Barbara Raskin, author of the best-selling novel "Hot Flashes," uses this common sign of menopause as a metaphor for her generation, women now in their fifties whose lives have been shaped by the freedom of the '60s and support from the women's movement in the '70s.

Raskin's graphic discussion of "the flash" reflects the openness with which women now talk about midlife changes.

Scientists take a more detached approach. Still, no one knows precisely what causes a hot flash, says Dr. Isaac Schiff, associate professor of obstetrics and gynecology at Harvard Medical School and director of reproductive endocrinology services at the Brigham and Women's Hospital, Boston. But some good theories exist, based on animal research, measurements of hormone levels and skin temperatures of women who experience flashes.

Schiff uses two terms: hot flash, which he says is "really the subjective part of it, almost like a premonition," and hot flush, "which describes the part we can measure."

According to Schiff, the sudden cessation of estrogen that comes with menopause reduces the level of the brain chemical norepinephrine. This leads to the release from the hypothalamus of GnRh -- a gonadatrophin-releasing hormone -- which in turn stimulates the release of LH, leutenizing hormone, from the pituitary gland.

"We know from animals that GnRh affects the temperature control center, which is also located in the hypothalamus," says Schiff. The body's own thermostat seems to be set mistakenly too high, forcing the body to take steps to reduce its inner temperature. The steps: dilation of the peripheral vessels (hence the reddening of the face and chest) and sweating.

Occasionally, women in their forties or even late thirties have hot flashes, Schiff says. When this happens and the woman is menstruating regularly, "it usually occurs around the time in her cycle of ovulation. This makes sense, because we know that during ovulation there's a huge outpouring of GnRh."