Women who like to lull themselves to sleep with a drink or two may want to think twice a new study shows alcohol can disrupt the sleep patterns of women more than those of men.

The study found that after drinking alcohol, women got fewer hours of sleep than men did after drinking. They woke more frequently and for more minutes during the night.

Women who drank alcohol at intoxicating doses got about 20 minutes less sleep than those who drank caffeine-free cola.

"This may not sound a lot," said study researcher Dr. J. Todd Arnedt of the University of Michigan, "but when you think about it, these are healthy, young adults with normal sleeping habits. For someone who sleeps eight hours a day, 20 minutes is equivalent to about 4 percent of total sleep time."

Although alcohol can increase feelings of sleepiness during the early part of the night, it can disrupt the continuity and quality of sleep in the second half of the night, making the person more wakeful. And this so-called "rebound effect" is most pronounced in young healthy women, Arnedt said.

"We're interested in seeing how alcohol affects sleep differently between men and women, because women metabolize alcohol differently," Arnedt said. Women tend to have less body water and more body fat than men. As a result, they tend to have a higher breath-alcohol concentration after drinking equal amounts of alcohol, even after their alcohol consumption is adjusted for body weight.

"However, women also eliminate alcohol faster than men. Women's breath-alcohol concentration declines more quickly as the night goes by," he said.

The researchers gave 93 men and women in their 20s either caffeine-free cola with bourbon or vodka, or a placebo of cola with tonic water. The participants drank enough to push their peak breath-alcohol content to about 0.11 percent well above the legal limit of intoxication in the United States. The participants then slept for eight hours under observation.

Similar to previous studies, this study found that alcohol could affect sleep continuity. However, unlike previous studies, it showed that there was no gender difference in sleep architecture, the nightly cycle of sleep stages that includes rapid eye movement and nonrapid eye movement sleep.

"It's actually reassuring that the data on sleep architecture conflicted with previous studies," said Dr. Mary Carskadon, a sleep researcher at Brown University who was not involved with this study. "Studies in the past, especially ones with smaller sample sizes, might have over-interpreted the effects of alcohol on sleep architecture."

This, by contrast, was the first large-scale study that directly compared the effects of alcohol in men and women, and it produced solid data for future discussions and references.

"The results may not be as meaningful for healthy adults, since they are not likely to drink heavily, regularly. However, there are some people with insomnia who like to self-medicate by drinking before they go to bed. Well, this research is telling you to not do that. Especially if you're a woman," Carskadon said.

The researchers also compared the findings between those with a family history of alcoholism and those with no family history, and found no difference in sleep disruption between the two groups.

Arnedt agreed that his research is likely to be a starting point for studying other groups, particularly people with insomnia or alcohol-use disorders.

"We can use this to understand which group of people would be most vulnerable to alcohol misuse and understand how sleep might factor in the development of problem drinking," he said.

This study will be published in the May issue of Alcoholism: Clinical & Experimental Research.

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