BAKERSFIELD, Calif. -- As many as 18 million Americans suffer from sleep apnea.

During sleep, the airways become temporarily blocked, often enough to cause the person to stop breathing for a brief period of time.

Previous studies have linked sleep apnea to an increased risk for heart disease and stroke, but new research has suggested the condition may be even more serious.

One study from Australia and another from Wisconsin found that middle-age adults with severe sleep apnea were four to six times more likely to die than those without sleep-breathing problems.

The research showed sleep apnea increases the chance of dying even beyond other risk factors such as advancing age or obesity levels.

Sleep apnea is often treated with a continuous positive airway pressure machine, which delivers air through a face mask while the patient sleeps. The device makes it less likely that a person's oxygen flow will be disrupted during the night.