Patients and doctors long have relied on a simple rule of thumb for seeking care after an ischemic stroke: “Time is brain.”

In other words, the longer it takes for patients to seek medical care after a stroke, the more their brain tissue is likely to be starved of blood and die – significantly raising the risk of permanent disability. The general rule is to seek care within six hours.

But a study published this month in the New England Journal of Medicine found that the window may be as long as 24 hours for some stroke patients. The study also indicated that outcomes were less likely to include a disability when treated with a combination of surgery and clot-busting medication.

Though doctors still agree that earlier treatment is better, the study offers hope to patients who have a stroke in their sleep or who live in rural areas where it can take longer to reach medical care, said Dr. Dileep Yavagal, chief of interventional neurology for the University of Miami Health System and a practicing physician at Jackson Memorial Hospital.

“It’s going to be quite impactful because of how many strokes happen in sleep and how many stroke patients get missed in the first few hours,” Yavagal said, adding that about 25 percent of strokes occur during sleep.

“In the past, we couldn’t offer them much because they would go to bed and then wake up with a stroke and that would mean basically more than seven or eight hours since they were last well,” he said. “And what that then meant was that they were out of luck, in terms of getting any therapy. But this study showed us that there were a number of people who, when they wake up with a stroke, had a fair amount of salvageable brain.”

The study followed the progress of 206 patients who had received medical care six to 24 hours after a stroke at medical centers around the world.