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

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.

You know you feel great after a good night of sleep. But, why is that? Does anything important happen during sleep? Growth hormone in children is secreted during sleep, and chemicals important to the immune system are secreted during sleep. You can become more prone to disease if you don't get enough sleep, and a child's growth can be stunted by sleep deprivation.

No one really knows why it is that we sleep, but there are all kinds of theories, including these:

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  • Sleep gives the body a chance to repair muscles and other tissues, replace aging or dead cells, and so on.
  • Sleep gives the brain a chance to organize and archive memories. Dreams are thought by some to be part of the process.
  • Sleep lowers our energy consumption, so we need three meals a day rather than four or five. Since we can't do anything in the dark anyway, we might as well "turn off" and save the energy.
  • Sleep may be a way of recharging the brain.