（英语类学生必做） Sleep is important to us because it helps restore organs and tissues in our body. But how much sleep do we actually need? For most of us, eight hours seems to be about the right amount. Yet we know that there are many people who get 【61】 perfectly with less sleep, and some who may need 【62】 A great deal depends on the 【63】 we live. But a good general rule 【64】 is to sleep as long as we have to in order to feel happy and be able to work 【65】 our best when we awaken. There are actually different 【66】 of sleep. There is a deep sleep and a shallow sleep. In a shallow sleep our body does not get the same kind of rest it gets in a 【67】 sleep, 【68】 after eight hours of a shallow sleep we may still feel tired. But a short, deep sleep can be very 【69】 Alexander—the Great Emperor was able to get a deep sleep 【70】 he needed it. Once, during the night before an important battle, he remained 【71】 longer than anyone else. 【72】 he wrapped himself in a cloak and 【73】 down on the earth. He slept so 【74】 that his generals had to wake him three times for him to give the 【75】 to attack! Normally when we go to sleep, our ' sleep center' blocks off nerves so that 【76】 our brain and body go to sleep. 【77】 prevents us from wanting to do anything, and the other 【78】 our internal organs and limbs go to sleep. 【79】 sometimes only one goes to sleep and the other does not. A very tired soldier can sometimes 【80】 asleep (brain sleep) and keep on marching, because his body is not asleep. (86)