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 morning，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”一in this case something like six hours，almost a full night’s sleep．
Sleep debt is the difference between the amount of sleep you should be getting and the amount you actually get．It’s a shortage that grows every time we skim some extra minutes off our nightly sleep．“People get more sleep debt gradually without being noticed，”says William C．Dement，founder of the Stanford University Sleep Clinic．Studies show that such short term sleep lack leads to a foggy brain，worsened vision，and trouble remembering．Long-term effects include obesity(肥胖)，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——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 all 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 rules 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．
53．If rou have short-term sleep lack，you may         ．   
A．put on weight    B．get heart disease
C．hate eating food D．have a poor sight
54．The example of sleep math is used to show         ．
A．in what case you build up a sleep debt
B．in what way you can make up for lack of sleep
C．why you are energetic even without enough sleep
D．why you need six hours’sleep every night
55．By saying the underlined sentence，Dement means that         ．
A．you need a lot of sleep to be a superman
B．you will be in a good state with enough sleep
C．you need to be a superman to repay sleep debt   
D．you will become a superman with more sleep debt
56．What might be the most suitable title for the passage?   
A．How can you keep energetic?    B．What is sleep debt?
C．Can you catch up on lost sleep?D．Can coffee refresh you?