What is sleep?

About Sleep

Introduction to sleep
When you're in a rush to meet work, school, family, or household responsibilities, do you cut back on your sleep? Like many people, you might think that sleep is merely a "down time" when the brain shuts off and the body rests. Think again.

What is sleep?
Sleep was long considered just a uniform block of time when you are not awake. Thanks to sleep studies done over the past several decades, it is now known that sleep has distinctive stages that cycle throughout the night. Your brain stays active throughout sleep, but different things happen during each stage. For instance, certain stages of sleep are indeed for us to feel well rested and energetic the next day, and other stages help us learn or make memories.
In brief, a number of vital tasks carried out during sleep help maintain good health and enable people to function at their best. On the other hand, not getting enough sleep can be dangerous—for example, you are more likely to be in a car crash if you drive when you are drowsy.

How much sleep is enough?
Sleep needs vary from person to person, and they change throughout the lifecycle. Most adults need 7–8 hours of sleep each night. Newborns, on the other hand, sleep between 16 and 18 hours a day, and children in preschool sleep between 10 and 12 hours a day. School–aged children and teens need at least 9 hours of sleep a night.
Some people believe that adults need less sleep as they get older. But there is no evidence to show that older people can get by with less sleep than younger people. As people age, however, they often get less sleep or they tend to spend less time in the deep, restful stages of sleep. Older people are also more easily awakened.

http://www.medicinenet.com/sleep/page4.htm