Judging from recentsurveys,most experts in sleep behavior agree that it is virtually an epidemicof sleepiness in the nation."I can't think of a single study that hasn'tfound

Americans gettingless sleep than they ought to,"says Dr. David.Even people who think theyare sleeping enough would probably be better off with more rest.

The beginning of oursleep-deficit crisis can be traced to the invention of the light bulb a centuryago.From diary entries and other personal accounts from the 18th and 19th

centuries,sleepscientists have reached the conclusion that the average person used to sleepabout 9.5 hours a night. "The best sleep habits once were forced onus,when we

had nothing to do inthe evening down on the farm,and it was dark." By the 1950s and 1960s,thatsleep schedule had been reduced dramatically,to between 7.5 and 8

hours,and mostpeople had to wake to an alarm clock."People cheat on their sleep,and theydon't even realize they're doing it,"says Dr. David."They thinkthey're okay

because they can getby on 6.5 hours,when they really need 7.5,8 or even more to feel ideallyvigorous."

"Perhaps themost merciless robber of sleep,"researchers say,"is the complexity ofthe day." Whenever pressures from work,family,friends and communitymount,many

people considersleep the least expensive item on his program."In our society,you'reconsidered dynamic if you say you only need 5.5 hours' sleep.If you've got toget 8.5

hours,people thinkyou lack drive and ambition."

To determine theconsequences of sleep deficit,researchers have put subjects through a set ofpsychological and performance tests requiring them,for instance,to add columns

of numbers or recalla page read to them only minutes earlier."We've found that if you're insleep deficit,performance suffers,"says Dr. David."Short-term memoryis

weakened,as areabilities to make decision and to concentrate."

It can beconcluded from the passage that one should sleep as many hours as is necessaryto________.

A.improve one'smemory dramatically

B.be considereddynamic by other people

C.maintain one'sdaily schedule

D.feel energetic andperform adequately