Sleep ,as we know, is important to us because it helps restore tired organs and tissues in

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】

A．Yet

B．Hence

C．Anyway

D．Besides