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For millions of years before the appearance of the electric light， shift work，all-night cable TV and the Internet， Earth’s creatures evolved on a planet with predictable and reassuring 24－hour rhythms. Our biological clocks are set for this daily cycle.Simply put，our bodies want to sleep at night and be awake during the day. Most women and men need between eight and eight and a half hours of sleep a night to function properly throughout their lives.(Contrary to popular belief，humans don’t need less sleep as they age.)
But on average，Americans sleep only about seven and a half hours per night，a marked drop from the nine hours day averaged in 1910.what’s worse，nearly one third of all Americans get less than six hours of sleep on a typical work night. For most people， that’s not nearly enough.
Finding ways to get more and better sleep can be a challenge. Scientists have identified more than 80 different sleep disorders. Some sleeping disorders are generic. But many problems are caused by staying up late and sleeping in， by traveling frequently between time zones or by working nights. Dr. James. F. Jones at National Jewish Medical and Research Center in Denver says that sleep disorders are often diagnosed at other discomforts. About one third of the patients referred to him with possible chronic fatigue syndrome actually have treatable sleep disorders. “Before we do anything else， we look at their sleep，” Jones says.
Sleep experts say that most people would benefit from a good look at their sleep patterns， “My motto (座右铭) is ‘sleep defensively’，” says Mary Carskadon of Brown University. She says people need to carve out sufficient time to sleep， even if it means giving up other things. Sleep routines－like going to bed and getting up at the same time every day－are important. Pre－bedtime activities also make a difference. As with Elsneer， who used to suffer from sleeplessness， a few lifestyle change－avoiding stimulants and late meals， exercising hours before bedtime， relaxing with a hot bath—yield better sleep.
90.What is TRUE of human sleep？
A. Most people need less sleep when grow older.
B. Most people need seven and a half hours of sleep every night.
C. On average， people in the U. S. today sleep less per night than they used to.
D. For most people， less than six hours of sleep on a typical work night is enough.
91.For our bodies to function properly， we should .
A. sleep for at least eight hours per night
B. believe that we need less sleep as we age
C. adjust our activities to the new inventions
D. be able to predict the rhythms of our biological clocks
92.According to the author， many sleeping disorders are caused by .
A. other diseases
B. pre－bedtime exercises
C. improper sleep patterns
D. Chronic fatigue syndrome
93.Which of the following measures can help you sleep better？
A. Staying up late.
B. Taking a hot bath.
C. Having late meals.
D. Traveling between time zones.
94.“Sleep defensively” means that .
A. people should go to a doctor and have their problems diagnosed
B. people should exercise immediately before going to bed every night
C. people should sacrifice other things to get enough sleep if necessary.
D. People should give up going to bed and going up at the same every day.
