Human beings spend on average one third of their lives asleep. We know we need to sleep but most of us have never really given a whole lot of thought to why. Why do we spend seven or eight hours a night immobile and unconscious? What really happens inside our brains and bodies while we're sleeping? We've known the purpose of our other biological drives for hundreds of years: we eat to give our bodies energy, we drink to keep hydrated, we procreate to perpetuate the species - among other things. But what is the biological purpose of sleep? It turns out no one really knows for sure. As correspondent Lesley Stahl reports, why we sleep is one of the biggest unanswered questions in all of science, which is why researchers all over the country are doing studies and coming up with some new and intriguing discoveries. "We don't sleep just to rest our tired bodies?" Stahl asks Matthew Walker, the director of the Sleep and Neuroimaging Lab at the University of California, Berkeley. "Well, that's been one of the long-standing theories. But I think what we're starting to understand is that sleep serves a whole constellation of functions, plural," Walker explains. One thing that's clear, says Walker, is that sleep is critical. In a series of studies done back in the 1980s, rats were kept awake indefinitely. After just five days, they started dying. Walker says they started dying from sleep deprivation. "In fact, sleep is as essential as food because they will die just about as quick from food deprivation as sleep deprivation. So, it's that necessary," he says.
And it's not just rats: every animal studied so far needs sleep, from the elephant right down to the fruit fly. But that's as far as the similarities go. Some animals sleep 20 hours a day, others only two or three. And still others sleep with half their brains at a time, all making it hard to figure out what exactly it is about sleep that makes it so essential, and that, in terms of evolution, makes it worth the risks. "You wonder why we developed this if survival is the whole point.
