IT SEEMS like a no-brainer. Junior doctors work very long hours and fatigue is known to impair judgement, so forcing hospitals to give doctors time off should cut down on medical errors.

The truth is more complex. In 2003, US resident doctors were ordered to work no more than 80 hours per week, and no more than 24 hours at a stretch. Since these restrictions came in, patient mortality rates have remained unchanged, say Kevin Volpp of the Philadelphia VA Medical Center in Pennsylvania and colleagues (Journal of the American Medical Association, vol 298, p 975). A related study by the same team found a small benefit, but only for certain hospitals and medical conditions (JAMA, vol 298, p 984). The concern is that the benefits of additional doctors are offset by doctors passing patients to each other more frequently, which can lead to misunderstandings about a patient's condition, says Volpp. Some doctors argue that trainee medics may not be getting enough experience of medical procedures.