Scientists have uncovered evidence of a link between a common sleep disorder and brain damage.
It is usually men who are affected by sleep apnoea
Sleep apnoea occurs when the airways become blocked by the tongue or soft palate, depriving the person of oxygen and briefly waking them.
New Scientist magazine says it leads to a loss of brain cells, potentially explaining the memory and learning problems linked to the condition.
But experts say more work is needed to confirm there is permanent damage.
Up to 2% of adult males - around 300,000 people - in the UK have sleep apnoea.
Researchers from the National Heart and Lung Institute at Imperial College, London, carried out Magnetic Resonance Imaging (MRI) scans of seven patients with sleep apnoea and seven healthy patients.
They looked at the patients' brain density.
It was found the sleep apnoea patients had less brain density in the left hippocampus, where the storage of memory is co-ordinated.
Mary Morell, who led the study, said: "What I found surprising was such a clear-cut result in such a small number of people."
In rats, it has been shown that a lack of oxygen at night was linked to problems in learning and memory.
Dr Ronald Harper at the University of California, Los Angeles, said: "There is now more ample evidence that the brain is affected by obstructive sleep apnoea.
"Therapies and preventative treatments must therefore consider brain damage, in addition to the usually treated breathing problems."
But Professor John Stradling, of the Oxford Centre for Respiratory Medicine, who has seen the Imperial College researcher, told BBC News Online they had only found subtle differences,
"I would only ever regard this as a pilot study.
"The thing about sleep apnoea is that even when you get low oxygen, it's back again in a minute."
He added: "There is an argument that it's the return of the oxygen to the area that causes the damage."
But Prof Stradling added: "It's unproven that sleep apnoea causes permanent, long-term, damage.
"If you have treatment for sleep apnoea, you tend to go back to normal."
The research is also due to be published in the journal Sleep Medicine.