Scientists say sleeping pills could help children with attention-deficit hyperactivity disorder (ADHD).

The condition's symptoms include lack of concentration, a need for almost constant attention and restlessness, even when asleep.

In a study involving children with restless legs syndrome and periodic limb movements of sleep, researchers found drugs used to stimulate a brain chemical known as dopamine led to better rest and curbed some symptoms of ADHD.

Classrooms may be disrupted by the disorder

Neurologist Steven Kugler at the Robert Wood Johnson Medical School in New Jersey, USA, said: "We got two sets of improvements: a marked improvement in their sleep and an improvement in their level of attention, their hyperactivity and their sociability."

He added that this might show ADHD is caused by a lack of the chemical dopamine in the brain.

The two drugs used in the study, L-Dopa and pergolide, are commonly used for people with Parkinson's disease, who lack dopamine.

The research could also help explain why Ritalin, a common drug for treating ADHD, improves hyperactive behaviour, said Arthur Walters, a neurologist at the Robert Wood centre.

"One of the major effects of Ritalin is to promote dopamine in the brain," Mr Walters said.

"What could be working is the dopamine-promoting properties, not the stimulant effect of Ritalin."

But, he added, because Ritalin is a stimulant that works on the central nervous system, many people are concerned about addiction and the drug's possible side affects.

Mr Walters presented the results of the study during the American Academy of Neurology's annual meeting in Minneapolis, USA.

Both scientists said the next step was to extend the research to include children without sleep disorders and increase the sample size of the study.

A recent review published in the American Medical Association's journal found that an estimated 3% to 6% of the US school-age population may have ADHD.

In Britain, a report from the Children's Society said that children are missing over half-a-million days in temporary exclusions from schools because of disruptive behaviour.

Although ADHD cannot be blamed for all of these, some experts say almost three in 10 children in the UK could be sufferers.