as fashion trends go, hot flash pajamas are not glamorous. But they stand a chance of holding on for more than one brief season. At least five brands have sprung up in the past five years and, perhaps as a result of the expanding ranks of women turning 50, sales are rising fast.

The various inventors of this new sleepwear all seem to have had essentially the same eureka moment ? on a night when they started getting hot flashes of their own. They were all active and fit women, with years of experience with perspiration. And they realised that the temperature fluctuations of menopause called for the same wicking fabrics as running, hiking or exercising in the gym.

“When I first had hot flashes, I would change my T-shirts through the night,” said Wendy McClung, a co-founder of HotCool Wear, in Toronto, which began making Hot Mama pajamas in 2000. “One night I grabbed one of my running shirts, and I thought, ‘My goodness, this is what it is like to sleep.’’’

Hot Mama sleepwear is made of CoolMax, a polyester fabric used in workout clothes, finely milled to make it light and soft, like cotton flannel. Wicking J. Sleepwear, from a company in Evergreen, Colorado, uses a similar fabric called Intera. And Wildbleu, a Seattle brand, uses one called Dri-release. The polyester fibers are designed to lift sweat from the body and allow it to evaporate quickly, said Helen Rockey, the founder of Wildbleu.

Whether that leads to a better night’s sleep is another question, one that’s harder to answer because it’s not clear that hot flashes disturb the sleep of menopausal women as much as they think. Recent laboratory research has found surprisingly little connection between hot flashes and sleep quality.

A 2004 study conducted at Wayne State University in Detroit looked at the sleep quality of 31 women ages 46 to 51, 12 of them experiencing an average of five hot flashes per night.

Sometimes the flashes woke them up, but more often it was the other way around.

“They awakened first and then they flashed,” said Robert R. Freedman, a professor of psychiatry and obstetrics and gynaecology, who conducted the study with Timothy A. Roehrs, a sleep researcher. “Perhaps their waking up is what triggered the flash.”

When the researchers measured the subjects’ daytime alertness ? by testing their reaction time, for example, and clocking how long it took them to nod off when they were allowed to nap ? they found that the women who had hot flashes were no sleepier than those who did not.

A study of 589 women at the University of Wisconsin in 2003 found that women who reported having hot flashes had no worse sleep quality than those who did not. Perhaps, the researchers said, the bleary-eyed exhaustion that women reaching menopause often complain of may be caused by other things ? like depression, pain or the various discomforts that sometimes come with age.

In a more recent study, not yet published, Freedman found that hot flashes woke his subjects only during the first half of the night, a period of slow-wave sleep. During the second half of the night, when the women were in REM (rapid eye movement) or dreaming sleep, they rested peacefully. If the subjects woke at all in the second half of the night, their awakenings preceded, rather than followed, hot flashes.

So Freedman now recommends that hot flash sufferers keep their bedrooms cool during the first half of the night. “Set the thermostat at 64?F for the first four hours,” he said. After that, he added, it will not hurt to let the air warm a bit (allowing anyone else in the room to quit shivering).

Estrogen supplements, now a subject of dispute, are an effective treatment for hot flashes. But herbal products like black cohosh, kava, red clover leaf, dong quai root and ginseng have not been shown to be effective, according to a report from a National Institutes of Health state-of-the-science conference this year.

Life after menopause: Hot flashes defy explanations

Some doctors also use prescription drugs like anti-depressants or blood pressure medications “off-label” to treat hot flashes, but the medications can have side effects.

As for wicking pajamas, the Good Housekeeping Institute evaluated two brands, HotCool Wear and Wicking J., by asking seven menopausal women to wear them for a week and then rate them. Five of the testers said the fabrics kept them more comfortable during and after night sweats.

The pajamas come in various styles ? gowns, nightshirts, kimonos, pants and T-shirts ? and all of them tend to be blousy rather than skimpy. “The fabric can’t work unless it’s touching your skin,” McClung explained.

The wicking fabrics are also odour-resistant.

Wildbleu nightwear comes in pastel solids and prints; Wicking J., in solids only. HotCool Wear offers some dark and bright colours. Wicking J. and HotCool Wear also sell pillowcases. All three of these brands are sold in small stores and online at websites like serenecomfort.com.

Scientists still do not know exactly why hot flashes occur, except that they seem to result from the body’s diminishing sensitivity to estrogen in the years before and just after menopause.

“It’s sort of a repositioning of the thermostat, if you will, where there’s less variation in temperature between sweating and shivering,” said Dr Rogerio Lobo, a professor of obstetrics and gynaecology at Columbia University.

About 80 per cent of women have hot flashes at the onset of menopause, according to a large survey of women conducted in the Netherlands in 1993. During this peak time, women may have a half-dozen hot flashes a day and the same number at night, doctors say, though the frequency varies widely. The record holder among Freedman’s subjects had 39 in 24 hours.