【正确答案】Although we are not sure why we sleep, there is no doubt that we need to do so and so do most other animals. The pattern of sleep and wakefulness is closely connected with our habits and senses. Animals which depend on sight for food, shelter and defense, like man, are for the most part are active during the day and asleep at night. The amount of sleep needed by a person to remain in full health varies considerably with age. Preschool children generally need ten to twelve hours' sleep, schoolchildren nine to eleven hours and adults seven to nine hours. Adults seem to need progressively less sleep as they grow older. The amount of sleep also varies with races. It has been said that the Japanese, both children and adults, sleep less than Europeans, but that may be due to habit rather than race.