Studies show that some people remain dangerously impaired into the morning hours after taking sleep drugs, even as they climb behind the wheel of a car. (Mau Kae Horng)

The Food and Drug Administration said Thursday it will require manufacturers of popular sleep medications such as Ambien to scale back their recommended dosages for women, in light of new data showing some people remain impaired well into the next morning, even as they climb behind the wheel to head to work.

The new edict will affect the labeling of drugs that contain zolpidem, a widely used ingredient in sleep aids. FDA officials settled on the new rules after driving simulations and laboratory studies showed that eight hours after taking the medication, as many as 15 percent of women and 3 percent of men still had enough of the drug in their systems to impair driving “to a degree that increases the risk of a motor vehicle accident.”

An even higher percentage of patients experienced a lack of mental alertness the morning after using extended-release zolpidem products, the agency found.

“After analyzing these data, we felt it necessary to add new drug safety information into the drug’s labeling,” Ellis Unger, director of the FDA’s Office of Drug Evaluation, told reporters Thursday. “We have a particular concern about driving.”

The move will require drugmakers to slash recommended doses by half for women, from 10 milligrams to 5 milligrams for immediate-release products and from 12.5 milligrams to 6.25 milligrams for extended-release products. The agency also said drugmakers should consider lowering recommended doses for men and should more elaborately describe on their labels the potential next-morning effects posed by zolpidem.

There have been numerous incidents in recent years involving accidents caused by drivers allegedly under the influence of sleep medications. Last month, for example, a New Jersey man was sentenced to more than three years in prison for causing a fatal head-on collision while impaired by Ambien. In 2006, then-Rep. Patrick J. Kennedy (D-Mass.) faced charges after he crashed into a barricade near the Capitol in the middle of the night. He said he was disoriented from taking prescription drugs, including Ambien.

Unger said Thursday that the FDA has received about 700 “adverse event” reports in recent years, but in most cases it was difficult to determine whether the driver’s impairment was specifically related to zolpidem or whether other factors played a role, such as the use of alcohol or other drugs.

He said the next-morning effects of zolpidem became clearer after driving simulations conducted during the approval process for another sleep medication, Intermezzo, which was approved last year. Those findings, combined with laboratory blood tests provided by manufacturers, showed that the level of drowsiness resulting from zolpidem can last longer than initially was expected.

The studies showed that women eliminate the drug from their bodies more slowly than men, although it isn’t clear why, Unger said. Officials also cautioned that people with high levels of zolpidem in their blood can remain impaired even if they feel wide awake.

“All insomnia drugs are potent medications, and they must be used carefully,” Russell Katz, director of the FDA’s Division of Neurology Products, said in a statement.

FDA officials said people taking zolpidem, which also includes popular drugs such as Edluar and Zolpimist, should continue with their prescribed doses until consulting with a doctor.

Unger said the FDA will continue to evaluate the risk of next-morning impairment with other insomnia drugs, and it will push manufacturers of new sleep medications to conduct more research during the approval process.

“We are asking sponsors of medications that will be used for sleep to do these kinds of driving simulation studies for new drugs that are being developed,” he said.

According to the agency, doctors dispensed about 39 million prescriptions for zolpidem products in 2011, and about 9 million patients received zolpidem products from U.S. outpatient retail pharmacies. Sixty-three percent of those patients were women.