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Recently, I saw a young friend who is training to be a surgeon. Extremely bright and the recipient of numerous medical school awards for her work with patients, she had been anxious as a student about the grueling hours she would face once she began working as a junior surgeon on the wards.

Now she was laughing over those old fears. “Training has changed a lot,” she said. “My life is different than yours was — I have a lot of time outside the hospital.” She described how she loved her work but was able to sleep at home most nights, go out regularly with friends, stay involved with her church and take an improv class.

As our conversation drifted to patient care, her voice suddenly turned less cheery. “To be honest, I don’t really know if this is better or worse,” she said, recounting how she felt she was signing over responsibility for her patients more often than she ever imagined she would, missing key events in their hospital course and even getting dismissed during the middle of a patient’s operation in order to stay within the limits on work hours.

“Sometimes it seems so counterintuitive to just sign out as if we were shift workers, but this is all any of us know right now,” she said. “We have nothing to compare it to.”

Few areas of medicine have changed as significantly over the last decade as the training of young doctors. While work-hour guidelines have been around since 1989, it wasn’t until 2003 that the organization responsible for accrediting American medical residency programs issued the first national guidelines limiting residents to no more than 80 hours of work per week. Last September, they issued a second mandate that will go into effect in July, maintaining the 80-hour restriction while adding a series of specialty-specific rules that detail everything from the length of shifts and rest periods to “optimal” clinical loads and acceptable exceptions to the work-hour rules.

Junior doctors in the European Union have faced even tighter regulations. Since 2009, doctors-in-training have been restricted to working no more than 52 hours per week; beginning in 2012, the limit will be 48 hours.

Amid these sweeping changes, experts’ opinions have remained both charged and split. All agree that fewer hours in the hospital will result in better-rested young doctors. But critics warn that the medical profession will suffer from a precipitous fall in the quality of training and thus patient care. And proponents remain adamant that the changes herald the coming of a new era in patient safety.

Now it looks like neither side is correct, according to a new report in the British medical journal BMJ that reviewed all the published data on the effects of restricting resident work hours in the United States and Europe, and both groups may be stuck opining to each another for a good deal longer.

The researchers found that while all the punch-clock sturm und drang has improved the lifestyle of junior doctors, decreasing their fatigue seems to have had little if any effect on how patients actually do.

“Intuitively, these work-hour directives seem like the obvious thing to do,” said Dr. Ramani Moonesinghe, lead author and a consultant in anesthesia and critical-care medicine at University College Hospital in London. “But improving patient care is much more complicated than reducing shift lengths or work time.”

The researchers found 34 published studies from the past decade that focused on how patients fared, but few were comparative and of a large enough scale to offer useful and definitive evidence. Only one was a large randomized controlled study. “In every other aspect of health care, we work on evidence rather than gut feelings or hunches,” Dr. Moonesinghe said. “It’s really a shame we don’t have enough research to guide us right now.”

The dearth of high-quality comparative studies is partly a result of the difficulty of finding doctors who could serve as a control group. “The work-hour rules went into place all at once and across the board,” said Dr. Leora I. Horwitz, author of the editorial reply that accompanies Dr. Moonesighe’s study and an assistant professor of general internal medicine at the Yale School of Medicine. “There are no residents who are still working 100-plus-hour weeks whom researchers can use to compare.” Researchers are often thus forced to compare trainees with physicians who are already fully trained, or with old data from the time before work-hour mandates went into effect.

But such research results in crude comparisons. The skills of an intern are not comparable to those of an experienced doctor. And retrospective data doesn’t take into account all the high-tech devices, novel drugs and patient safety initiatives that have appeared in the last decade.

Even the way junior doctors care for patients has changed. Patients are handed off from one doctor to the next more frequently than ever before. And while such handoffs have become more streamlined since the first work-hour mandates, it remains unclear how physician discontinuity affects patient care. “Every patient is different from other patients,” Dr. Horwitz said. “They aren’t interchangeable like airplanes, where pilots can be confident that one 747 jet has pretty much the same buttons and toggle switches as another 747 jet.”

While Dr. Moonesinghe’s work offers few solutions, it does point clearly to the need for more research on how the sea changes of the last decade are affecting patients. And that research must examine not only the number of resident work hours but also the implications of frequent shift changes, shift lengths and patient handoffs.

“There’s no simple path from regulations to outcomes,” Dr. Horwitz said. “This is a real wake-up call. It’s not good enough to do what seems obviously right.”