In order to answer this, let's look at some basics about sleep. The amount you need decreases with age. A newborn baby might sleep 20 hours a day. By age 4, the average is 12 hours a day. By age 10, the average falls to 10 hours a day. Senior citizens can often get by with 6 or 7 hours a day. And most adult people seem to need 7 to 9 hours of sleep a night. This is an average, and it is also subjective. You, for example, probably know how much sleep you need in an average night to feel your best.

The average adult needs 7 to 9 hours of sleep a night.

You know you feel great after a good night of sleep. But, why is that? Does anything important happen during sleep?