attention for issues such as habitual snoring, daytime sleepiness and the breathing interruptions of sleep apnea can have a huge impact on the life of a child or adult," Dr. Chervin notes.

More information: The National Sleep Foundation.