Many junior doctors make mistakes due to tiredness even when working schedules designed to give them plenty of rest, according to research.

A study found that two-thirds of doctors had made an error at some point in their careers due to exhaustion and 42% had done so in the preceding six months.

At the time of the study, the doctors were working 40 or more hours a week on schedules designed to cut weekly working hours, give adequate rest breaks and reduce the risk of errors.

But the authors, writing in the journal Occupational and Environmental Medicine, said cutting junior doctors' working hours and giving them more rest breaks would not, by itself, reduce tiredness or errors.

The researchers questioned 1,366 junior doctors on work patterns, sleepiness and errors related to tiredness through an anonymous questionnaire.

A quarter (24%) of doctors said that, since the start of their career, they had fallen asleep at the wheel of their car while driving home. Two-thirds (66%) said they had come close to falling asleep at the wheel in the previous year, and one in five said it had happened on at least five occasions.

Each doctor was given a "fatigue risk score" by combining 10 different aspects of their work and sleep, such as access to rest and support, in the preceding week. Almost a third (30%) were ranked as "excessively sleepy", and researchers found that all the doctors were twice as likely to be "excessively sleepy" as the general population.

The higher a doctor's fatigue risk score, the higher the chance they had fallen asleep while driving or made a mistake in the last six months. The New Zealand-based doctors were more than twice as likely to report nearly falling asleep at the wheel as British male drivers, the study found.

Working night shifts or having unscheduled changes to rotas were more closely linked to tiredness and errors than the total number of hours worked, it added. But regular access to supervision reduced the chance of sleepiness and errors, irrespective of the total hours worked.

Only 13% of the doctors were working more than 70 hours a week, but just 30% were working fewer than 50 hours, the study 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 authors said. "The findings support the view that a more comprehensive risk management approach is needed to reduce doctors' sleepiness and improve patient safety."