The results of the clinical study by Australian and Icelandic researchers, led by RMIT University, has been published in the journal Sleep.

It has widespread implications with around one-in-four Australian men over 30 who have some degree of sleep apnea - a serious condition that occurs when a person's breathing is repeatedly interrupted during sleep.

The cause of Alzheimer's disease is still unknown but amyloid plaques that are toxic to brain cells are known indicators of the disease.

The latest research shows these plaques start in the same place and spread in the same way in the brains of people with obstructive sleep apnea, just like those in people with Alzheimer's.

Study leader Professor Stephen Robinson said scientists already knew the two diseases were somehow linked.

"We know that if you have sleep apnea in mid-life, you're more likely to develop Alzheimer's when you're older, and if you have Alzheimer's you are more likely to have sleep apnea than other people your age," he said.

"The connection is there but untangling the causes and biological mechanisms remains a huge challenge,"

The study is the first to find Alzheimer's-like amyloid plaques in the brains of people with clinically-verified obstructive sleep apnea.

"It's an important advance in our understanding of the links between these conditions and opens up new directions for researchers striving to develop therapies for treating, and hopefully preventing, Alzheimer's disease," Prof Robinson said.

The severity of sleep apnea is linked with a corresponding build-up of amyloid plaques.

The study found that using a continuous positive airway pressure machine - a standard treatment for people with moderate to severe sleep apnea - made no difference to the frequency of the plaques found in the brain.

Obstructive sleep apnea affects more than 936 million people worldwide and up to 30 per cent of elderly people.

Alzheimer's is the most common form of dementia, affecting up to 70 per cent of all people with dementia, with age the biggest risk factor for developing the disease.

The study investigated the extent of Alzheimer's-like indicators in autopsy tissue of the hippocampus - the part of the brain associated with memory - of 34 people and the brainstems of 24 people with obstructive sleep apnea.

Australian Associated Press