[单项选择] If you want to increase your odds of having a long and lively lifetime, scientists say, work on getting an average of seven or eight hours of sleep a night. If you get only six hours or less, you stand a 70 percent chance of dying before your time. Scientists are finding that if you don’’t get enough sleep, you are putting yourself at increased risk of heart trouble, digestive disease, or a serious, even fatal (致命的) accident. When tired, your sight, hearing, and attention are slow to alert you to dangers--and once you are aware of them, your reactions are also slow. H. Craig Heller--a professor of biology at Stanford University--is one of a small army of scientists studying the reasons why many people can’’ t sleep and wake refreshed, why the brain puts you to sleep, and why lack of sleep causes such powerful effects. Heller notes that most major accidents have occurred in the early morning, when workers probably were still sleepy from deficient (不足的) sleep. He includes
A. Getting too little sleep is the commonest sleep trouble with most Americans who have sleep trouble.
B. At least twenty percent of Americans don’’ t have enough sleep every night.
C. Our brains will force us to sleep when we have piled up too large a sleep deficit
D. Seven or eight hours of sleep every night is enough for people of different ages.