It's been 15 years now, but Dr. Sanjay Desai remembers the brutal hours he worked as a young medical intern and how he struggled with fatigue while treating patients.

"There were days we were easily working 36 hours straight and you couldn't remember how you got home - if you got home," Desai said. "It wasn't safe."

Times have changed. Regulations now demand that teaching hospitals limit first-year trainees to 16-hour shifts. By reducing work hours, medical authorities reasoned, interns would get more sleep, suffer less fatigue and commit fewer mistakes.

But a pair of studies published Monday in JAMA Internal Medicine suggest this may not be the case. Researchers concluded that interns were making more mistakes and learning less after the shift restrictions.

The problem, according to researchers, was that interns were required to do the same amount of work but in fewer hours. Shorter shifts also require more patient "handoffs" from one doctor to another. Handoffs are a common source of medical errors, previous studies have demonstrated.

"I was surprised," said Dr. Srijan Sen, a University of Michigan Medical School psychiatrist and lead author of one of the studies. "It was clear that interns do not function optimally after working 24 hours in a row. So there was real hope that reducing the maximum shift length would help both interns and patients. However, our results suggest that the negative unintended consequences of the reforms may outweigh any positives."

The studies no doubt will add fuel to the debate over duty-hour reforms.

In 2003, the Accreditation Council for Graduate and Medical Education reduced the maximum shift length for first-year residents, or interns, to 30 hours from 36. In 2011, the council reduced it further to 16 hours. The changes marked an enormous change in medical training: Beginning in the late 19th century, interns and residents were called "house staff" because they lived full time in their hospitals.

Sen and his colleagues surveyed more than 2,300 interns from 2009 to 2011 - a period that included the most recent shift change. Trainees were asked whether they were sleeping more, whether they felt depressed and how many serious medical errors they thought they had made.

"Despite the decrease in duty hours, there were no significant changes in hours slept, depressive symptoms or well-being," the study found. "The percentage of interns who reported concern about making a serious medical error increased from 19.9 percent to 23.3 percent."