Narcoleptics can fall asleep at any time - even at the wheel of a car. But what causes the overwhelming need to drop off?

Anyone who has gone two nights without sleep will know what it is like to suffer from extreme doziness. The siren call of slumber beckons irresistibly. Sleep creeps up to set the head nodding and the eyelids fluttering. Nothing can defeat the desire to lie down.

Anyone who has gone two nights without sleep will know what it is like to suffer from extreme doziness. The siren call of slumber beckons irresistibly. Sleep creeps up to set the head nodding and the eyelids fluttering. Nothing can defeat the desire to lie down.

For sufferers from narcolepsy, a rare sleep disorder, fighting weariness is a way of life. They live every day as if the previous 48 hours have been sleepless. Cathy Hughes knew things were serious when she slept through her GCSE English exam at the age of 16. "I put my head on the desk and that was it. I failed the exam and I had got an A in the mock."

In the years since - she is now 32 with two children - her life has been a perpetual struggle against being overwhelmed by sleep. "From when I get up in the morning I am permanently fighting sleepiness. My greatest fear is falling asleep with my children. I woke up once with my son, then a baby, lying across my shoulder. I remember making a bottle and going to wake him and that was it. I had been asleep for hours."

For years, doctors put Ms Hughes's complaints of being tired all the time - known by its acronym TATT by GPs, so common is it among their patients - down to depression. But her tiredness was of a different order from the run-of-the-mill kind. "I woke up on a train once, miles past my stop, in the pitch dark after I had fallen asleep. It was quite scary. Once, I woke up in the street with a black eye. I had been out with friends but I am not a big drinker."

About 2,500 people have been diagnosed with narcolepsy in the UK, but up to 10 times that number are thought to be affected by the disorder. Failure to diagnose patients can blight their lives if they are dismissed as lazy or drunk.

Ms Hughes, who is setting up a support group, NAPPS, to increase awareness of the condition, said problems caused by sleepiness and sleep deprivation were being overlooked. "Given that 90 per cent of sufferers are not being diagnosed or being wrongly diagnosed it begs the question of how many road accidents are caused or kids become social rejects. There is a huge issue here."

Falling asleep at the wheel of the car was what eventually drove Kerry James to seek medical help. Aged 38, from west London, she had to retire from her job as a college lecturer, after her condition became so bad that she frequently fell asleep during meetings, over meals and once even had to find an empty teachingroom in which to take a nap on the floor.

Ms James, who lives with her partner Stuart and two children, was eventually diagnosed four years ago, since when she has been taking drugs and can now go virtually through the day without needing to nap.

"The need to sleep affected every aspect of my life. I have had to accept that I will never lead a normal life. But it was such a relief to have the condition diagnosed and finally get help for it," she says.

The causes of narcolepsy are little understood but new research suggests it may be a disorder of the immune system. Sufferers are deficient in the hormone hypocretin, which controls behaviour including sleep and is produced by the hypothalamus, a gland inthe brain.

Researchers at Stanford University School of Medicine in California who studied the brains of people with narcolepsy which had been removed and preserved shortly after their deaths found they had lost the cells that produce the hormone. Writing in the journal Nature Medicine this month they suggest that the cells may have been destroyed by the sufferer's own immune system. Narcolepsy may be an auto-immune disease like diabetes and rheumatoid arthritis.

The discovery will aid the search for more effective treatments. For many victims of narcolepsy, existing treatments leave their symptoms inadequately controlled. Sedatives that improve sleep at night - narcoleptics often have disrupted sleep - have a hangover effect which leaves them sleepier next day. Stimulants, on the other hand, keep them alert during the day but give them insomnia the next night.

A drug licensed two years ago for the treatment of narcolepsy, called Provigil (chemical name modafinil), targets the hypothalamus although its exact mode of action is not understood. Tests in normal volunteers showed it boosts cognitive performance, especially memory, and reaction times in the sleep-deprived. Military organisations in France, the US and Britain are understood to have shown interest. There would be an obvious military advantage in a pill that could help armies fight through the day and march through the night.

NAPPS, PO Box 113, Pudsey, Leeds, LS28 7XG. Also at www.napps.cwc.net.

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