【案例分析题】Sleep 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 halfhours of sleep a night to function properly throughout their lives.（Contrary to popular belief，humans don’t need less sleep as they age.） But in average，Americans sleep only about seven and a half hours per night，a marked drop from the nine hours they 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 80different sleep disorders.Some sleeping disorders are genetic.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 as 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 Elaner，who used to suffer from sleeplessness，a few lifestyle.changes—avoiding stimulants and late meals，exercising hours before bedtime，relaxing with a hot bath—yield better sleep.

What is TRUE of human sleep? （）.