Over 80 percent of the sexually active people in our nationally representative survey of 1,000 adults said they sometimes avoided sex last year. And women were more likely than men to say that. Here are six top reasons, along with some advice about what, if anything, you should do about them.

1. TOO TIRED OR NEED SLEEP: 53 percent

You may feel amorous heading to bed, but once you're there the need for sleep can overpower all other desires. And worrying about not being able to fall asleep can make sex seem unappealing. Addressing insomnia is one obvious first step, but before turning to sleeping pills, it's usually best to adopt good sleep habits. And that can include having sex, since many people report that actually improves their sleep, says Jean Koehler, Ph.D., a certified sex therapist in Louisville and past president of the American Association of Sexuality Educators, Counselors and Therapists. But she also points out that when people say they're too tired they're sometimes really saying they're just not interested. In that case, it's important to talk honestly with your partner, and possibly with a therapist.

2. NOT FEELING WELL OR HEALTH PROBLEMS: 49 percent

"Not tonight dear, I have high blood pressure." It's not just the proverbial headache that can interfere with sex. In fact, Koehler says that your first step toward better sex should be a checkup, so your doctor can rule out the many diseases and drugs that can cause sexual dysfunction. Problems with your thyroid or pituitary gland, for example, can sap your sex drive. Conditions that can interfere with blood flow, such as arteriosclerosis or hardening of the arteries, can impair sexual function, as can nerve damage from diabetes or multiple sclerosis. And 9 percent of the people in our survey said they sometimes avoided sex because they felt overweight or unattractive.

3. NOT IN THE MOOD: 40 percent

While supplemental testosterone can sometimes boost libido in men and women, that step is rarely worthwhile. For one thing, many people who experience diminished sexual desire say they aren't bothered by it, according to a study published in the November 2008 issue of Obstetrics & Gynecology. And when they are, other factors—including illness and psychological issues—are often the real cause.

4. TAKING CARE OF CHILDREN OR PETS: 30 percent

5. WORK: 29 percent

These two are in a statistical dead heat. While taking care of children was a slightly more common excuse among women (34 percent vs. 27 percent), work was the only one that was cited more often by our male respondents (30 percent vs. 28 percent). "A lot of people say they are just too busy for sex," Koehler says. "But if sex is important to you, you may have to just make it more of a priority." One solution she recommends: Put it on your schedule. That, in fact, is something that nearly half of our respondents say they already do. Seven percent have actually done so using a calendar, smart phone, or PDA.

6. WATCHING TV OR A MOVIE: 19 percent

Other forms of entertainment that people sometimes find more diverting than sex include going out to eat (10 percent), spending time on the computer (8 percent), talking on the phone (8 percent), and exercising (7 percent).