After a day’s hard work, we need some sleep. During the sleep, the fatigue (疲劳) of the body disappears and recuperation (恢复) begins. The tired mind gathers new energy, and the memory improves: Some adults require little sleep; others need eight to ten hours in every twenty-four hours. Infants (婴儿) sleep sixteen to eighteen hours dally, the amount gradually diminishing as they grow older. Young students may need twelve hours; university students may need ten. A worker with a physically demanding job may also need ten, whereas an executive working under great pressure may manage on six to eight. Many famous people have little sleep. Napoleon Bonaparte, Thomas Edison and Charles Darwin apparently averaged only four to six hours a night.

Whatever your individual need, you can be sure that by the age of thirty you will have slept for a total of more than twelve years. By that age you will also have developed a sleep routine: a favorite hour, a-favorite bed, a favorite position, and a rule you need to follow in order to rest comfortably.

Investigators have tried to find out how long a person can go without sleep. Several people have reached more than 115 hours—nearly five days. Whatever the limit, it is absolute. Animals that kept awake for from five to eight days have died of exhaustion. The limit for human beings is probably about a week.

By writing this passage, the writer intends to tell us that（）

A. sleep is important for good mental and physical health

B. a light sleep is as restful as a deep sleep

C. memory is improved during sleep

D. sleep is relatively unimportant for human beings