Sleep apnoea usually jolts people awake

They say a cumulative loss of cells in the area of the brain that controls breathing is to blame - triggering a condition called central sleep apnoea.

However, they believe many such deaths in elderly people are misdiagnosed as heart failure.

The study, by the University of California, Los Angeles, is published in Nature Neuroscience.

SLEEP APNOEA Central sleep apnoea: Triggered by problems with the brain's breathing centre Obstructive sleep apnoea: Breathing stops when the airway collapses

The researchers had previously pinpointed a region of the brainstem they dubbed the preBötzinger complex (preBötC) as the command post for generating breathing in mammals.

They had also identified a small group of cells within this area as being responsible for issuing the commands.

Cells killed

In the latest study, they injected rats with a compound to kill more than half of these cells - and then monitored the animals' breathing patterns.

When the animals entered the rapid eye movement phase of sleep - when dreaming occurs - they stopped breathing completely, and were jolted into consciousness in order to start again.

Over time, the breathing lapses increased in severity, spreading to other phases of sleep, and eventually occurring when the animals were awake as well.

Rats possess 600 of the specialised cells. The researchers believe humans have a few thousand, which are slowly lost over a lifetime.

Lead researcher Professor Jack Feldman said: "We speculate that our brains can compensate for up to a 60% loss of preBötC cells, but the cumulative deficit of these brain cells eventually disrupts our breathing during sleep.

"There's no biological reason for the body to maintain these cells beyond the average lifespan, and so they do not replenish as we age.

"As we lose them, we grow more prone to central sleep apnoea."

The UCLA team believes that central sleep apnoea may pose a particular risk to elderly people, whose heart and lungs are already weaker due to age.

They also suspect the condition strikes people suffering the late stages of neurodegenerative disorders, such as Parkinson's disease.

These people often have breathing difficulties during sleep, and the researchers believe their bodies eventually reach a point where they are unable to rouse themselves from sleep when they stop breathing.

The UCLA team plans to analyse the brains of people who die from neurodegenerative diseases to determine whether these patients show damage in their preBötzinger complexes.

Frank Govan, of the UK Sleep Apnoea Trust, told the BBC News website that previous work had linked cot death to obstructive sleep apnoea - caused by collapse of the airways.

However, he said science had failed to prove the link.

He said: "These chaps may well be right, that the link is between central sleep apnoea - rather than obstructive sleep apnoea - and cot death, and unexplained adult death."