Sleep
　　Why is it so difficult to fall asleep when you are overtired? There is no one answer that applies to every individual. It is possible to feel “tired” physically and still be unable to fall asleep, because while your body may be exhausted, you do not feel sleepy. It is not so easy to simply “turn off”.
　　Lack of sleep complicates matters even more. Experts say adults need at least seven to eight hours of sleep a night to function properly. When you get less sleep than that on consecutive three nights, you begin to accrue four “sleep debt”. As sleep debt increases your body experiences a stress response. Now a vicious cycle has been created: You experience the feeling of being more and more tired, but your body is increasingly stimulated. “Power sleeping” for more hours on weekends is only a temporary solution. There is no substitute for getting a good night's sleep on a regular basis.

　　Useful Words and Expressions:
　　1. overtire 使过度疲劳
　　2. apply to 将……应用于
　　3. consecutive 连续的，连贯的
　　4. accrue 自然增加，产生
　　5. vicious恶的
　　vicious cycle 恶性循环
　　6. stimulate 刺激，激励
　　7. substitute for 代替……，替换……