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When all else fails, take a nap. - A.A. Milne, author of Winnie-the-Pooh

That advice may sound impractical to an overworking American. But according to circadian neuroscientist Russell Foster, sleep is an essential part of being a functional human being.

"Sleep is an incredibly important part of our biology," Foster said, "and neuroscientists are beginning to explain why it's so very important."

There are "dozens of different ideas about why we sleep," Foster says. Here are three of the main contenders.

Restoration

According to the director of the Sleep Medicine Program at NYU Langone Medical Center, David M. Rapport, M.D., our bodies use sleep as a time to "tuneup." During this time, we're essentially healing ourselves from the inside out, by releasing hormones that encourage tissue growth.

"A whole raft of genes have been shown to be turned on only during sleep, and those genes are associated with restoration and metabolic pathways," Foster said. "So, there's good evidence for the whole restoration hypothesis."

Energy Conservation

We save approximately 110 calories per night, which according to Foster is about the equivalent of a hot dog bun.

"Now, I would say that a hot dog bun is kind of a meager return for such a complicated and demanding behavior as sleep," Foster added. "So, I'm less convinced by the energy conservation idea."

Memory Consolidation

Did your grandma ever advise you to sleep on a problem, saying, "the answer will come to you in the morning"? If so, you should pat your granny on the back. She's right again.

"Sleep and memory consolidation is also very important," Foster noted. "However, it's not just the laying down of memory and recalling it. What's turned out to be really exciting is that our ability to come up with novel solutions to complex problems is hugely enhanced by a night of sleep."

"In fact, it's been estimated to give us a threefold advantage," he continued. "Sleeping at night enhances our creativity. And what seems to be going on is that, in the brain, those neural connections that are important, those synaptic connections that are important, are linked and strengthened, while those that are less important tend to fade away and be less important."

The National Sleep Foundation recommends getting at least eight hours of sleep per night. Teens, however, require 9.25 hours or more per night.