The Yale University School of Medicine study found that obstructive sleep apnea causes a decreased blood flow to the brain during episodes. When this happens, the organ's blood pressure rises. And when it happens repeatedly, the brain's ability to protect itself in such situations eventually wears down.

An earlier study by the Yale team found that people with sleep apnea are three times more likely to have a stroke or die than people of similar health who don't have the breathing disorder.

More than 18 million Americans have been diagnosed with sleep apnea, a condition in which the upper airway becomes blocked so that breathing is hampered or even ceases. This causes blood oxygen levels to drop and blood pressure to rise, but order is restored when the person momentarily awakens. The cycle repeats itself throughout the night.

The study found that those with sleep apnea had lower cerebral blood flow velocity and notably lower blood oxygen levels during sleep than did people without the condition. They also took longer to recover from a drop in blood pressure and to re-establish normal blood flow to the brain. This, the researchers said, points to problems with cerebral autoregulation -- the process in which the brain regulates blood flow to meet its needs.

The findings were published in the December 2008 issue of the Journal of Applied Physiology.

Sleep apnea can be treated in several ways, including use of an airway pressurization mask -- a method known to normalize cerebral autoregulation.

More information

The U.S. National Heart, Lung, and Blood Institute has more about sleep apnea.