Junior doctors regularly work at night
However, sleep deprivation does not appear to compromise their ability to recognise that they might be wrong.
Fifteen were senior house officers (SHOs) with around five years experience since qualifying, the remainder were house officers with an average of six months experience since qualifying.
After a night on call, averaging just under five hours sleep for the house officers, and just over three for the SHOs, they were asked a range of clinical questions.
Exam questions
These were selected from final exam papers and published questions typical of membership exams for the Royal College of Physicians.
The doctors were also asked to score their confidence in the answers they had given.
Energy levels and overall confidence were much lower than would be expected for the general population.
Doctors who had longer periods of unbroken sleep, rather than total hours slept, answered significantly more questions correctly.
But shorter periods of unbroken sleep did not impair doctors' ability to doubt the accuracy of an incorrect answer, or to say that they didn't know.
And sleep loss did not undermine knowledge and experience.
SHOs answered significantly more questions correctly than did their more junior colleagues, and gave fewer 'don't knows.'
They were also significantly more confident of their answers.
The authors conclude that although doctors deprived of sleep are more likely to make mistakes, their ability to recognise that they have done so is not impaired.
And they suggest that this insight may help doctors compensate, by deliberately taking longer over a task or asking for a second opinion.
Encouraging
Professor Sir George Alberti, president of the Royal College of Physicians, said: "This interesting study confirms that sleep deprivation, not surprisingly, is not good for junior doctors.
"However, it is encouraging that the impact was less that we might have expected. It also shows that more experienced doctors perform better than less experienced ones - which we would hope and expect.
"Unfortunately, we don't know how much better these same doctors would do given a full nights' uninterrupted sleep.
"Overall the study confirms that all doctors should be allowed a proper night's sleep - or be off immediately after one night on call."
This will be covered for the introduction of the new European Working Time Directive.
Dr Trevor Pickersgill, chairman of the British Medical Association's junior doctors committee, said the research emphasised how important it was that junior doctors were given protected periods of rest.
However, he said many juniors were working more than they were supposed to under the terms of the "New Deal" negotiated in 1991.
He said: "The conclusion of this research is clear. Doctors are more likely to make errors if they are sleep deprived."
The research is published in the Postgraduate Medical Journal.