What if you had breast cancer and discovered that timing surgery to coincide with a particular point in the menstrual cycle might make a difference in your prognosis?

Or what if you had diabetes and learned that insulin sensitivity varies with menstrual rhythms?

And what if you were plagued by other miseries, like migraine headaches or yeast infections or systemic lupus erythematosus, that also seem to wax and wane along with the menstrual cycle?

Would you use menstrual cues to time your surgery, to watch your blood sugar extra closely, or to predict the onset of troubling symptoms?

Chances are you would — in these and many other cases.

In fact, if you’re among the millions of women who have not yet hit menopause, you’ve probably already become a kind of amateur scientist, searching for connections between your own monthly rhythms and the ups and downs of your health.

The problem is, the medical establishment hasn’t exactly shared in this enthusiasm for finding links between menstrual rhythms and patterns of disease. Until recently.

Like the circadian, or daily, rhythms now known to be part of many basic biological processes, like hormone secretion and temperature control, our bodies also have longer rhythms, none more obvious than the menstrual, or monthly, cycle.

To be sure, the question of why humans and many other primates menstruate — pass blood from the uterus periodically when conception does not occur — is still an open one.

Perhaps evolution just isn’t done with us yet. In fact, if women held the evolutionary cards, maybe we’d get rid of menstruation, especially if we could have babies without it.

Or perhaps menstruation, just as the textbooks say, is what happens when something else — conception — doesn’t happen, though in energy costs to the body, it seems a high price to pay for a fancy reproductive design.

Or perhaps, as maverick scientist and MacArthur “genius” award recipient Margie Profet contends, menstruation is the female’s way of getting rid of all sorts of nasty bacteria and other pathogens that ride into the body on the tails of sperm.

Profet, an enthusiastic visiting scholar at the University of Washington in Seattle and author of a controversial article on menstruation three years ago in the Quarterly Review of Biology, thinks menstruation, far from being an evolutionary accident, is a nifty adaptation to the threat of infection.

Citing evidence from numerous species, including those that may menstruate invisibly, she says that “sperm are vectors of disease” and that menstruation exists “to protect the uterus and oviducts from colonization by pathogens.”

Whatever its reason for being, the menstrual cycle in humans is a lot more complex — and potentially more closely linked to the rhythms of health and disease — than scientists used to think.

Susceptibility to AIDS infection, for instance, may vary as hormone levels fluctuate across the menstrual cycle.

In May, scientists at the Aaron Diamond AIDS Research Center in New York and the National Institute of Child Health and Human Development found that monkeys given a high dose of progesterone were more likely than other monkeys to become infected by SIV, the monkey AIDS virus, following vaginal exposure to the virus.

Cindy Pearson, executive director of the National Women’s Health Network, notes in a soon-to-be-released newsletter that this study involved monkeys, not women; SIV, not HIV; and progesterone implanted in pellets under the monkeys’ skin, not the body’s own natural surges of the hormone.

Still, the network is “extremely concerned” because the study showed that when progesterone is high, vaginal tissue thins, perhaps making it easier for the AIDS virus to gain entry.

This suggests that women who have sex with HIV-positive men might be more vulnerable during the second half of their cycles, and that women using progesterone-based birth control, such as Norplant and Depo-Provera, might also be at extra risk.

And there are other potential links between health and menstrual rhythms, though the quality of the research varies from study to study, says epidemiologist Sioban Harlow of the University of Michigan, who has combed the medical literature looking for such links.

Basic metabolic rate, for instance, appears to be higher in the second half of the cycle, with women often consuming — and expending — 500 extra calories a day. “You’re hungrier and with reason, not because of lack of will power,” says Harlow.

Immune function also seems to fluctuate with the time of the month — in still-perplexing ways. Some researchers say progesterone acts as an immune suppressant; others have found that infection-fighting white cell counts are lowest when estrogen is highest.

In one study, Dr. Barbara Mittleman, an immunologist at the National Institute of Mental Health, found that the kind and degree of immune fluctuation across the menstrual cycle seems to vary from woman to woman.

Yet some auto-immune conditions clearly wax and wane with menstrual rhythms, among them rheumatoid arthritis and systemic lupus erythematosus, says Dr. Balu Athreya, a rheumatologist at Thomas Jefferson University in Philadelphia. The worst flare-ups of lupus, for instance, often come in the luteal phase.

And with arthritis, some hormones — like those in pregnancy — often offer relief, while those of lactation may make it worse.

Asthma, too, seems to flare just before menstrual periods, many women say, though a 1989 review found that detailed studies on the subject showed no clear pattern.

It has also been shown that progesterone, the dominant hormone of the second half of the cycle, can trigger adverse changes in cholesterol, says Dr. JoAnn Manson, co-director of women’s health at Brigham and Women’s Hospital. Because younger women are at low risk of heart disease to begin with, however, these fluctuations may not pose a huge risk.

Far more likely to cause grief are migraine, acne, panic attacks, changes in bowel function and an increased tendency toward epileptic seizures — all of which have been shown to get worse before a woman’s period, says Dr. Karen Carlson, director of Women’s Health Associates at Massachusetts General Hospital and a co-author of the Harvard Guide to Women’s Health.

Diabetic women, too, may have an extra tough time just before their periods, she says, because progesterone increases resistance to insulin. This means it takes more insulin to get sugar into cells — so diabetic women may need to increase insulin as their periods approach.

But of all the potential links between menstrual rhythms and health, perhaps none has greater implications than the timing of breast cancer surgery to ovulation.

“This is really where the rubber meets the road,” says Dr. William Hrushesky, senior attending oncologist at the Samuel Stratton Veteran’s Administration Medical Center in Albany, N.Y.

For years, says Hrushesky, the idea that the timing of breast surgery might be important was greeted “with ridicule; then [it] went to outright hostility, then has been ignored.”

But even skeptics like Dr. Kent Osborne, chief of medical oncology at the University of Texas Health Science Center in San Antonio, now say “what’s really needed is a prospective trial” in which menstrual data are correlated with timing of surgery and results are compared.

So far, there have been about 20 retrospective studies in women with breast cancer. About a dozen of them have concluded that, at least in women whose cancer has spread to underarm lymph nodes, the best time for surgery may be just after ovulation, when estrogen is falling and progesterone is surging. This is the early part of the luteal phase, or second half, of the menstrual cycle.

In 1991, an American study of nearly 300 women found the luteal phase was best for surgery, as did a British study of nearly 250 women. In a 1994 study, the British team noted that the high progesterone in the luteal phase seems to contribute to “significantly better survival.”

A 1994 Italian study of nearly 1,200 women came to a similar conclusion, finding that women who had surgery in the luteal phase had a “significantly better prognosis” than those who had surgery in the follicular, or first half of the month.

Some other studies have found no relationship between the timing of surgery and prognosis, but there are theoretical reasons why there might be a connection.

In the first half of the cycle, women have high levels of estrogen without any accompanying progesterone. Estrogen can trigger a tumor growth factor called IGF-1 and may increase enzymes like cathepsin-D that help cancer spread.

By contrast, there are hints that progesterone may damp down such enzymes and keep small blood vessels from leaking cancer cells into the bloodstream. Both estrogen and progesterone may also influence immune response to cancer, though how this plays out across the menstrual cycle is difficult to pin down.

The bottom line, as the Society for Menstrual Cycle Research has been saying for nearly 20 years, is that more research is needed into the menstrual cycle’s effects on health.

Until that research is done, one of the best things you can do for your own health is to keep records of recurring symptoms and chart them against your menstrual cycle.

Whichever way your data turns out, says Carlson, you’ll get valuable information. You may find some connections with menstrual rhythms that you never suspected.

And some troubles that you might have linked to monthly periods may turn out to be utterly random strokes of fate.

SIDEBAR:

The premenstrual syndrome paradox

One of the most controversial conditions linked to menstrual rhythms is PMS, or premenstrual syndrome, now also called premenstrual dysphoric disorder.

Politically, PMS represents a no-win situation for feminists.

The risk in legitimizing it is in “medicalizing” the normal menstrual process and potentially branding all women as sufferers of emotional distress before their periods, says Ann Voda, director of the Tremin Trust Research Program on Women’s Health at the University of Utah College of Nursing.

On the other hand, failing to recognize the genuine suffering of some women — perhaps 5 to 8 percent, according to Nancy Fugate Woods, director of the center for women’s health research at the University of Washington in Seattle — is no solution either.

Clearly, some women do have a terrible time as their periods approach, with bloating, breast tenderness, irritability, changeable moods, depression and fatigue among the symptoms.

One hypothesis is that this misery is caused by low levels of the brain chemical serotonin, though this is not proved.

Still, for some women, the premenstrual time does seem to be “almost a serotonin-deficiency state,” says Dr. JoAnn Manson, co-director of women’s health at Brigham and Women’s Hospital. And some, she says, get “a good response to Prozac,” an antidepressant drug that increases serotonin levels.

For others, the anti-anxiety Xanax taken for a week before the period may also help, says Dr. Karen Carlson, director of Women’s Health Associates at Massachusetts General Hospital.

The night belongs to asthma.

If you’re one of America’s 10 million asthmatics, you may find that your symptoms vary like, well, night and day, with the odds of an attack vastly greater in the wee hours — about 4 a.m.– than in daylight.

But if it’s heart attacks and strokes that worry you, early morning is definitely prime time. That’s when cardiovascular problems — as well as more minor troubles like allergies and rheumatoid arthritis — are most likely to strike.

And if pain is your worst enemy, you probably already know when the witching hour comes: late evening.

For years, scientists have known that nearly everything in biology seems to be imbued with its own distintive circadian — or daily — rhythms, from the lowliest bacterial colony to the deadliest of cancer cells to the most complicated human hormones that ebb and flow through our veins according to timetables laid down long ago by evolution.

They’ve found, for instance, that adrenal glands in a test tube secrete hormones at precise times of day, just as they do in the body. And that plants, stashed in closets, furl and unfurl their leaves according to deep, genetically-set rhythms.

Researchers are now making the leap from chronobiology, the study of these basic rhythms, to chronotherapy, the tailoring of drugs to the rhythms of the body and of disease.

“In the old days, doctors used to think that a `steady state’ of drugs in the system was best,” says Michael Smolensky, a physiologist at the University of Texas-Houston School of Public Health, largely because they had been indoctrinated with concept of homeostasis — the idea that the body likes to keep its internal milieu reasonably constant.

But with 24-hour monitoring of blood pressure, hormone secretion and other variables, chronobiologists have shown that homeostasis is more complicated than that. It’s now clear there is fluctuation in many body rhythms every 24 hours — and that it can pay to synchronize treatments to these rhythms.

This insight has led to a such a scramble among researchers and drug companies that doctors are struggling to keep up. According to a Gallup survey done for the American Medical Association, only one doctor in 20 really understands chronobiology.

But most said they were eager to learn, recognizing there’s more to chronotherapeutics than, say, just popping a sleeping pill or even melatonin, the popular but controversial hormone, in the evening to get a better night’s sleep.

Consider asthma, for instance, an inflammatory lung disease with a clear circadian pattern — and treatment strategy.

One reason asthma flares at night is adrenalin, a stress hormone that is also a bronchodilator, or airway opener. Adrenalin peaks in mid-afternoon, then sinks to its nadir 12 hours later, so that airway function is best in mid-afternoon and worst at 4 a.m., says Dr. Richard Martin, head of the division of pulmonary medicine at the National Jewish Center for Immunology and Respiratory Medicine in Denver.

Histamine, a natural substance that constricts airways, makes things worse. At night, when adrenalin is low, histamine soars, adding to the tendency of airways to narrow.

And then there’s cortisol, a hormone that dampens immune response and inflammation. Cortisol peaks in the early morning, but during the night, when it would help to have cortisol available to curb inflammation in the airways, levels are low.

The result is that airway function varies somewhat in everybody over a 24-hour period. In people with severe asthma, the difference can be as much as 50 percent — potentially the difference between life and death.

To combat this pattern, researchers years ago developed the first genuine chronotherapeutic drug, Uniphyl. If the drug is taken in the evening, its efficacy peaks when it’s needed most — in the middle of the night — then wanes during the day.

Other asthma remedies have also been shown to be most effective at particular times of day, including oral steroids, which seem to work best when given around 3 p.m., says Martin. But he cautions that patients should not alter the timing of these medications without talking to their doctors.

Other chronotherapeutics are fast becoming available.

Last month, Searle, the pharmaceutical arm of Monsanto — began marketing its recently-approved drug, Covera-HS, for high blood pressure and the chest pain of angina.

Doctors have long known that the odds of heart attack, angina or stroke peak in the early morning, as blood pressure soars and platelets become sticky and more likely to form clots.

One obvious solution is to get blood pressure medications into the patient’s system before he wakes up and enters this danger zone. But this can be trickier than it sounds, says Dr. William B. White, chief of hypertension and vascular disease at the University of Connecticut Health Center in Farmington.

Taken at bedtime, some anti-hypertensive drugs may lead to a too-severe drop in blood pressure during sleep, when blood pressure is already comparatively low, and leave too little in the system to be effective in the morning, he says.

Covera-HS, a once-a-day bedtime drug, is designed to get around that problem. The pill has coatings that delay absorption for four to five hours, so that it kicks in early in the morning, says White, who has studied the drug.

Other studies show it can help to take cholesterol-lowering drugs at night, when the liver is busiest making cholesterol.

And just 10 days ago, a Charlestown company, Ergo Science, released tests of a new diabetes drug designed with chronobiology in mind.

Scientists have long known, say Ergo physiologists Anthony Cincotta and Albert Meier, that in healthy, lean animals, certain brain chemicals — notably dopamine — surge in the early morning, then taper off at night. The same pattern is believed to hold for people.

But in Type II diabetes, a condition in which the body becomes resistant to the sugar-transporting hormone insulin, the body clock seems to go haywire: Dopamine drops in the morning.

Ergo’s new drug, still unapproved, seems to fix this errant biorhythm. Taken at breakfast, it dissolves fast, providing a quick blast of a dopamine substitute just as the body needs it.

In 400 people with diabetes, the drug has improved blood levels of glucose, cholesterol, triglycerides and fatty acids.

Timing of treatment may also be important in cancer chemotherapy, says Dr. William Hrushesky, senior attending oncologist at the Samuel S. Stratton Department of Veterans Affairs Medical Center in Albany.

Hrushesky, a pioneer in the study of biorhythms in cancer treatment, says there is a distinct rhythm for cell division, for cell suicide, for the growth of new blood vessels around tumors and for the metabolism of drugs given to combat cancer.

Because of these rhythms, he says, “timing is everything” — with different drugs having different windows of effectiveness.

Adriamycin, for instance, may work best in the early morning, 5-FU (Fluorouracil) does best at night and platinum-based drugs like Cisplatin in late afternoon, he says. Pumps worn by the patient make it relatively easy, he adds, to take drugs on a schedule that’s most effective, not just convenient for doctors.

Though the timing of chemotherapy to fit biorhythms is not yet mainstream practice, Hrushesky and French researchers have evidence that survival rates can be quadrupled in ovarian cancer and doubled in advanced colon cancer if chemotherapy is timed right.

And Canadian researchers have shown that children in remission from acute lymphoblastic leukemia who take the drug 6-MP (mercaptopurine) in the evening are three times less likely to relapse than those who take it in the morning.

As with any other new field, of course, much research still needs to be done. But the field is clearly exploding, says Smolensky of Texas. Ten to 15 years ago, when chronobiologists began promoting their findings, “people would look at us as if we had stepped out of a spaceship,” he says.

Now, they’re eager to know more.

SIDEBAR:

With medication, timing is everything

While researchers are developing new drugs to take advantage of the daily rhythms of the body or to fix errant rhythms, research in chronobiology suggests you may be able to enhance the effectiveness of old ones by taking them at just the right time.

If you suffer from hot flashes, for instance, taking estrogen replacement therapy at night — when hot flashes can wake you — may make sense, says Dr. Gary Richardson, director of the neuroendocrine clinic at Brigham and Women’s Hospital.

Rheumatoid arthritis, an inflammatory disease, is often worst in the morning, says Dr. Martha Barnett, a rheumatologist at Beth Israel Hospital. Though she believes morning stiffness stems from inactivity at night, others think it may be a result of circadian rhythms in hormone secretions and other factors.

Whatever the reason, taking nonsteroidal anti-inflammatory drugs (NSAIDS) may help most in the evening. But be sure to take them with food, to help prevent ulcers and other stomach problems. In fact, ulcers have their own circadian rhythms, with many flare-ups occuring during the early hours of sleep.

On the other hand, osteoarthritis, which results from wear and tear on joints, is worse at night, after a day of activity, so taking medications in the morning may be most effective.

Runny noses, colds and nasal allergies are often worst in the morning, notes Michael Smolensky, a physiologist at the University of Texas-Houston School of Public Health. Thus, time-release anti-histamines taken at nighttime may help.

Aerobic exercise, while not a drug, does impact health and can also be timed for optimal effect. Studies by the US and German armies suggest that the biggest increase in endurance comes if you exercise in the afternoon and early evening, says Dr. William Hrushesky, a chronobiologist in Albany, N.Y. It’s also much easier to exercise at this time of day, he adds.

Sunlight isn’t exactly a drug, either. Though it can increase the risk of skin cancer, it is also beneficial because it suppresses the sleep hormone melatonin and helps re-set circadian rhythms so you sleep better at night, says Dr. Michael Holick, chief of endocrinology at the Boston Medical Center.

In fact, one explanation for “winter depression” is that the weaker sunlight in winter may not suppress melatonin well enough.

And there’s another reason sunlight may make you happy. In skin cells at least, sunlight boosts levels of the feel-good hormone, beta-endorphin.

Three years ago, Ruth Hertz, 66, a self-described “LOL” or little old lady, began feeling lousy.

A normally avid tennis player, she found herself dragging around the court. “The tiredness sort of seemed to come on suddenly,” she recalled last week.

In fact, Hertz, who lives in Framingham, was “more than tired. I was lethargic. I was doing a lot of sleeping,” she says, “and I was beginning to be a little depressed.”

She finally confided her growing list of symptoms to another woman in her water aerobics class who immediately — and correctly — diagnosed her problem: an underactive thyroid gland.

Thyroid troubles, especially the mild, early stages of hypothyroidism or underactive thyroid, are among the most common and easily missed of medical problems, especially for older women, whose complaints of fatigue or malaise may be dismissed as anxiety, loneliness or part of menopause.

Yet more than 6 million Americans, most of them women, suffer from hypothyroidism. Another million have the opposite problem, hyperthyroidism, in which the butterfly-shaped thyroid gland in the neck makes too much, not too little, hormone.

And while some women do encounter thyroid problems right after childbirth, many more — an estimated 10 to 20 percent — first begin having thyroid abnormalities around age 60.

But increasingly, doctors are getting better not just at diagnosing these problems, but also at avoiding what used to be a major pitfall: Overtreating hypothyroidism, which can cause osteoporosis, or bone loss.

Today, thanks to increasingly sophisticated versions of a sensitive test called TSH, doctors can adjust medications precisely, allowing virtually all patients to maintain normal thyroid function.

Researchers also have a more detailed picture of what causes most thyroid troubles in the first place. As we age, the immune system, for unclear reasons, sometimes begins to attack organs in the body, especially in women.

In hypothyroidism, immune cells mistakenly attack the thyroid gland, a process known as Hashimoto’s thyroiditis.

Hyperthyroidism can also be caused by an auto-immune disorder — Graves’ disease — in which proteins called antibodies attack the thyroid.

The half-ounce thyroid gland is far more important than its diminutive size would suggest. In fact, the iodine-laced hormone it makes, thyroxine, is essential — and not just in humans.

It is thyroxine that turns tadpoles into frogs, notes Dr. Reed Larsen, chief of the thyroid division at Brigham and Women’s Hospital. If tadpoles’ thyroid glands are removed, they keep growing — up to a foot long — but never change into frogs.

Thyroid hormone makes kids grow, too, and is essential for normal brain development as well. A baby born with a defective thyroid will, unless treated, develop cretinism, a form of mental retardation endemic in parts of the world where iodine deficiency causes goiter, an enlargement of the thyroid.

Thyroid hormone is also essential for bone growth and for maintaining the basal metabolic rate — the speed at which the body uses sugars, fats and proteins.

With increasing age, many of us — perhaps 50 percent — develop nodules in the thyroid that doctors can feel. These usually do not interfere with thyroid function and are almost always benign, but occasionally, larger nodules may contain cancer. This is rarely fatal and can be treated by removing the affected part of the gland and treating with replacement hormones.

For most of us, however, it’s the more mundane problems that are the biggest troublemakers in later life.

If your aging immune system goes bonkers and attacks your thyroid, you may suffer fatigue, weight gain, intellectual dullness, depression, a tendency to feel cold, constipation, dry skin and other signs of an underactive thyroid. An overactive thyroid is no better — it can cause fatigue, heart failure and heart palpitations, mood swings, nervousness, weight loss, heat intolerance and other problems.

Either way, you may be miserable, though it’s worth remembering that few people die of either over- or underactive thyroid problems.

But in rare caes, hypothyroidism can cause such a sluggish metabolic rate that coma and death result. And hyperthyroidism can produce a fatal “thyroid storm,” in which the heart races and temperature soars.

But it’s usually the tiny, non-life-threatening changes in hormone function that cause most people misery. And how much hormone you produce is controlled by chemical signals sent out by the pituitary gland, which lies just underneath the brain.

If the pituitary senses that the thyroid gland is not making enough hormone, it puts out extra TSH, or thyroid stimulating hormone. If the pituitary senses the thyroid is making too much hormone, it decreases TSH to damp the system down.

In the old days, before TSH testing allowed doctors to measure miniscule changes in thyroid function, the only way doctors could assess function was by measuring blood levels of a thyroid hormone called T4. And the only rememdy they could offer was a dessicated powder made from the thyroid glands of pigs or cows, which varied greatly in strength from dose to dose.

“But now there are better ways both to test for and treat thyroid problems,” says Dr. Gilbert Daniels, co-director of Thyroid Associates at Massachusetts General Hospital.

Now, most patients’ blood is tested for TSH levels, which is “much more sensitive” than T4 testing at detecting miniscule changes in thyroid function, he says. If the test shows you have hypothyroidism, the solution is straightforward.

“Just give back thyroxine,” says Larsen of the Brigham. Nowadays, most doctors prescribe synthetic hormones such as Synthroid, which costs roughly $10 for a month’s supply, or Levoxyl and Levothroid, which cost $5 and $7 respectively.

But getting the dose right can be tricky, so doctors advise taking the hormone for 4 to 12 weeks, then having your TSH measured again. Once you’re on a medication, some doctors say you should stick with that brand, because formulations vary, though others say all the major drugs are equivalent.

“Don’t let people play musical brands with you, because one brand may not be 100 percent equivalent to another,” says Dr. Harold Rosen, an endocrinologist at Beth Israel Hospital.

And definitely don’t fool around with the dosage. If you take too much thyroid hormone, you may wind up with hyperthyroidism.

“Some people think overtreating is a neat way to lose weight,” says Rosen, and some people do feel better if they are slightly hyperthyroid.

But thyroid therapy rarely turns a chubby person into a svelte one. Even if you lose five or 10 pounds on hormones, you may gain the weight back. Overtreating with thyroid hormone can also cause dangerous thickening of the heart muscle.

And hyperthyroidism, whether it arises spontaneously or from overtreatment of hypothyroidism, can also cause osteoporosis, the bone-thinning disease that women already face at menopause from falling levels of another hormone, estrogen.

Thyroid hormone acts directly on bone, and too much can result in bone loss, says Dr. Meryl Le Boff, director of the Brigham’s skeletal health and osteoporosis program. The rate of loss can be slowed by estrogen therapy or other drugs.

For hyperthyroidism not caused by overtreatment, the best long-term solution is often radioactive iodine, says Larsen of the Brigham. Some patients panic at the idea of taking radioactive chemicals, he says, but this treatment “has been used for 50 years without any radiation-related side effects.”

For some patients, surgery to remove the thyroid gland can also help. And others benefit from drugs like propylthiouracil (PTU) or methimazole (Tapazole) that block the thyroid from making hormone.

The take-home lesson, says Larsen, is that because thyroid problems are so common and the symptoms can be so easily dismissed, patients should not be afraid to ask their doctors for a TSH test.

Ruth Hertz emphatically agrees. Taking thyroid hormone, she says, has “changed my life. It gave me much more energy. And I’m more interested in doing things.”