(HealthScoutNews) -- There may be a biological cause for those snorts, chokes and gasps that come from the man's side of the bed in the middle of the night. And biology may explain why more men than women have the disorder called sleep apnea.

The cause? Hormones.

A new study finds that estrogen seems to protect women against the type of oxygen deprivation common in obstructive sleep apnea, at least in rats.

"For many years, we have assumed that males and females are the same with respect to normal physiology," says one of the study's authors, Mary Behan, professor of medicine at the University of Wisconsin's School of Veterinary Medicine, in Madison. But, when it comes to breathing during sleep, Behan and her colleagues say men and women are not the same.

The researchers say learning which hormones react to oxygen deprivation could help scientists come up with new treatments for obstructive sleep apnea. The disorder affects up to 12 million people, reports the American Sleep Apnea Association. Sleep apnea occurs when a person temporarily stops breathing while sleeping. The interruptions can happen as often as 60 times an hour. The disorder can cause excessive sleepiness, high blood pressure and poor concentration. The biggest risk factors for the disorder are being male, overweight and over age 40.

Behan, Gordon Mitchell, also a professor of veterinary medicine, and graduate student Andrea Zabka studied young and middle-aged female rats to see how they respond to periods of oxygen deprivation. The rats in their study were comparable in age to women well before their menopausal years.

Previous work by the researchers found that male rats became less responsive to oxygen deprivation as they aged. The opposite seemed to occur with female rats in the latest study, which appears in the December issue of the Journal of Applied Physiology. As the female rats aged, their bodies responded more to episodes of oxygen deprivation by taking deeper and more frequent breaths.

"We think that estrogen has a protective role against [oxygen deprivation]," says Zabka. To test this theory, the researchers removed the ovaries of the rats to decrease estrogen levels and simulate menopause. These rats responded more like older male rats and had a reduced response to oxygen deprivation.

The next step in their research will be to test much older female rats and to see how male rats that have had their gonads removed react to oxygen deprivation.

The studies aren't the first to focus on the role of hormones in sleep apnea. "A recent large study found that post-menopausal women given hormone replacement therapy had similar rates of sleep apnea to pre-menopausal women," says Dr. Rochelle Zak, an instructor in psychiatric neurology at the Sleep-Wake Disorders Center at Weill Cornell Medical Center, in White Plains, N.Y. But, Zak says other small studies have failed to confirm that hormones work as a treatment for the disorder. Zak says researchers likely will continue to focus on the role of hormones in sleep apnea. "On one hand we have epidemiological data that suggest hormones do play a role, yet when you put that in a treatment model, it doesn't always work," she says.

What To Do

If you snore a lot, or wake up choking, and your partner says you seem to stop breathing in your sleep, you may have sleep apnea. Other signs include unrefreshing sleep and daytime sleepiness, says Zak. If you suspect you have sleep apnea, Zak says you should talk with your doctor about going to a sleep disorders center to be evaluated.

To learn more about the sleep disorder, go to the American Sleep Apnea Association or the American Academy of Family Physicians.

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