If you've ever pulled an all-nighter or gone without sleep for more than two days in a row, you know what happens. You start to feel groggy, and eventually have trouble concentrating. You might even hallucinate. Go without sleep for too long and, as studies using rats have shown, you can die. Clearly, our bodies need sleep.

The question is, why? What happens during sleep that helps keep us stay alert, healthy, and alive?

Oddly, nobody knows for sure. However, there are several theories as to why sleep is essential for good health. One theory suggests that during sleep the body repairs itself by repairing muscles and other tissues and replacing dead cells with new ones.

Similarly, according to another theory we need to sleep because that's when the brain rests and recharges itself. The brain doesn't completely shut down, of course, since we still dream, breathe, move around, and do other things that wouldn't be possible if the brain were completely out of action. The notion that the brain needs to rest goes along with a third theory that we sleep in order to conserve energy.

What all of these theories have in common is the practical notion that without sleep we rob ourselves of the ability to recharge our batteries, as it were. What we know without a doubt is that not sleeping enough will eventually catch up with you. How much is enough? It depends on you. Some people can't function without at least eight hours, while some are fine with six or fewer. However, too many sleepless nights will lay anyone low.