The sleep debt is the difference between the amount of sleep you should get and the amount you actually get. It grows every time we skim some extra minutes off our nightly sleep. “People accumulate sleep debts gradually,” says Dr. William Dement, founder of the Stanford University Sleep Clinic. Studies show that such short-term lack of sleep leads to a foggy brain, impaired vision, worsened driving and troubled remembering. Long- term effects include obesity, insulin(胰岛素) resistance, and heart disease.
A 2007 survey by the National Sleep Foundation reports that Americans sleep for 6.9 hours per night, 6.8 hours on average during the week and 7.4 hours on the weekends. Generally, experts recommend eight hours of sleep per night, although some people may require only six hours of sleep while others need ten. That means on average, Americans lose one hour of sleep each night more than two full weeks of sleep every year.
The good news is that the sleep debt can be repaid with some work, though it won’t happen when you sleep longer for once. Adding an extra hour or two hours of sleep a night is the way to catch up. For the long-term sleep shortage, it takes a few months to get back to natural sleeping pattern, says Lawrence J. Epstein, medical director of the Harvard Sleep Health Center.
Go to bed when you are tired, allowing your body to wake you in the morning, with no alarm clock. You may find yourself uncomfortable at the beginning of the recovery cycle. Expect to bank to ten hours shut-eye per night. As the days pass, however, the amount of sleeping time will gradually decrease. For recovery sleep, both the hours of the sleep and the intensity(强度) of the sleep are important. The most refreshing sleep occurs during deep sleep, which is generally considered a restorative (促使健康的) period for the brain. And when you sleep for more hours, you allow your brain to spend more time relaxing.
As you reduce the sleep debt, your body will come to rest at a sleeping pattern that is specifically right for you. Sleep researcher believe that genes determine our individual sleeping patterns. So you can’t train yourself to be a “short sleeper”. A 2005 study in the journal Sleep found that the more tired we get, the less tired we feel.
46. Which of the following is NOT the side effect of sleep debt?
A. Putting on weight.  B. Having a bad memory. 
C. Having trouble eating food.  D. A temporary loss of eyesight.
47. We learn from the 2007 survey that _______.
A. Americans generally don’t have enough sleep
B. Americans sleep too much over the weekends
C. everyone is supposed to sleep for eight hours
D. most people lack two weeks of sleep every year
48. What should we do to make up for the sleep shortage?
A. We should go to sleep when we are free.
B. We should sleep for at least ten hours every day.
C. We should sleep for one or two more hours at night.
D. We should sleep day and night during the holidays.
49. Why is the intensity of the sleep important to us?
A. Our brain is resting when we are sleeping deeply.
B. Deep sleep helps our brain to fully recover.
C. We feel more relaxed when sleeping soundly.
D. Short sleep makes our body more refreshing.
50. The author seems to believe that _________.
A. the more tired we are getting, the more sleep we need 
B. the sleeping patterns have nothing to do with our genes
C. it is possible for us to reduce our sleep time by training
D. it is wise for us to adapt to our natural sleeping patterns.