By the time we (1)________old age most of us have (2)________ twenty years sleeping. Yet nobody knows why we do it. Most scientists believe that by resting our bodies, we allow time for (3)________ maintenance work to be done. Any (4)________ that there is can be put right more quickly if energy isn’t being used up doing other things. Sleep is controlled by certain chemicals. These build up during the day, eventually reaching peaks that cause tiredness. We can control the effects of these chemicals to some extent. Caffeine helps to (5)________us awake while alcohol and some medicines make us sleepy. By using electrodes, scientists are able to (6)________what goes on in people’s heads while they sleep .They have (7)________ that when we first drop off everything slows down. The heart (8)________ more slowly and our breathing becomes shallow. After about 90 minutes our eyes start to twitch and we go into what is called REM (Rapid Eye Movement) sleep, which is a (9)________that we’ve started to dream .You have dreams every night, even if you don’t remember them. There are many theories about why we dream, none of them conclusive. A lot of people say they have to have eight hours’ sleep every night while others seem to (10)________on a lot less . One thing’s for sure – we all need some sleep. Going without it can have some very strange effects. 1. A. arrive B. become C. reach D. get 2. A. passed B. used C. occupied D. spent 3. A. essential B. elementary C. needed D. main 4. A. harm B. suffering C. damage D. hurt 5. A. stay B. maintain C. make D. keep 6. A. exhibit B. study C. work D. think 7. A. seen B. researched C. watched D. discovered 8. A. beats B. hits C. moves D. trembles 9. A. clue B. proof C. sign D. signal 10. A. manage B. need C. get D. deal