单项选择题

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

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单项选择题

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.

单项选择题

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