In cases where insomnia proves to be fatal, it is usually because of some inherited genetic feature. This is called fatal familial insomnia.

But scientists have discovered a case where a victim had no family history of the disorder, and evidence of prion activity in his brain tissue.

This means the rare condition could be infectious.

There are five other similar cases on record.

Gradual development

Reporting in the New England Journal of Medicine, researchers from California examined the case of a 44-year-old man who died of insomnia.

He gradually developed sleeping problems, and after six months had trouble walking. His eyes watered and sleeping pills were of little use.

Within a year he was confined to a wheelchair and his memory had deteriorated. He had problems telling the difference between dreams and reality.

After 16 months, hounded by delusions and hallucinations, he died of pneumonia.

The researchers added some of the man's brain tissue to mice, and found that it produced similar symptoms - a characteristic of prion diseases.

A new form of the disease

Prions are also thought by some scientists to be behind Creutzfeldt-Jakob disease - the human version of BSE.

The team, led by Dr James Mastrianni of the University of California at San Francisco, called the disease sporadic fatal insomnia.

In an accompanying editorial, Dr Pierluigi Gambetti and Dr Piero Parchi of the Case Western Reserve University in Cleveland said the research was enough to establish the condition as a new form of the disease.

"One might question the establishment of a novel disease on the basis of a single case," they said, "but this is not a concern since we have also recently described five patients with similar features."