Snuggling under the covers is one of life's great pleasures, topped only perhaps by the satisfaction of waking up after a good night's sleep. But sleep is something we're getting too little of: While we need anywhere from 7-9 hours a night to feel rested, Americans sleep an average of 6 hours and 40 minutes during the week and about 7-and-a-half hours on weekend nights, according to the National Sleep Foundation (NSF). Depending on their age, kids need 11-18 hours, but up to half of them aren't getting enough, parents told the NSF in a poll. Why do we need to get more sleep? Besides helping you feel rested, a good night's sleep can make you smarter, happier, slimmer and improve your memory.