【正确答案】 D

【答案解析】[听力原文]
   We spend about one-third of our lives asleep. This may sound like a lot of time, but it is not wasted. Our bodies need sleep to stay healthy.  Not getting enough sleep can hurt the immune system and make us sick. When we sleep, our bodies release proteins that support sleep and help us fight infections and diseases. A long-term lack of sleep shuts down parts of the immune system. If sleep-deprived people are exposed to a virus, they are more likely to get the virus. Sleep is important to overall health and well-being and especially important to immune health. To stay in good health, adults should get seven to eight hours of sleep. Teenagers need nine to ten hours. And school-aged children may need more than 10 hours of sleep a night. The quality and quantity of our sleep becomes poor as we age.  This happens because brain circuits that control sleep slowly begin to weaken. The process begins in our 30s. By the time we reach our 50s, the average person has lost 50 percent of their ability to have a restful sleep. These people also have trouble falling asleep and staying asleep through the night. From middle age on, these problems only get worse, and they can affect our health.
   There are several signs that may mean you are not getting enough sleep. You're always hungry or you've gained weight. A lack of sleep can affect the hormones that tell your brain you have eaten enough. You have trouble making decisions. A lack of sleep may affect your higher level brain functions, such as problem solving. You have trouble remembering things. Sleep helps prepare the brain for new memories. You feel very emotional. People who are sleep-deprived may feel unable to control their emotions, whether they are angry, sad or happy.
   There are things people can do to get better sleep. Health experts recommend exercise and avoiding work on computers and electronic devices before bed. A cool and quiet room is also a better environment for sleeping.
   What is released in our sleep to help fight infections and diseases?