A new drug is being launched to treat one of the most baffling of medical conditions - the sleeping disorder, narcolepsy.

2,500 people in Britain are receiving treatment, but doctors believe five times as many are affected by the condition, which can cause them to fall asleep involuntarily many times during the day, while suffering insomnia at night.

The new drug, Modafinil, is thought to have fewer side-effects than existing treatments using amphetamines.

Daniel Morgan could not hold a job down before getting treatment

Daniel Morgan, a narcolepsy sufferer, was nearly expelled from school because teachers thought he was lazy.

On leaving school, before he sought help, he used to fall asleep driving and found it hard to hold down a job.

"If I wasn't on medication and I was talking to you now I'd probably start falling asleep, people would think I'd be very very rude, but I wouldn't be able to help it at all," he said.

He used to be prescribed amphetamines to stay awake, but he found them addictive and he had to keep increasing the dose. Now he is taking Modafinil.

But the new drug is not the complete answer to the problem of narcolepsy. Treatment costs �3,000 a year, and some health authorities are refusing to pay.

2,500 patients are being treated in Britain

Dr Adrian Williams from St Thomas's Hospital Sleep Disorder Centre in London says there are practical measures patients can take "making the night sleep as good as it can be, the narcoleptic tends to be an insomniac, making adjustments at work so that patients can nap," he says.

"Napping is very refreshing, quite effective in the mild narcoleptic."

Unlike amphetamines which work by stimulating the nervous system, Modafinil targets the hypothalamus, thought to be responsible for wakefulness. Neither does it produce the emotional highs and lows associated with amphetamines.

In time, Modafinil might become a treatment for jet lag, or for people whose sleep patterns are disrupted by shift work.