At a news conference during the annual meeting of the American College of Chest Physicians, Dr. Barbara Phillips of the University of Kentucky at Lexington presented results of the annual sleep poll conducted by the National Sleep Foundation. Phillips is president of the Foundation.

Researchers conducted a telephone survey of some 1,500 randomly selected adults aged 18 and older. Their average age was 49.

Symptoms of RLS were reported by 9.7 percent of the participants - 8 percent of all men and 11 percent of all women.

Residents of the Southern and Western US had a higher risk of RLS than those living in the Northeast US. Other risk factors were heavy smoking, unemployment status, hypertension, gastroesophageal reflux disease, arthritis, diabetes, depression and anxiety.

Sleep apnea and insomnia appear to be other risk factors for RLS, along with difficulty falling asleep (taking more than 30 minutes), driving while drowsy and excessive daytime fatigue.

Subjects with self-reported RLS also had a higher incidence of being late for work, missing work, making errors at work and missing social events because of fatigue more often than those without RLS.

"There is definitely a circadian rhythm," Phillips told Reuters Health. Patients describe their symptoms as more of an urge to move rather than actual pain, Phillips said. Sleep labs are not actually necessary to make a diagnosis of RLS, she added.

"No one really knows what causes RLS," Dr. Phillips said. "RLS is probably not a single thing...A lot of things look the same but aren't the same."

"Primary RLS probably has some genetic basis," she said. "The brain content of iron is different in RLS...Iron and dopamine stores are low...Treating iron deficiency can correct the symptoms," she pointed out.

This past summer, FDA approved the dopamine agonist ropinirole (Requip) as first-line therapy for RLS. Phillips predicts that other similar drugs will soon receive similar approval.

She called for studies to better define the diagnosis of restless leg syndrome.

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