One way to understand why we 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 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 of reality. Rats forced to stay awake continuously will eventually die, proving that sleep is essential.

A person who gets just a few hours of sleep per night can experience many of the same problems over time.