单选题 .  If you are worried about things and are under a lot of stress at work or school, then you are probably not sleeping well. Worry can keep you awake,     1    over and over in bed until the early hours of the morning when you eventually     2    asleep. When you wake up, you don't feel     2    , but tired and worn out and     4    to face a new day.
    Dr. Henry Winkle, in a recent newspaper article     5    Stress and Sleep,     5    that stress and lack of sleep are directly     7    . Dr. Winkle says, "The more we worry, the     8    we sleep, the more we are unable to deal with     9    ." "If we can find a way to get a good night's sleep," he     10    , "we can often find the     11    to deal with what's worrying us."
    So, what is a good night's sleep? Research shows that the amount of sleep     12    people need in order to keep healthy     13    a lot. Seven hours is about the average amount, though strangely enough, sleeping longer often gives you a headache instead     14    king you feel more refreshed.
    Dr. Winkle believes that preparing for sleeping is important. People who work late should try to give themselves a short     15    and do something restful before going to bed. This could be watching TV or listening to music. Doing some exercise     16    in the day should help you to feel physically as well as     17    tired. A bedtime drink can also help, but coffee or tea should be avoided as they contain caffeine and will keep you     18    . "When you turn the light     19    ," Dr. Winkle says, "concentrate on relaxing your muscles, slowly up from your feet, and you'll be asleep     20    you know it."1.