In an episode of the “Seinfeld” TV show, Kramer complained that his new romantic interest had the “jimmy legs.”

“She’s throwing off my whole sleep,” he said, and then demonstrated by raising one leg and juddering it in the air.

Turns out that the likelihood of having jimmy legs — or restless legs syndrome, as doctors call this sleep disorder — can vary greatly by sex and race, researchers at the University of Missouri-Kansas City have discovered.

The UMKC researchers presented their findings earlier this month at an international conference in San Diego.

Researchers interviewed 190 patients at a primary care clinic and found that 23 percent had the syndrome. That’s far more than the 3 to 10 percent rate found by other studies and may have been due to the kinds of patients the clinic sees.

“There will likely be a difference between populations attending medical clinics as opposed to those who are well and healthy,” said researcher Ammar Alkhazna.

The UMKC data also show that only 12 percent of the African-American men and women who were interviewed had restless legs syndrome. Among non-African Americans, 29 percent of men and 40 percent of women had the jimmy legs.

Women are more likely than men to have risk factors such as iron deficiency or rheumatoid arthritis, researchers said. But the racial disparity remains a mystery.

“There are significant ethnic differences in the prevalence of restless legs syndrome, but the exact causes of higher prevalence among Caucasians are unknown,” Alkhazna said.