Read the article and select the best answer. Article: 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"----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 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 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 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 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 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. Question: The example of sleep math is used to show_. Options: 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 have enough sleep D: you should drink coffee to keep energetic when you don't have enough sleep
The answer to this question is:
A