March21hasbeendeclaredWorldSleepDay，atimetorecognizeandcelebratethevalueofsleep．Manysleepexpertshopeitwillbeawake-upcall．Cdordingtoa2013poll（民意调查）bytheNationalSleepFoundation，nearly4in5Americans

　　March 21has been declared World Sleep Day，a time to recognize and celebrate the value of sleep．Many sleep experts hope it will be a wake-up call．

　　Cdording to a 2013poll （民意调查）by the National Sleep Foundation，nearly 4in 5Americans don't get as much sleep as they should during the workweek．On average，adults are thought to need at least eight hours of sleep a night，although some can manage with less and some won't do well without more．But the survey found that，on workdays，only 21% of Americans actually get a full eight hours of sleep，and another 21% get less than six．

　　To many of us，the thought of spending more time sleeping is，well，a big yawn．On the other hand，the thought of being smarter，thinner，healthier and more cheerful has a certain appeal．And those are just a few of the advantages that can be ours if we consistently get enough sleep，researchers say．Also on the plus side：We're likely to have better skin，better memories，better judgment，and，oh，yes，longer lives．

　　"When you lose even one hour of sleep for any reason，it influences your performance the next day，"says Dr．Alon Avidan，director of the UCLA Sleep Disorders Center．

　　A study published last year found the same to be true even of children．When kids aged 8to 12slept for just one hour less for four nights，they didn't function as well during the day．

　　But sleeping has an image problem．"We see napping or sleeping as lazy，"says Jennifer Vriend，a clinical psychologist in Ottawa，Canada，and the leading author of the study with children．"We put so much emphasis on diet，nutrition and exercise．Sleep is in the back seat．"In fact，she adds，no matter how much we work out，no matter how well we eat，we can't be in top physical shape unless we also get plenty of sleep．

　　31．The underlined part in Paragraph 1means___．

　　A．To wake up the sleepers

　　B．To serve as a morning call

　　C．To draw people's attention

　　D．To declare