Katie Thomas / New York Times News Service

The first test for a new sleep drug is — unsurprisingly — how safely it puts people to sleep. Now comes a second test: how safely it lets people wake up.

The Food and Drug Administration is taking heightened interest in the issue, as new evidence suggests what many people have long suspected: the effects of common prescription sleep aids like Ambien can persist well into the next day. Of particular concern is whether people who take the drugs before bed can drive safely the next morning.

Consumer advocates have warned for years about possible links between sleep drugs and car accidents. In one prominent example, Kerry Kennedy, the former wife of Gov. Andrew M. Cuomo, was arrested last year after tests showed she had taken a generic version of Ambien before swerving her car into a tractor-trailer.

The FDA’s actions are part of a robust national conversation about how to cope with the throngs of drivers who take to public roads every day under the influence of prescription drugs. Law enforcement authorities have struggled with how to prosecute those who are impaired, especially when they have a prescription. A government survey in 2007 found that nearly 5 percent of daytime drivers tested positive for prescription or over-the-counter medications.

Doctors wrote close to 60 million prescriptions for sleep aids in the United States last year, according to the research firm IMS Health, but experts say testing how these drugs affect driving is not easy. Nonetheless, the FDA has been unusually active.

Last month, it rejected an application by Merck to approve a new sleep drug, suvorexant, in part because tests showed that some people had trouble driving the next day. In May, the agency warned patients taking common allergy drugs like Benadryl against driving, noting that the sedating effects can sometimes last into the following day. In January, citing similar concerns, the FDA took the unusual step of requiring that all manufacturers of zolpidem, the generic name of Ambien, cut in half the dosage for women.

The agency has since said that it is taking a closer look at all insomnia drugs on the market, and will ask manufacturers to conduct more extensive driving tests for all new sleep drugs. It will also more closely scrutinize any drug that causes drowsiness.

Part of the problem is that the standard warnings on many drugs — against driving or operating heavy machinery — are often brushed aside. “It would be so convenient and it would be so good if you could just tell people, don’t drive unless you feel OK,” Dr. Ronald Farkas, the clinical team leader for the FDA’s division of neurology products, told a group of industry experts at a conference in February. “I think this has penetrated now that this is not adequate. It is still good advice that, if you feel impaired, don’t drive. But if you feel fine, you might be impaired.”

For years, traffic safety officials have called, without success, for a so-called “safe list” of drugs that do not affect driving. Some people react more strongly to drugs than others do, making it difficult to predict a person’s individual risk. And driving tests can vary significantly, like studies conducted over public roadways or those done in computer simulators.

“It seems like a really simple question, but it’s really difficult,” said Dr. Matthew Rizzo, a professor of neurology at the University of Iowa, who studies driving abilities in people with cognitive impairments caused by conditions like Alzheimer’s disease.