Go to the movies or click through the channels on your TV any evening, and you're likely to see a woman seemingly enjoying passionate sex.

But in real life, many women struggle to feel any sexual desire at all. In fact, this lack of libido--dubbed Hypoactive Sexual Desire (HSD) by clinicians--is the most common female sexual problem in America. "At least a third of all sexually active adult women suffer from it at some point in their lives," says sex therapist Gerald Weeks, Ph.D., co-author of Hypoactive Sexual Desire: Integrating Sex and Couple Therapy (W.W. Norton, 2002).

Studying women's sexual desire has been problematic for researchers: Because we're capable of going through the motions in bed whether we want sex or not, our sexual desire doesn't necessarily correlate with the frequency or quality of our sexual encounters. And often the cause of a waning libido may not be immediately obvious to us.

The human sexual response has three distinct phases: desire, arousal and orgasm. Sex researchers define desire as psychological, while arousal and orgasm are physical processes. When you're aroused, your nipples become more erect and you feel a tingling in your genitals from increased blood flow. But sexual desire is mental: You're thinking about sex, and therefore you're feeling sexy.

Desire is essential to experiencing real passion. "A woman's sex drive is biologically normal--just like her hunger for food," says certified sex therapist Jean Koehler, Ph.D., president of the American Association of Sex Educators, Counselors and Therapists. "But if you don't have any sexual desire, that means something is definitely wrong."

Of course, every woman experiences times in her life when her sexual desire dips, due to stress, illness, relationship problems or a recent pregnancy. However, if your relationship is on track and you're in good physical health, yet your sex drive has taken a nose dive, one or more of these libido killers may be to blame.

1. Not taking care of yourself

Nothing sinks the libido of a physically and emotionally healthy woman faster than lack of sleep, poor eating habits and/or a sedentary lifestyle. Anything that hampers your general physical health also may take an immediate, and perhaps sustained, toll on your sex drive because you feel ill or depleted, Koehler adds.

The sexperts' Rx You simply feel sexier when you take care of yourself, and that includes getting enough shut-eye (at least eight hours a night), eating nutritiously and, of course, exercising. "Exercise is the closest thing we have to a fountain of youth," says Sallie Foley, M.S.W., co-author of Sex Matters for Women (Guilford Publications, 2002). "It helps our bodies maintain nutritional and hormonal balance, it elevates our mood, and it offers sexual benefits such as improved pelvic muscle tone and physical flexibility."

A complete physical exam might help ferret out reasons why you're just too tired, achy or blue to have a sexual appetite. Ask your physician to check for anemia, thyroid deficiencies, diabetes and other chronic illnesses, which all sap your energy. Finally, your healthcare provider can craft a nutrition and exercise program to maximize your physical well-being.

2. Medication

"Many prescription, nonprescription or recreational drugs can cause sexual side effects," says Gerald Weeks. For young women, the most likely culprit is the pill. "Oral contraceptives can definitely lower your libido, though not necessarily in all patients on the pill," says Alan M. Altman, M.D., co-author of Making Love the Way We Used to ... or Better (Contemporary Books, 2002). Hormones in birth-control pills can diminish your free-testosterone levels--and testosterone is an essential ingredient for sexual desire, arousal and orgasm.

The other most common prescription medications linked to low libido are the anti-depressant SSRIs (selective serotonin reuptake inhibitors) such as Paxil, Prozac and Lexapro. Indeed, nearly 65 percent of all SSRI prescriptions are now written for women. In one study, up to half of patients taking SSRIs reported a decline in libido.

The sexperts' Rx For starters, talk to your physician about switching medications. In the case of oral contraceptives, each woman's response to a particular pill is chemically unique, Weeks explains.

Treating HSD in women on SSRIs may include "waiting to see if the symptoms remit, lowering the dose, substituting another anti-depressant or adding a supplementary medication," says Weeks. Some psychiatrists report that taking bupropion hydrochloride (Wellbutrin SR) in conjunction with an SSRI results in a slightly improved libido in women, he adds.

3. Hormonal changes

Just as a decline in testosterone production can negatively affect all three phases of the sexual response (desire, arousal and orgasm), so, too, can the natural decline in estrogen experienced by women in their 30s and 40s. "The gradual decline in circulating estrogen has been associated with body changes that can cause discomfort during sex, such as vaginal dryness and loss of elasticity," Weeks explains, the result being diminished desire.

The bottom line? "A lot plays into your sex drive, and if you don't have at least minimal levels of estrogen and testosterone, other contributions to sexual desire will usually not be enough to overcome those deficiencies," says Jean Koehler. "Hormones are one of the essential building blocks for desire."

The sexperts' Rx Hormone deficiencies should be addressed medically. Your physician can order hormone testing, especially for free and total testosterone and estrogen levels, if needed. Depending on the nature of the problem, your doctor may refer you to an endocrinologist or sexual-medicine specialist, who will evaluate whether you're a candidate for testosterone and/or estrogen replacement. Most clinicians are very cautious about hormone replacement therapy (HRT) for pre-menopausal women; such women need to use highly effective birth-control measures with every sexual encounter so as not to risk a pregnancy while on testosterone.

If you're suffering a loss of estrogen due to breastfeeding or perimenopause, estrogen creams or vaginal tablets (which are locally absorbed) can help with lubrication--increasing the pleasure factor of intercourse. Keep in mind, though, that taking estrogen orally (either in birth-control pills or in HRT) can lower testosterone below levels needed for proper sexual function.

4. Sexually transmitted diseases

STDs are among the most common infectious diseases in the United States, affecting more than 15 million men and women in this country alone each year, according to the Centers for Disease Control and Prevention. Such infections have a disproportionate impact on women because a good number of STDs cause no initial symptoms, so many women don't seek treatment until serious problems have already developed. STDs also have more severe health consequences for women than for men. Left untreated, for instance, some can spread into the uterus to trigger pelvic inflammatory disease, a major cause of both infertility and ectopic pregnancy.

Sexually transmitted infections like chlamydia, genital herpes, HPV, genital warts, gonorrhea, syphilis and trichomoniasis can cause physical discomfort or pain, leading to a decreased libido. What's more, even if a woman has been treated for an STD, she may feel shame and sexual inhibition. "STDs are like the new scarlet letter," Foley says. "Women often feel branded and worry about how to talk with subsequent partners about their condition."

The sexperts' Rx Schedule regular checkups for STDs even in the absence of symptoms, especially if you're having sex with a new partner. Seek medical help immediately if any suspicious symptoms develop, such as burning during urination or red bumps in or around the vaginal area. In case you are diagnosed with an STD, notify all recent sex partners (and urge them to get a checkup), follow your doctor's orders for treatment and, if you're feeling inhibited or ashamed about having sex with your partner (or a new partner) following treatment, try joining a support group. The American Social Health Association (www.ashastd.org) provides support and free information about STDs, and keeps lists of clinics and doctors who treat them.

5. Depression

One of the most significant health risks for women is depression, especially if a woman is of childbearing age. Clinical depression affects twice as many women as men; one out of every four women will suffer depression at some point in their lives.