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To answer this, we need to look at some basics about sleep. The amount you need depends on your age. A newborn baby might sleep 20 hours a day while a 4-year old might need only 12 hours. The average for 10-year-olds is 10 hours a day and most adults seem to need seven to nine hours of sleep a night. And the average senior citizen can often get by with just six or seven hours a day. Whatever amount you need, you know you feel great after a good night of sleep. But, why is that? Does anything important happen during sleep? Yes, scientists have found that two key things happen during sleep: growth hormone in children is secreted and chemicals important to the immune system are secreted. If you don't get enough sleep, you're more prone to disease, and a child's growth can be stunted by sleep deprivation.

Beyond the immune system and growth hormone factors, no one really knows why it is that we sleep, but there are all kinds of theories, including the following. For one, sleep gives the body a chance to repair muscles and other tissues, replace aging or dead cells. Also, sleep gives the brain a chance to organize and archive memories (dreaming is probably a part of that process). Furthermore, sleep may be a way of recharging the brain. And finally, in very early times, sleep made sense in that people couldn't really do anything in the dark anyway, so they might as well "turn off" and save the energy.

A good way to understand why you sleep is to look at what happens when we don't get enough. If you've ever pulled an all-nighter, you know that missing one night of sleep isn't fatal. A person will generally be irritable during the next day and will become tired easily or will be totally wired because of adrenalin. If a person misses two nights of sleep, it gets worse. Concentration is difficult and attention span falls by the wayside. Mistakes increase. After three days, a person will start to hallucinate and clear thinking is impossible. With continued wakefulness, a person can lose grasp on reality. A person who gets just a few hours of sleep per night can experience many of the same problems over time.

It only takes three days of sleep deprivation to cause a person to hallucinate. Obviously, if you were to go for a longer period of time, the symptoms would worsen and in time, would most likely prove fatal. Rats forced to stay awake continuously will eventually die, proving that sleep is definitely essential. So, unless you were being forced to stay awake, you'd probably fall asleep before something as drastic as death could happen.

It's interesting to know that some people can function on very little sleep if necessary. A portion of a Navy SEAL's rigorous training program is a good example of this phenomenon. During a particularly rigorous week of training, the trainees must engage in highly physical activities for about six days – all of their hard work is accomplished on about four hours of sleep for the entire week!