Children with at least one parent who snores are three times more likely to snore themselves than kids whose moms or dads are silent sleepers, a study of one-year-olds has found.

The study by the Cincinnati Children's Hospital also determined that youngsters with sensitivities to environmental and food allergens such as dust, pollen and eggs were twice as likely to snore.

Although ragged, stertorous breathing while tots are in the Land of Nod may seem annoying at worst, it can actually be unhealthy.

"Frequent snoring is not benign," lead author Dr. Maninder Kalra, a specialist in pulmonary medicine, said in an interview from Cincinnati. "Frequent snoring in children has been shown to be associated with behavioural problems, learning difficulties as well as cardiac and metabolic problems."

Snoring occurs when air flow is reduced, usually because of obstruction by the tongue or other parts of the mouth and throat, which relax during sleep. This causes tissues or muscles in air passages to vibrate. The narrower the airway becomes, the greater the vibration - and the louder the snoring.

In children, a low-set, soft palate or enlarged tonsils and adenoids can narrow the airway, leading to snoring, as can nasal blockages caused by allergies.

Kalra said hyperactivity is more common among children who habitually snore. In fact, earlier research has shown that children who snore are four times as likely as non-snorers to have attention-deficit hyperactivity disorder, or ADHD.

A recent study reported that having enlarged tonsils surgically removed eliminated symptoms of ADHD in some children.

But the greatest concern related to snoring is sleep apnea, a condition in which relaxed tissues and muscles narrow the airway so much that it closes, causing breathing to stop for up to 10 seconds at a time - or even more, in some cases. Sufferers awake briefly when their brain detects a resulting drop in oxygen and their breathing resumes. This process can occur several times during the night.

Besides disturbing sleep, the interrupted breathing of sleep apnea can eventually lead to serious heart problems, because the repeated bouts of inadequate oxygen make the heart pump harder than normal. In extreme cases, apnea-induced oxygen deprivation can cause brain damage.

"So frequent snorers should be evaluated for sleep apnea," said Kalra.

The study evaluated 681 children, aged 12 months on average, and found that 15 per cent had frequent snoring, defined as occurring at least three times a week. Twenty-nine per cent of the kids studied had a sensitivity to allergens, a condition called atopy that can lead to allergies and asthma.

An increased prevalence of habitual snoring was found in youngsters with sensitivities to allergens (21.5 per cent versus 13 per cent of non-sensitive kids) and in children with a parental history of frequent snoring (almost 22 per cent compared to eight per cent).

"The fact that this aggregates in families suggests that there are hereditory factors that are important in the development of snoring," Kalra said. "We still don't know what are the genes that lead some children to develop snoring and sleep apnea and others not to have it."

But he said the genetic link appears to be more strongly related to males: among parents, habitual snoring was reported in 46 per cent of fathers compared to just 20 per cent of mothers. However, the propensity to gasp and snort during sleep was roughly equal among girls and boys, said Kalra, whose study is published in the April issue of the journal Chest.

"Untreated, childhood sleep-disordered breathing can have a significant effect on a child's health, behaviour and cognitive development," said Dr. Michael Alberts, president of the American College of Chest Physicians, commenting on the study.

"By knowing potential risk factors for sleep-disordered breathing in children, clinicians can identify high-risk groups and educate patients and families on how to modify risk factors before they have a long-term impact on health."

The study of one-year-olds is the first step in determining the natural evolution of childhood snoring, said Kalra, adding that his team will follow the children over time to see if snoring progresses as they age.

"Does it stop, continue or escalate?" he said. "We will continue to follow these children through age five years to answer these questions."