Nurses in U.S. hospitals about 40 percent of the time are working long shifts that raise the risk of medical mistakes such as giving the wrong medication or the wrong dose, a study released on Wednesday said.

The likelihood of a hospital nurse making a mistake was three times higher once a shift stretched past 12.5 hours, according to findings published in the journal Health Affairs.

The study, led by researchers at the University of Pennsylvania, tracked 393 hospital nurses and found about 40 percent of their shifts exceeded 12.5 hours over a four-week period.

Extended nursing shifts are growing more common as hospitals struggle with a shortage of registered nurses. Working more than 40 hours per week or working unscheduled overtime also made errors more likely.

The researchers called for fewer long shifts and less overtime. “Routine use of 12-hour shifts should be curtailed, and overtime - especially that associated with 12-hour shifts - should be eliminated,” they wrote.

Data on the impact of long nursing shifts have been limited, although many studies have linked sleep-deprived physicians to medical mistakes, the study’s lead author said.

“There are over 50 studies of physicians’ work schedules, but we have never looked at our largest group of health-care providers, which are registered nurses,” Ann Rogers, the study’s lead author, said in an interview.

Rogers, an associate professor at the University of Pennsylvania’s School of Nursing, and colleagues reviewed logbooks from 393 registered nurses throughout the country who worked full time in hospitals. The study analyzed data from 5,317 shifts over a four-week period. The longest shift lasted 23 hours and 40 minutes.

During the 5,317 shifts, there were 199 medical errors. Most were medication errors such as administering the wrong drug or the wrong dose, or giving medication later than scheduled. The study did not assess the impact of the errors on patients.

The nurses also reported 213 near errors, cases in which the nurses caught themselves before they made a mistake.

Working overtime raised the chances of making at least one error, regardless of how long the shift was originally scheduled to last, the study said.

In most cases, overtime was not scheduled in advance, Rogers said. On average, the nurses worked 55 minutes longer than scheduled each day. One-third of the nurses worked overtime every day they worked during the four weeks.

The research was funded by the federal Agency for Healthcare Research and Quality and the Robert Wood Johnson Foundation, a philanthropy devoted to health issues.

The Institute of Medicine, an independent organization that advises the federal government on medical matters, also has recommended easing nurses’ schedules, including limiting their workday to 12 hours.