Our bodies seem wonderfully deft at maintaining balance. We sweat to cool down, and our hearts pound when blood pressure falls. As it turns out, though, our natural state is not a steady one. Researchers are finding that everything from blood pressure to brain function varies rhythmically with the cycles of sun, moon, and seasons. And their insights are yielding new strategies for warding off such common killers as heart disease and cancer.    In medical school, most doctors learn that people with chronic conditions should take their medicine at regular rates. “Everyone does it, but it's a terrible way to treat disease,” says Dr. Richard Martin. For example, asthmatics are most likely to suffer during the night, when mucus production increases, airways narrow, and inflammatory cells work overtime. Yet most patients strive to keep a constant level of medicine in their blood day and night. In recent studies, researchers have found that a large midafternoon dose of a steroid or bronchodilator can be as safe as several small doses, and better for warding off nighttime attacks.    Dr. William Hrushesky has shown that many cancer drugs are less toxic if they're used only at certain times of day. In his clinic and others, patients getting rhythmic chemotherapy through portable injection pumps have suffered less heart, stomach, and bone-marrow damage than those getting continuous infusions.    Daily rhythms aren't the only ones that could affect cancer treatment. Hrushesky analyzed the records of 41 women who'd undergone surgery for breast cancer and found that those operated on midway through the menstrual cycle enjoyed better 10-year survival rates than those treated at other times of the month.    Unlike most new treatments, this one would cost no more than what it replaced. Time, after all, is free.