B

　　体裁：说明文　话题：医学　词数：271　时间：6′

　　As cold season approaches, many Americans will buy a lot of vitamin C and echinacea(紫锥菊)in order to avoid catching a cold.But paying attention to an old piece of advice about having enough sleep every day might be more important.

　　Research suggests that poor sleep can increase the chance to colds.Studies have shown that poor sleep and a chance to catch a cold go hand in hand, and scientists think sleep plays an important role in the body's defenses.

　　In a recent study for The Archives of Internal Medicine, scientists followed 153 men and women for two weeks, keeping an eye on their quality and duration(持续性)of sleep.Then, during a five－day period, they exposed them to cold viruses.Those who slept an average of fewer than seven hours a night, it turned out, were three times as likely to get sick as those who averaged at least eight hours a night.

　　Sleep and immunity(免疫), it seems, are tightly linked.Studies have found that mammals that require the most sleep also produce greater levels of disease-fighting white blood cells—but not red blood cells, even though both are produced in bone marrow(骨髓) and stem from the same precursor.And researchers at the Max Planck Institute for Evolutionary Anthropology have shown that species that sleep more have greater resistance against pathogens(病原体)．

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