Sleep plays a vital role in our daily regimen, as during this dormant period, it recharges and rejuvenates weary body organs and tissues, and restores the body to an alert state. A person's lifestyle is another variable in the amount of time he needs to achieve this healthy balance, and to feel well rested upon awakening.

The quality of a person's sleep also factors into the sleep equation. When we drift off to sleep, we either fall into a deep, restful sleep, or into a shallow, light sleep. One who normally requires eight hours of sleep, but sleeps shallowly, will most likely awaken feeling tired. In contrast, if the same individual sleeps fewer hours, but lapses into a state of deep sleep, he may well awaken refreshed and invigorated.

A lucky few can bypass shallow sleep altogether, and fall directly into deep sleep on an as needed basis. Alexander the Great's military prowess may have been due, in part, to his ability to take a "cat nap."

Yet one more variable factors into the amount of sleep we require: that of the brain and of the body sleeping simultaneously. Under normal circumstances, the brain's sleep center blocks off the nerves to the brain and to the body to make sleep possible. The brain has no interest in doing anything, and could not even if it did, because the sleep center has shut down the internal organs, the arms, and the legs. Sometimes, however, the sleep center is unsuccessful in shutting down both the brain and the body at the same time. For example, in times of war, it is not unusual for soldiers suffering from sleep deprivation and from combat fatigue to continue to march, despite the fact that their brains have halted. Thankfully, such a situation is an anomaly, and does not occur with great frequency.