Sleep apnea is tied to a higher risk of stroke, heart attack and early death.

Those suffering from sleep apnoea are at increased risk of cancer mortality, researchers from Valme University Hospital, the University of Barcelona and the La Fe University Hospital in Valencia, Spain have found.

Researchers examined the health condition of more than 5,600 patients from seven different sleep clinics in Spain to find out the link between sleep apnoea and cancer mortality.

They examined the severity of sleep apnoea using a hypoxaemia index which measures the amount of time during night that a person suffers from low levels of oxygen in the blood (less than 90% oxygen saturation).

People with sleep apnoea had approximately double the relative risk of death due to cancer (odds ratio 1.94), than people without sleep apnoea. The results showed that this association was even higher in men and younger people.

"We found a significant increase in the relative risk of dying from cancer in people with sleep apnoea. This adds to evidence presented earlier this year that found for the first time a link between cancer and sleep apnoea mortality. Our research has only found an association between these disorders but this does not mean that sleep apnoea causes cancer," said Dr Miguel Angel Martinez-Garcia, researcher at the La Fe University Hospital.

Similar results were also found in another study which showed an increase in all-type cancer incidence in people with severe sleep apnoea. Both the studies were presented at the European Respiratory Society's (ERS) Annual Congress in Vienna recently.

In the third study, researchers had conducted an experiment on mice with skin cancer (melanoma) to find out how tumour spread (metastasis) was associated with sleep apnoea.

The study found that the spread of cancer was more abundant in mice suffering from sleep apnoea compared to those with normal breathing.

"The data from this study in animals strongly suggests a link between the spread of cancer and sleep apnoea. This provides strong evidence to encourage further study in this area to understand in more detail the links between sleep apneoa and cancer," said Professor Ramon Farre, researcher at the University of Barcelona.

Sleep apnoea patients can be treated using continuous positive airway pressure (CPAP) therapy, which generates a stream of air to keep the upper airways open during sleep.

"Further studies are necessary to corroborate our results and analyse the role of CPAP treatment on this association. We hope the findings of our studies will encourage people to get their sleep apnoea diagnosed and treated early to help maintain a good quality of life," said Dr Francisco Campos-Rodriguez from Valme University Hospital.