A study by doctors in Austria has concluded that the attacks, which have been the subject of controversy, actually do exist.

They concluded that the attacks which can happen at any time are caused by the medication taken by patients to alleviate symptoms of the disease.

Dr Carl Nikolaus Homann and colleagues at the Karl Franzens University Hospital in Graz, Austria, reviewed studies of 124 patients with Parkinson's disease.

They found that 30% had suffered at least one sleep attack.

Medication link

After studying the type of medication these patients were taking, the doctors concluded that all types of dopamine drugs were associated with sleep attacks.

They found that the risk of attacks did not change if the patients were taking low or high doses of the drugs.

However, the doctors also found that some people suffered attacks while driving.

In 10 incidents, the attack led to road crashes.

Writing in the British Medical Journal, Dr Homann said: "In 17 cases the sleep event happened during driving, leading to road crashes in 10."

He said those figures highlighted the need for doctors to continue to warn patients with Parkinson's about the risks of driving.

No drive ban

Dr Homann said: "Despite the potential danger from driving, experts believe that sleep attacks are too infrequent to recommend that patients taking dopamine stop driving.

"Studies suggest that whether or not patients with Parkinson's take dopamine drugs they do not cause more road crashes than age matched controls.

"Recommendations for driving other than informing patients of a potential risk, should be made with caution until more data are available."

The Parkinson's Disease Society urged patients to discuss any concerns with their doctors.

A spokesman added: "Sleep problems are often caused when people experience a change in their response to Parkinson's medication."

A spokesman for Pharmacia, which manufactures drugs for Parkinson's, said its product information leaflets advise patients against driving.

"Our labelling clearly advises doctors and patients being treated with these medicines that they should not drive or engage in other activities where impaired alertness could put themselves or others at risk of serious injury or death."