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Why do we sleep? So the brain can forget

Why do we sleep? Is it to allow a weary mind to recover from a long day of making tens of thousands of decisions? Maybe. But two scientific teams working on different aspects of the question have come up with a startling, new answer. We sleep in order to forget some of what we learned during the day, they wrote in a pair of studies in Science.

When we learn, neurons in our brain form links between each other. They're called synapses, and these connections between neurons form memories and allow us to access information. But they also can become a tangled bundle of connections that are quite messy – much like the way in which wires become a tangled mess behind our cable TV system.

When we sleep, these researchers discovered, our brain prunes these newly formed synapses and untangles them so that the important parts of what we learned during the day are stored and accessed more efficiently. Because our memories are stored in these neural networks, the work to clean them up while we sleep is important.

Researchers have known for some time that these connections between neurons can grow wildly during the day as our brains process massive amounts of information.

What they didn't know until these news studies were conducted, however, was what happened to this explosive growth of these synapses in brains. So they studied the brains of mice, and found that these connections shrank nearly 20% overnight. That's an extraordinary amount, and confirms that this is clearly a vital part of the brain's effort to streamline neural networks.

In the second study in Science, researchers literally created a tiny window that allowed them to peer into a mouse brain while it slept. They then placed chemicals on top of the synapses so that they could watch how it functioned overnight. They watched, through that tiny window, as a particular surface protein shrank the synapses.

That convinced the researchers that this streamlining of the neural networks allowed us to forget some portion of what we process each day so that we can store memories more efficiently and actually learn. "You can forget in a smart way," one of the researchers, Dr. Giulio Tononi, a biologist at the University of Wisconsin-Wisconsin, told Carl Zimmer of The New York Times.