A good way to understand why you sleep is to look at what happens when we don't get enough:
As you know if you have ever pulled an all-nighter, missing one night of sleep is not fatal. A person will generally be irritable during the next day and will either slow down (become tired easily) or will be totally wired because of adrenaline.
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- 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. 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 what is commonly called "Hell Week," the trainees must engage in highly physical activities for about six days - all of their hard work is accomplished on about 4 hours of sleep for the entire week!