Cutting junior doctors' working hours will not reduce fatigue and the risk of errors on its own, a study says.
Junior doctor working hours are being reduced
Researchers found changes in rotas and working nights were at the heart of the problem in a poll of 1,400 medics.
UK doctors agreed more needed to be done, and warned badly designed rotas were a concern.
The Occupational and Environmental Medicine journal study was done by researchers at Massey and Auckland universities in New Zealand.
The study is published as many western countries look to reduce the hours their junior doctors work.
In the UK, they are no longer allowed to work over 58 hours and this must be reduced to 48 by 2009 under the European Working Time Directive.
The doctors who were quizzed were working a variety of hours from 40 a week to over 70.
Each participant was given a fatigue risk score by combining 10 aspects of work patterns and sleep in the preceding week.
Some 30% of respondents were classified as "excessively sleepy" - twice the rate of the general population.
One in four said that since becoming a doctor they had fallen asleep at the wheel of their car while driving home and two thirds said they had come close to falling asleep in the past year.
Two thirds also said that they had made a mistake associated with fatigue at some point in their careers, while more than four out of 10 said that this had happened in the preceding six months.
But the study found night shifts and unscheduled changes to rosters were more consistently linked to sleepiness and errors than the total number of hours worked.
For example, while hours worked did reduce the risk of sleepiness it did not change the risk of feeling sleepy at the wheel or a fatigue-related clinical error.
Whereas night work increased the risk of sleepiness at the wheel by a half and shift changes increased the risk of error by a third.
Dr Phillip Gander, of Massey University's Sleep Research Centre, said: "We conclude that long work hours are not the only aspect of work patterns that needs to be managed to reduce sleepiness and fatigue-related clinical errors among junior doctors.
"The findings support the view that a more comprehensive risk management approach is needed to reduce doctors' sleepiness and improve patient safety."
Dr Masood Ahmed, deputy chairman of the British Medical Association's junior doctors committee, said: "When designing rotas, the NHS must recognise the importance of doctors having enough time to rest beforehand and afterwards.
"They must also provide adequate rest rooms. Unfortunately, too many are more focussed on saving money than improving patient safety, and are withdrawing rest facilities."
A Department of Health spokesman said the government was working hard to get a balance for junior doctors.
"Working patterns must provide a sensible balance between supporting patient services, doctors working lives and their training."