The Food and Drug Administration is worried that a sleeping pill you take tonight could make for a riskier drive to work tomorrow.

In its latest effort to make sure that the millions of Americans taking sleep medications don't drowsily endanger themselves or others, the agency on Thursday said it will require the manufacturer of the popular drug Lunesta to lower the recommended starting dose, after data showed that people might not be alert enough to drive the morning after taking the drug, even if they feel totally awake.

The current recommended starting dose of eszopiclone, the drug marketed as Lunesta, is 2 milligrams at bedtime for both men and women. The FDA said that initial dose should be cut in half to 1 milligram, though it could be increased if needed. People currently taking 2 and 3 milligram doses should ask a doctor about how to safely continue taking the medication, as higher doses are more likely to impair driving and other activities that require alertness the following morning, the agency said. 

“To help ensure patient safety, health care professionals should prescribe, and patients should take, the lowest dose of a sleep medicine that effectively treats their insomnia,” Ellis Unger, of FDA's Center for Drug Evaluation and Research, said in a statement.

In 2013, the FDA said, approximately 3 million prescriptions of Lunesta were dispensed to nearly a million patients in the United States. Lunesta, made by Sunovion Pharmaceuticals, also recently became available in generic form. The new rules, including changes to existing labels, will apply both to the brand-name and generic forms of the drug.

FDA officials said the decision came, in part, after seeing findings from a study of 91 healthy adults between the ages 25 and 40. Compared to patients on a placebo, those taking a 3 milligram dose of Lunesta were associated with "severe next-morning psychomotor and memory impairment in both men and women," the agency said. The study found that even people taking the recommended dose could suffer from impaired driving skills, memory and coordination as long as 11 hours after taking the drug. Even scarier: The patients often claimed that they felt completely alert, with no hint of drowsiness.

Drowsiness is a common side affect of all insomnia drugs. The FDA said it would continue to examine the risks to mental alertness of all sleep drugs, even those sold over the counter, and could take broader action in the future.

Thursday's news comes nearly a year and a half after the FDA said it would begin requiring manufacturers of popular sleep medications such as Ambien to scale back their recommended dosages for women, in light of data that detailed some people remain impaired well into the next morning after taking the drugs. 

That move affected 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 discovered.

At the time, the FDA said it would require drugmakers to reduce 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.

The agency's concern, at least in part, involves the disturbing problems that can arise from sleep medications that don't wear off before people climb behind the wheel to head to work in the morning. In one instance, 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.