Sleep gives your body a rest and allows it to prepare for the next day. It’s like giving your body a mini-vacation. Sleep also gives your brain a chance to sort things out. Scientists aren’t exactly sure what kinds of organizing your brain does while you sleep, but they think that sleep might be the time when the brain sorts and stores information, replaces chemicals, and solves problems. Time for Bed? The amount of sleep a person needs depends a lot on his or her age. Babies sleep a lot — about 14 to 15 hours a day! But many older people only need about 7 or 8 hours of sleep each night.
Source: http://sleepfoundation.org/how-sleep-works/how-much-sleep-do-we-really-need?page=0%2C1