Ever wonder why she always gets into bed in winter with feet like icicles? Ever wonder why he winds up a summer day drenched in sweat while she is only slightly soggy? If you take your temperature, do you know when to be alarmed -- when you should reach for aspirin or leave a fever alone?

The answers lie in understanding how the human body struggles to maintain a constant internal body temperature, no matter what the weather outside brings.

The internal average of 98.6 degrees Fahrenheit didn't arise by chance. The enzymes in the body, which regulate every metabolic reaction from digestion to cell repair to thinking, work best at that temperature.

Deviation from the norm of 98.6 can be dangerous. If the internal temperature drops below 94, the enzymatic reactions proceed indolently, "the brain loses its ability to control the temperature, and that's hypothermia," says William Kaufman, professor of human adaptability at the University of Wisconsin.

Similarly, at high temperatures the body is unbalanced: The heat changes the structure of the enzymes so they fail to function properly, if at all. When temperatures rise to 105, infants can suffer brain seizures.

There is, however, growing medical opinion that one deviation, fever, may actually be therapeutic. Sometimes trying to bring a fever down -- the standard medical practice in this century until very recently -- may do more harm than good, a number of experts believe.

That doesn't mean we should never meddle with this largely automatic temperature-regulating system. But timing, in health as in humor, is crucial.

To know when to intervene and when not to, the first step is to recognize the three mechanisms the healthy body summons to sustain its internal tropics: dilation or constriction of blood vessels, sweating and shivering.

When the body starts getting hot -- for example, as the air temperature rises above 72 or during vigorous exercise -- the hypothalamus (a tiny portion of the brain that regulates sleep, water balance, food consumption and body temperature) directs the blood vessels to dilate, slowing the blood flow so that heat can escape from the blood to the skin's surface and into the atmosphere. The sweat glands moisten the skin, and evaporation further cools the body.

If the body begins to feel cold (usually when the air temperature drops below 68), the blood vessels constrict, concentrating the warming blood flow on the internal organs and letting the appendages cool. Skin pores tighten to stem the flow of moisture and retain the heat that would be lost by evaporation.

When the pores of furry animals close tightly against the cold, their fur stands on end, trapping an insulating layer of air next to the skin. Goosebumps in humans are a vestigial -- and useless -- reminder of hairier days.

And finally, the body shivers, making tiny muscular contractions that generate heat.

Men and women, in general, respond slightly differently to heat and cold. Women, says Jan Stolwijk, professor of epidemiology at Yale University who has spent a lifetime examining human temperature regulation, tend to complain more about the cold but actually adapt to it more quickly than men.

"There appears to be in women a greater tendency to constrict the peripheral blood vessels sooner than men when exposed to cool air: the result is cold fingers and toes," says Stolwijk. "It may be unpleasant and they will complain -- but their internal body temperature will be preserved better than men's."

On the other hand, in the heat, men seem to sweat sooner and more profusely than women. And while men complain about the discomfort, it effectively cools them off and puts them at less risk for hyperthermia (dangerous overheating) than women, says Stolwijk.

Ironically, women have greater potential to cope with the heat than men because they have more sweat glands per square inch than men have. (The average adult has about 2 1/2 million sweat glands.) "But these extra glands are dormant in most women" says Richard Gonzalez, chief of biophysics at the U.S. Army Research Institute of Environmental Medicine in Natick, Mass. This appears to result from the fact that women are traditionally less active than men. Sweat glands join the ranks of the body organs that "you either use or lose."

Physical exercise can overcome most of these gender differences. "Active men and women are about equal" in adjusting to temprature changes, says Gonzalez. Fit individuals sense temperature change more quickly than average and make energy-saving adjustments sooner.

Humans may function most efficiently at 98.6, but most bodies regularly deviate from this "ideal." It is perfectly normal to have the body's thermostat set half a degree above or below. Furthermore, within any individual, temperature fluctuates as much as a degree throughout the day -- with the low in the morning and the high late in the afternoon.

Women's temperatures can also rise one or two degrees during ovulation. Temperature even varies within the body: The brain and the liver are always the hottest internal organs.

But the most dramatic temperature extreme comes with fever. Fever is not a disease -- it is a symptom of the presence of an infection. And far from being a cause for alarm, new research suggests it is an encouraging sign that the body is mobilizing the immune system to protect itself.

When the body senses infection, white blood cells release a protein called endogenous pyrogen (EP) or Interleukin 1. This protein tells the hypothalamus to reset the thermostat at a higher level. Then the body swings into action with chills and shivering to raise the temperature.

The higher temperature creates an inhospitable environment for invaders: It inhibits many bacteria and viruses from multiplying; it improves the action of interferon, a substance cells produce to inhibit the spread of viruses within the body; and it speeds the production of T lymphocytes, which attack viruses and release bacteria-killing antibodies, says Matthew Kluger, a physiologist at the University of Michigan Medical School and one of the leading researchers on the therapeutic aspects of fever.

"The EP also causes the body to remove iron and zinc from the blood. This deprives invading organisms of the iron they need to grow," says Kluger. "It's called nutritional immunity."

So why do doctors advise taking aspirin to bring the fever down as if it were harmful? "Until about 100 years ago physicians did not think of fever as being harmful," Kluger says. Indeed, until very recently doctors would deliberately induce fever (by infecting a patient with malaria) to treat otherwise untreatable diseases, such as syphilis, he says.

But once aspirin-related drugs became commercially available in the 1800s, attitudes changed, says Kluger. Aspirin effectively reduced pain; it also happened to reduce fever. The two effects became confused.

"People took these drugs and felt better -- and if you feel better, you assume you're being cured," Kluger says.

Now a growing number of physicians are returning to the old notion that moderate fever is adaptive -- that it may help the species survive. "To rigidly beat down any fever that comes along," says Stolwijk, "may rob us of the natural way we have to fight off [invading] organisms."