Want to know if a doctor-in-training is stressed out or tired and about to make a big mistake? Just ask.

A study published on Tuesday finds that resident doctors know when they are exhausted, upset or overwhelmed and when they are, they are far more likely to admit they made an error.

What they are admitting is that sleepiness is not the only factor. Above and beyond that is general distress and mental fatigue, researchers reported in the Journal of the American Medical Association.

"While fatigue is important, there is this whole domain of distress beyond fatigue that also demands attention," Dr. Colin West of the Mayo Clinic in Rochester, Minnesota, who led the study, said in a telephone interview.

The findings may help point to ways to better reduce the burdens on resident doctors, known in some countries as junior doctors, and in turn prevent mistakes.

"I think this is going to have an impact on healthcare reform," West said. "We need (to put) resources into training and medicine to control work hours and maintain physician well-being."

The U.S. Institute of Medicine reported in 1999 that between 48,000 and 98,000 Americans die each year from preventable medical errors ranging from drug overdoses to infections caught in the hospital.

And doctors, unions and other experts have been clamoring to cut the hours worked by residents, who once routinely put in 100 to 120 hour work-weeks and who still are required to work at least 80 hours a week at most training hospitals.

West and colleagues surveyed 356 residents at 163 medical schools globally.

They found that 39 percent reported making at least one major medical error during the study period, and these doctors were also more likely to say they were sleepy, fatigued or stressed.

"What we have shown in these data is that fatigue is important ... but it's only part of the issue and previous studies have not paid much attention to the distress factors," West said.

"Over the course of their training and the course of their career everyone has made a major error. Everyone tries to be perfect but no one is."

He said merely limiting work hours may not be enough.

Teaching hospitals across the United States have moved to limit residents' work weeks to 80 hours to reduce fatigue-related errors. The Rand Corporation in Santa Monica, California, estimates this could cost $171 million to $487 million a year, depending on whether additional residents are hired or substitute providers are brought in.

"Targeting work hours may not be enough," West said.