Most patients worry before an operation about waking up during the surgery unable to move or cry out and their fears are not without grounds.
According to Britain's top anaesthetists, as many as one or two people per 1,000 experience awareness after being given a general anaesthetic.
Though this equates to between 1,000 to 2,000 cases in the UK each year, many anaesthetists believe that this is an under-estimate and that anaesthetic awareness is under-reported by those who experience it.
Awareness during surgery usually happens because the anaesthetist has misjudged the amount of anaesthetic required to keep the patient unconscious, and the patient has been given a muscle relaxant to make sure they remain absolutely still during surgery.
Dr. Keith Myerson, a consultant anaesthetist and a spokesman for the Royal College of Anaesthetists, says anaesthetic drugs can lower blood pressure and reduce the supply of blood to the brain and other organs and certain patients are more at risk from anaesthesia.
For patients with a weak heart the side-effects of anaesthetic drugs may be more dangerous in certain types of operation and when general anaesthesia is used during an emergency Caesarean a delicate balance is needed so that while sufficient drugs must be given to the mother during the major surgery they must not affect the baby.
Also whether patients smoke, take prescribed or illegal drugs, have a high proportion of body fat or are habitual drinkers can affect how their response to anaesthetic drugs.
Research suggests that people who have undergone an episode of awareness are more likely to suffer another and that one patient in three who experiences awareness feels pain.
Experts say when the experience is pain-free, waking up on the operating table is still extremely traumatic and can cause lasting psychological damage - flashbacks and nightmares are experienced and many have panic attacks and develop phobias.
Many patients' lives are completely and tragically undermined and some suffer a lifetime of problems.
Surgical and anaesthetic techniques have improved tremendously in the last decade and the likelihood of waking up on the operating table is far less likely.