Scientists believe this may mean that women are less sensitive than men to anaesthetic drugs that promote unconsciousness and could need more anaesthesia during surgery.

The finding may also help explain why three times more women than men have complained of being conscious curing surgery.

Dr Tong Gan, an anaesthesiologist at Duke University Medical Center, North Carolina, said: "This is the first time we have seen a difference in how men and women wake up after anaesthesia is withdrawn, and that gender effect is both unexpected and strong.

"It shows that women have a higher chance of being aware during surgery than men, and indicates women may need significantly more anaesthesia than men to keep them asleep."

Currently, doses of anaesthetic are based on body size, which means that women are typically given less anaesthesia than men, although drug levels are often adjusted during the operation if a patient shows signs of returning consciousness.

Anaesthesiologists who took part in this study used a new standardised technique that delivered the same concentration of an anaesthetic drug to patients, and then monitored their level of consciousness and how quickly they awoke after the drug was stopped.

Conservative approach

Dr Gan said the phenomenon of patients being aware during surgery was rare because many doctors tend to give more anaesthetics than is needed to ensure the patient stays asleep.

But he said that when a patient did regain consciousness during surgery the effect could be "devastating".

Dr Gan said the reason why women appeared to wake earlier was unknown.

It could be that they are less sensitive to anaesthetics than men, or that they metabolise them faster.

Anaesthesiologists constantly maintain a delicate balance between keeping patients asleep, but as lightly sedated as possible, for fear of developing side effects such as reduced blood pressure.

The study was based on monitoring 96 men and 178 women who underwent a range of different types of surgery.

They were all given the same doses of a drug called propofol, along with a painkiller called alfentanil and nitrous oxide, which also controls pain and awareness.

The researchers discovered that female patients woke up in an average of about seven minutes, compared to more than 11 minutes for men in the study.

Professor Leo Strunin, president of the Royal College of Anaesthetists, said the readings produced by the equipment used in the US research was open to different interpretation.