The debate on hormone replacement therapy has centered on its effects on heart disease and cancer. But at a recent medical conference in Seattle, researchers presented a hormone study that focused on a different question: sleep.

The researchers, from Stanford University, found that estrogen improved the breathing of postmenopausal women who had sleep apnea, a condition involving repeated breathing pauses.

The study, by Dr. Tracy Kuo, a postdoctoral fellow, and Dr. Rachel Manber, the director of the insomnia program at the Stanford Sleep Disorders Center, was small, but it is one of a number of recent investigations into how sex hormones may disturb or improve sleep.

Sleep researchers are finding that women’s sleep problems differ from men’s and that hormones may explain part of the difference.

Many women complain of poor sleep. In a poll released in April by the National Sleep Foundation, women were more likely than men to feel they were not getting enough sleep (28 percent versus 19 percent), to report daytime sleepiness (20 percent versus 13 percent) and to have had symptoms of insomnia (63 percent versus 54 percent).

Now there is wider recognition that such complaints are real, said Dr. Joyce Walsleben, the director of the New York University School of Medicine Sleep Disorders Center. “Instead of being told, ‘You’re just crazy, dear,’ someone may actually listen,” she said.

In the past, Dr. Walsleben said, a woman who told her doctor she was tired and groggy might be labeled depressed, when she was really describing the daytime effects of apnea.

Dr. Walsleben thinks that half of women’s sleep problems can be attributed to factors unique to women: menstrual cycles, pregnancy, menopause and even motherhood. “A large part of being female is we’re supposed to know what’s going on with the kids at night,” she said.

A study by Dr. Rosalind Cartwright, the chairwoman of the department of psychology at the Rush-Presbyterian-St. Luke’s Medical Center in Chicago, found that a baby’s cries woke a woman sooner than a man.

In a baby’s first year of life, a woman may lose as much as 700 hours of sleep.

Until the 1990′s, most sleep research was done on men. In the early days, Dr. Cartwright said, “it was not considered polite for women to go to sleep in front of men” in laboratories.

Dr. Terry Young, an epidemiologist at the University of Wisconsin, found that although nine times as many men as women were being treated in sleep clinics for apnea, the disorder actually occurred at a ratio of less than 3 to 1 — 24 percent of men to 9 percent of women.

In a preliminary review published this spring, Dr. Young found the risk of apnea in postmenopausal women was four times that of premenopausal women, possibly because of declining sex hormones.

Dr. Manber of Stanford conducted several studies of the menstrual cycle’s effect on sleep and found that at least 15 percent of women suffered significant sleep disruption before menstruation.

Researchers are also looking into sleep and depression — which is diagnosed twice as often in women as in men — and other diseases that occur more often among women but whose connection to sleep may at first glance seem tenuous. They include multiple sclerosis, lupus and fibromyalgia, in which fatigue has long been considered an unfortunate side effect.

“Now people are looking for treatable sleep disorders in these diseases,” said Dr. Beth A. Malow, an associate professor of neurology at the University of Michigan.

“We tend to think of sleep in a vacuum, but it really isn’t. Through the study of sleep and sleep disorders, we’re going to open up a whole new dimension to understanding women’s health.”