Reading Comprehension 
从下列每篇短文的问题后所给的四个选择项中选出一个最佳答案。
     Sleep is generally divided into five stages．In the first two stages, a person sinks into a light sleep; muscles relax, breathing and pulse rate slow down．In the third and fourth stages of deep sleep, a sleeper can only be awakened by a loud and unusual noise; the heart beats slowly and blood pressure drops. It is in the fifth stage that the body regenerates (恢复体力) and a person dreams. Dreams are an important part of healthy sleep. They help the brain adapt and respond to life experience．
     The brain is almost as active at this time as when a person is awake. The secrets of a good night's sleep are a good mattress (床垫) and reasonable eating habits. A heavy meal and too much wine will destroy a good night's sleep. When buying a mattress, you should test it to make sure that when lying on it you are comfortable and your body sinks into a side position easily. The mattress should be of the right size—20 to 30 centimeters longer than the sleeper's stretched out body and at least 95 centimeters wide.
     A comfortable bed and a slightly darkened bedroom are important to a good sleep, too. It is true that a good night's sleep must be long, deep and with no disturbance, so that both the brain and the body will regenerate during sleep. Therefore, the bed should be placed at least 60 centimeters away from a window. Furthermore, if you feel hungry before bedtime, have a little milk.