More than three quarters of American women suffer from hot flashes during the menopause transition. This means, of course, that a lucky minority of women don't. Our question is: who are these women, and where are they hiding? Everyone we know has experienced the unwelcome sensation of sudden heat more than once, and often in an embarrassing situation: in the middle of a conversation, during an introduction to someone new, rushing to meet a deadline. The heat spreads from your torso to your face. Some women appear flushed; many experience rapid heartbeat and a rush of anxiety. This can happen a few times a day or almost every hour. There's no rule. You may feel as if you're on fire, but your internal body temperature doesn't change. What does heat up is the temperature of your skin, and it can rise as much as seven degrees, although between one and four degrees is more typical. Generally, you'll cool down in a few minutes, although some women have individual hot flashes that last as long as a half hour. (See, it could be worse!) The average length is between 30 seconds and five minutes.
No one knows exactly what happens to your body during a hot flash, but it appears that changes in our brain chemistry have something to do with it. One theory is that these changes may affect the hypothalamus, a region of the brain that controls all kinds of things: blood pressure, fluid and electrolyte balance, and body temperature. The hypothalamus gets input from all parts of your body as it goes about the job of keeping you running efficiently. When it senses a problem, it springs into action, adjusting heart rate and blood flow to the skin, among other things. It may be that during a hot flash, fluctuating estrogen levels and hormones from the pituitary gland confuse the hypothalamus. Even before you feel the heat, the hypothalamus may be getting an incorrect message that your skin is too hot, so it tries to cool you off. Your heart pumps faster, the blood vessels in your skin dilate to circulate more blood (and get rid of the heat) and your sweat glands spring into overdrive. The "flushed" effect comes from the dilated blood vessels.
More recent studies suggest another mechanism. Dr. Robert R. Freedman and his colleagues at Wayne State University in Detroit have used functional magnetic resonance imaging (fMRI) to observe women's brains during a hot flash. The area that showed the most activity was the insular cortex, which is responsible for understanding internal body events. Mysteriously, no activity was seen in the hypothalamus. Freedman's theory is that women who get hot flashes have a very narrow "thermoneutral zone"—the temperature at which the body is neither sweating nor shivering. Estrogen appears to widen the thermoneutral zone. More research is underway, raising hopes that we may soon understand not only what causes hot flashes but also how to prevent them.