The grind. (GL Body: Q&A).

I grind my teeth in my sleep, it keeps my sister up at night, but I can't help it. Plus, I wake up with really painful headaches. Why do I grind, and how can I stop doing it?

So, you're a bruxer. That's the medical term for teeth-grinder. Grinding can cause serious probs--broken or cracked teeth, gum problems, tooth sensitivity and even loose teeth. Plus, facial muscle spasms and tension headaches can result from clenching back teeth (grinders squeeze their back teeth 14 times harder than non-grinders). So, why do you grind? "Grinding starts when your bite is not in harmony. If your teeth don't perfectly fit in your mouth, the lower jaw moves more to compensate by clenching and biting,' says dentist Dr. Chris Kammer of the Center for Cosmetic Dentistry in Madison, Wis. Some bruxers aren't aware they grind, but dentists can detect the wearing down of teeth. Dr. Kammer recommends an NTI appliance--a front teeth splint--to keep your teeth from grinding. Check with your dentist to see if she is familiar with this treatment.

Is it safe to wear a tampon overnight?

Unless you sleep 15 hours straight, there's no reason you can't. "It's totally safe to leave a tampon in overnight," says New York gyno Dr. Elizabeth Eden. She suggests changing the tampon just before bed and right after waking up. The fear surrounding tampons has to do with Toxic Shock Syndrome (TSS), a potentially fatal disease in which the tampon traps bacteria and releases poisonous toxins into the bloodstream. "TSS is really rare and is associated with very super absorbent tampons which have been taken off the market, or wearing tampons longer than overnight," notes Dr. Eden. Sweet dreams!

My nipples stick in, not out. Is this normal?

It's called an inverted nipple, and it occurs in 2 percent of girls, says Dr. Kathleen Kaufman of Santa Monica, Calif. They can pop out as your breasts continue to grow. The breasts are fully developed about four years after you start having menstrual periods, says Dr. Kaufman. There's no medical reason to be concerned about inverted nipples, but it can restrict you from future breast-feeding. Surgery is an option if they remain inverted, but Dr. Kaufman suggests discussing concerns with your doc.

I'm 13 and have really bad eyesight. I hate glasses, and contacts are too hard to put in. Am I too young to get laser eye surgery?

Laser surgery is for people who have standard vision problems, like nearsightedness or farsightedness, says Dr. Jonathan Lewis of Atlanta. You also have to have a prescription that basically has stayed the same over a period of time. At 13, your eyesight might still change. Laser eye surgery isn't 100-percent perfect yet either. The American Academy of Ophthalmology reports three of 10 patients don't achieve 20/20 vision after surgery. It's also expensive, and most insurance companies don't pay for it. We suggest giving contacts another go or shopping around for a cute new pair of glasses for back-to-school.

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