The student, 20, who collapses every time she giggles

They say laughter is the best medicine, but not in Kay Underwood's case. In fact, a fit of the giggles is all it takes to cause the 20-year-old student to collapse on the spot.

Miss Underwood suffers from cataplexy - a muscular weakness triggered by emotion which can lead to anything from slackening of the facial muscles to total collapse.

Exhilaration, anger, fear, surprise, awe and even embarrassment can also cause similar reactions, but for Miss Underwood, it is a fit of the giggles which does the trick.

Victims are often left paralysed for several minutes, although they always retain the ability to hear what is going on around them.

Today she told how the condition is no laughing matter - and once caused her to collapse more than 40 times in a single day.

Miss Underwood, of Barrow-upon-Soar, Leicestershire, said: 'People find it very odd when it happens, and it isn't always easy to cope with strangers' reactions.

'Once, when I collapsed on some stairs, a woman walked past, hit me over the head and said I should have collapsed in a more convenient place!

'But I've learnt to live with it. I can tell when it's going to happen and have learnt to fall in a comfortable position or find something to lean on.'

She said the problem means she also often drops off to sleep without warning.

Cataplexy is not a condition in its own right - rather it's a symptom found only in people suffering from narcolepsy.

This is a medical condition affecting around 20,000 people in the UK which results from a fault in the mechanisms of the brain that control wakefulness and sleep.

Narcolepsy causes slurred speech and impaired vision, but hearing and awareness remain intact and the sufferer remains fully conscious during each attack, which usually last less than two minutes.

Miss Underwood has struggled with the condition since she began displaying the most common early symptom - excessive daytime sleepiness - five years ago.

The student told how, aged 15, she would arrive home from school and immediately sleep for one hour. At night, she would often wake but be unable to move while she was still dreaming.

She said: 'It was very scary. There were times, too, when I would hallucinate - seeing chairs lift off the ground and things like that.'

Miss Underwood was forced to give up her driving licence because of the condition, which was diagnosed at the specialist sleep disorders service at Leicester General Hospital, run by Dr Andrew Hall.

She is now on new medication which she hopes will transform her life and allow her to resume driving once more.

Dr Hall, a consultant in anaesthesia, intensive care and sleep disorders, said the centre treats about 200 people with narcolepsy.

He said that in the past patients were given amphetamines to help them sleep and to help keep them awake.

But over the past three years about 15 of his patients have been helping trial new drugs which could help others in the near future.

Dr Hall said: 'It is not just laughter which can make people collapse', he said.

'It is usually extreme emotion. I had one patient who it happened to whenever he had an overwhelming feeling of smugness.'

Last year, cataplexy and narcolepsy sufferer Wendy Richard told the Daily Mail how simply saying 'I love you' could be enough to cause her to collapse.

Mrs Richards, 53, from Sheffield, said jokes or funny situations could also trigger her to faint.