One of the easiest and cheapest ways to help your health is just to sleep eight hours or more every night, but more and more people in the world are not sleeping enough. According to the World Health Organization, over half the people in the world may be _ . The result of this is not just a lot of tired people; in the United States alone, sleepy drivers cause at least 100,000 car accidents, and 1,500 deaths a year. Problems with sleep can also cause mental   problems, as well as medical problems such as high blood pressure and heart problems. American culture values work and often doesn't value sleep. In fact, people who sleep a lot are often called "lazy". Many famous men say proudly, "I only have time to sleep four or five hours a night." Students, especially college students, often sleep only a few hours a night. They often say, "I'm used to sleeping only a little." But according to experts, sleep is like money. If you sleep only five hours a day, you don't "get used to it", but instead, build up a "sleep deficit  ". "It's like a credit card (a card used to buy things or services)," says Dr. James Maas, "You are only borrowing time. You always have to pay it back." The more hours you don't sleep, the more hours you should sleep to "pay back" the hours on your "sleep credit card". For example, if you sleep four hours on Monday and then sleep eight hours on Tuesday, on Wednesday you will still have a "sleep deficit" of four hours. This deficit can continue for months or even years. People with a "sleep deficit" are sleep-deprived. Driving and making decisions can be dangerous for these people. Another research shows people should sleep nine or ten hours every night. More than half the people in the world may be running a "sleep deficit". You didn't do your homework last night? Maybe you can tell your teacher that you were working hard on your sleep deficit. "Don't think you will be doing less work because you sleep during the day. That's a foolish idea which is held by people who have no imagination. You will be able to do more," said Winston Churchill. From the passage we know sleep-deprived people   _  .
are easily tired most likely cause traffic accidents can have both mental and medical problems have all the above problems