[单项选择]Adults typically need seven to nine hours of sleep each night to feel fully rested and function at their best. (1) , Americans are getting less sleep than they did in the past. A 2005 National Sleep Foundation poll found that Americans (2) 6.9 hours of sleep per night, which represents a drop of about two hours per night (3) the 19th century, one hour per night over the past 50 years, and about 25 minutes per night just since 2001.
(4) , we are not very good at perceiving the (5) effects of sleep deprivation. Researchers at the University of Pennsylvania restricted (6) to less than six hours in bed per night for two weeks. The volunteers (7) only a small increase in sleepiness and thought they were (8) relatively normally. However, formal testing showed that their cognitive abilities and reaction times (9) declined during the two weeks. By the end of the two-week test, they were as (10) as subj