Why is it so difficult to fall asleep when you are overtired？ There is no one answer that【

Why is it so difficult to fall asleep when you are overtired？ There is no one answer that【C1】______ to every individual. But many people fail to note the【C2】______ between fatigue — physical tiredness — and sleepiness, the inability to【C3】______ awake. It is possible to feel tired【C4】______ and still be unable to fall asleep, because while your body may be exhausted, you dont feel【C5】______. To fall asleep, you need adequate time to unwind,【C6】______ you feel fatigued, not so easy to【C7】______ "turn off." According to Carl E. Hunt, director of the National Center on Sleep Disorders Research in Bethesda, Maryland, most people do not allow themselves【C8】_____ deceleration. Lack of sleep【C9】______ matters even more. Experts say adults need at least 7 to 8 hours of sleep a night to【C10】______ properly. When you get less sleep than【C11】______ on consecutive nights, you begin to accrue "sleep debt".【C12】______ sleep debt increases and functionality decreases, your body experiences a stress response and begins to release adrenaline. Now a vicious【C13】______ has been created: You experience the feeling of being more and more tired,【C14】______ your body is increasingly stimulated. Power sleeping for more hours on weekends is only a【C15】______ solution. "There is no【C16】______ for getting a good nights sleep on a【C17】______ basis," says Hunt. Most of us,【C18】______, dont get the sleep we need. According to the 2002 National Sleep Foundation, Americans sleep a(n)【C19】______ of 6.9 hours per night during the week, and 58 percent of adults experience【C20】______ of insomnia a few nights a week or more.

【C1】

A．adapts

B．adopts

C．applies

D．conduct

请帮忙给出正确答案和分析，谢谢！