Many people would reply by saying a minimum of eight hours of rest. But the answer doesn't depend solely on how many hours you log in bed. Night after night, you need deep uninterrupted sleep in a bed that provides adequate comfort support and space. What matters most of all is how you feel in the morning. If you wake up full of renewed energy, you've had a good night's sleep.
There is no one formula for how much sleep is enough for you. Expecting all people to need the same amount of rest would be as absurd as expecting them to eat the same amount of food every day. Each of us seems to have an innate sleep "appetite" that is as much a part of our genetic programming as hair color, height and skin tone. Normal sleep times range from five to ten hours; the average is 7 1/2. About one or two people in one hundred can get by with just five hours; another small minority needs twice that amount.
How much sleep is enough for you? To figure out your sleep needs, keep your wake-up time the same every morning and vary your bedtimes. Are you groggy after six hours of shut-eye? Does an extra hour give your more stamina? What about an extra two hours? Since too much time in bed can make some people feel sluggish, don't assume that more is always better. Listen to your body's signals and adjust your sleep schedule to suit them.
Keep in mind that sleep needs change with age. And the older you are, the less total sleep time you may need. A newborn may spend 18 hours asleep. From infancy to adulthood, sleep decreases by more than half. Throughout the middle decades of life, seven or eight hours of sleep generally are needed to provide adequate rest. For older individuals, six hours may suffice.