If you've had one, there's no mistaking it: the sudden, intense, hot feeling on your face and upper body, perhaps preceded or accompanied by a rapid heartbeat and sweating, nausea, dizziness, anxiety, headache, weakness, or a feeling of suffocation. Some women experience an "aura," an uneasy feeling just before the hot flash that lets them know what's coming. The flash is followed by a flush, leaving you reddened and perspiring. You can have a soaker or merely a moist upper lip. A chill can lead off the episode or be the finale.

What causes them

Hot flashes are mostly caused by the hormonal changes of menopause, but can also be affected by lifestyle and medications. A diminished level of estrogen has a direct effect on the hypothalamus, the part of the brain responsible for controlling your appetite, sleep cycles, sex hormones, and body temperature. Somehow (we don't know how), the drop in estrogen confuses the hypothalamus — which is sometimes referred to as the body's "thermostat" — and makes it read "too hot."

The brain responds to this report by broadcasting an all-out alert to the heart, blood vessels, and nervous system: "Get rid of the heat!" The message is transmitted by the nervous system's chemical messenger, epinephrine, and related compounds: norepinephrine, prostaglandin, serotonin. The message is delivered instantly. Your heart pumps faster, the blood vessels in your skin dilate to circulate more blood to radiate off the heat, and your sweat glands release sweat to cool you off even more.

This heat-releasing mechanism is how your body keeps you from overheating in the summer, but when the process is triggered instead by a drop in estrogen, your brain's confused response can make you very uncomfortable. Some women's skin temperature can rise six degrees Centigrade during a hot flash. Your body cools down when it shouldn't, and you are miserable: soaking wet in the middle of a board meeting or in the middle of a good night's sleep.

Who gets them

Eighty-five percent of the women in the United States experience hot flashes of some kind as they approach menopause and for the first year or two after their periods stop. Between 20 and 50% of women continue to have them for many more years. As time goes on, the intensity decreases.

If you have had breast cancer, your hot flashes can follow the same pattern as for women in general, or they can be more intense and last longer, particularly if menopause was premature, or if you are taking hormonal therapy and your body hasn't adjusted to it. Rarely, women may not have hot flashes until they stop taking tamoxifen — an unpleasant surprise. In these women, tamoxifen develops an unusual estrogen-like ability to combat hot flashes.

There is considerable variation in time of onset, duration, frequency, and the nature of hot flashes, whether you've had breast cancer or not. An episode can last a few seconds or a few minutes, occasionally even an hour, but it can take another half hour for you to feel yourself again. The most common time of onset is between six and eight in the morning, and between six to ten at night.

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