Sleep ,as we know, is important to us because it helps restore tired 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.【C1】______we know that there are many people who get【C2】______perfectly with less sleep, and some who may even need【C3】______. A great deal depend on the【C4】______we live. But a good general rule to【C5】______is to sleep as long as we have to in order to feel happy and be able to work【C6】______our best when we awaken. There are actually different【C7】______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 deep sleep, so that after eight hours of a【C8】______sleep we may still feel tired. But a short, deep sleep can be very【C9】______. Alexander the Great was able to get a deep sleep【C10】______he needed it. Once, during the night before an important battle, he remained【C11】______much longer than anyone else.【C12】______he wrapped himself in a cloak and【C13】______down on the-earth. He slept so【C14】______that his generals had to wake him three times to give the【C15】______to attack! Normally when we go to sleep, our!' sleep center' blocks off nerves so that【C16】______our brain and body go to sleep.【C17】______prevents us from wanting to do anything. And the other【C18】______our internal organs and limbs go to sleep.【C19】______sometimes only one goes to sleep and the other does not. A very tired soldier can sometimes【C20】______asleep(brain sleep)and keep on marching, because his body is not asleep. 【C1】