A、They are more health-conscious. B、They are changing their living habits. C、They get less and less sleep. D、They know the dange

The massive decline in sleep happened so slowly and quietly that few seemed to notice the trend. Was it because of the growing attraction of the Internet, video games and endless TV channels? Never disconnecting from work?(16)No matter how it happened, millions of Americans are putting their health, quality of life and even length of life in danger.
    New evidence shows why getting enough sleep is a top priority. Some 40% of Americans get less than 7 hours of shut-eye on week nights. "The link between sleep and health, and bad sleep and disease is becoming clearer and clearer. " says Lawrence Alberstaine, a sleep expert at Harvard University.(17)For example, sleep duration has declined from some 8 hours in the 1950s to 7 in recent years. At the same time, high blood pressure has become an increasing problem.(18)Blood pressure and heart rate are typically at their lowest levels during sleep. People who sleep less tend to have higher blood pressure, heart attack, diabetes, weight gain and other problems.
    Sleeping better may help fight off illness. "When people are sleep-deprived, there are higher levels of stress hormones in their bodies, which can decrease immune function. " says Doctor Felice of Northwestern University in Chicago. A university of Chicago study shows people who sleep well live longer. So say good night sooner and it may help you stay active and vital to a ripe old age.
Questions 16 to 18 are based on the passage you have just heard.
16. What is the speaker mainly talking about?
17. What do we learn from the talk about today’s Americans?
18. What does the speaker say will happen to people who lack sleep?