听力原文： You may grind your teeth at night. According to a recent survey, at least 8 to 10 percent of the adult population share this malady. It's called sleep bruxism. There's a waking version, too. But the origins are different and the effects are seldom as bad as during sleep. 'It's much like having a large football player standing on the tooth,' says Dr. Noshir Mehta, chairman of general dentistry at Tufts University School of Dental Medicine. During sleep bruxism, he explained, the upper and lower teeth may come into direct contact as much as 40 minutes per hour, and with a force of about 250 pounds. You can compare that with normal circumstances, when a person's teeth make contact for about 20 minutes a day while chewing, and with only 20 to 40 pounds of pressure. Sleep bruxism is not a disease, but a sleep disorder, the third most common one behind sleep talking and snoring. It is more prevalent in children, and its origins may be different in adults. 'The exact causes are unknown,' said Dr. Mehta. He explains that in the 1960s bruxism was thought to be the body's response to 'malocclusion', or problems with how the upper and lower teeth fit together but that theory was discredited for lack of clinical evidence. Stress was later thought to be the cause, but this failed to explain why not everyone with sleep bruxism was stressed and not everyone with stress ground their teeth. More recent research indicates some relationship of sleep bruxism to neurochemicals, but there is still disagreement on how significant a role they play. Whatever underlying causes science may show in time, the more immediate contributing factors for sleep bruxism are better understood. The medical literature shows that stress, smoking, alcohol, caffeine and other factors may set off or worsen the condition. So what should a grinder do? Some people tried relaxation techniques like yoga, exercise, biofeedback and hypnotherapy. Much as these may have helped their overall health, they didn't seem to kick the football player out of their beds. Right away, dentists suggest fitting the mouth with a mouth guard, a small plastic device with a price tag of $ 300. It covers some or all of the teeth to protect them against damage, but does not stop the grinding or clenching itself. You can also get an over-the-counter version at the drugstore for $20. However, 'the over-the-counter guards are usually better than nothing,' said Dr. Mehta. They protect the teeth, but may also be more likely to induce a chewing response and increase bruxism they can also cause irreversible damage to the arrangement of the teeth, and so should be used only temporarily. Dr. Lavigne of the Canadian Sleep Society says people who wake up with headaches or jaw pain may clench or grind their teeth, and should consult their dentist, or doctors. There, patients may be directed to try some of the techniques or others. These approaches may reduce bruxism, but since there is no known cure, only the effects can be reliably treated. In the end, mouth guards are still the best defense. After all, if you're going to square off against a football player every night, you need padding. Questions: 16.What is sleep bruxism? 17.Which of the following statements is true about sleep bruxism? 18.Which of the following is NOT suggested as an immediate cause of sleep bruxism? 19.The doctor suggests that the over-the-counter mouth guard can only be used occasionally. Why is it so? 20.What does the speaker mean by saying 'if you're going to square off against a football player every night, you need padding'? (36)