Each collapse lasts between 30 seconds to five minutes and at its worst Claire can endure up to 100 attacks each day. Claire, from Cambridge, described how she first loses her speech and vision before her body buckles -- but she remains completely "awake".

Dr Allen, 35, a scientist, suffers from bouts of "cataplexy", a rare symptom of narcolepsy, triggered by strong emotions -- such as fear, surprise or laughter.

However, the new drug, Xyrem puts Claire into a deep sleep for three-and-a-half hours so she must take a second dose in the middle of the night to get a full seven hours sleep.

She said: "The attacks are caused by any emotional surprise or shock but laughter is definitely the strongest trigger."

The attacks cause her to lose control of her body and she collapses into a deep sleep-although she remains conscious throughout.

She said: "For the last five years I hadn't slept for more than an hour at a time so the first time I slept for more than three hours it was incredible."

Researchers have found that narcolepsy may be caused by an irregularity to the brain cells which control the sleep hormone hypocretin.

An estimated 25,000 people in the UK suffer from narcolepsy but many remain undiagnosed due to the varying nature of the symptoms.