Let’s do some sleep math．You lost two hours of sleep every night last week because of a big project due on Friday．On Saturday and Sunday，you slept in，getting four extra hours．On Monday morningi, you were feeling so bright-eyed that you only had one cup of coffee,instead of your usual two．But don’t be cheated by your energy．You’re still carrying around a heavy load of sleepiness, or what experts call“sleep debt”——in this case something like six hours，almost a full nights’ sleep．
Sleep debt is the difference between the amount of sleep you should be getting and the amount you actually get．It’s a deficit(缺乏)that grows every time we skim some extra minutes off our nightly sleep．“People accumulate sleep debt gradually without being noticed，”says William C．Dement，founder of the Stanford University Sleep Clinic．Studies show that such short—term sleep deprivation leads to a foggy brain，worsened vision，and trouble remembering. Long-term effects include obesity，insulin(胰岛素)resistance, and heart disease.A survey by the National Sleep Foundation reports that we’re losing one hour of sleep each night一一more than two full weeks of sleep every year.
The good news is that，like all debt, with some work，sleep debt can be repaid. Adding an extra hour or two of sleep a night is the way to catch up．For the long—term lack of sleep，take it easy for a few months to get back into a natural sleep pattern．
Go to bed when you are tired，and allow your body to wake you in the morning(no alarm clock allowed)．You may find yourself catatonic(有紧张症的)in the beginning of the recovery cycle：expect to have ten hours shut-eye per night. As the days pass, however，the amount of sleeping time will gradually decrease.
So earn back that lost sleep——and follow the dictates of your innate(固有的)sleep needs. You’ll feel better．“When you put away sleep debt,you become a superman，”says Stanford’s Dement,talking about the improved mental and physical capabilities that come with being well rested．
51．If you have short—term sleep deprivation, ____________.
A．you can think and remember things clearly
B．you can still see everything very clearly
C．you can drive your car easily
D．you may have a poor sight
52．The example of sleep math is used to show____________
A．in what case you build up a sleep debt
B．why you need six hours’ sleep every night
C．why you are full of energy even when you don’t haye enoug sleep
D．you should drink coffee to keep energetic when you don’t have enough sleep
53．The author begins Paragraph 3 with ____________.
A．an order          B．a story             C．a definition            D．an example
54．By saying the undedined sentence in the last paragraph，Dement means ____________
A．a superman always needs a lot of sleep
B．you will be in a good state with enough sleep
C．you can become superman after you repay your debt
D．You will become superman if you don’t make up for sleep debt
55．What might be the most suitable title for the passage?
A．Can you catch up on lost sleep?             B．How can you keep energetic?
C．Can you have a good sleep?                  D．What is sleep debt?