Why We Sleep

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What McNaughton's recordings have shown is that many of the same neurons that fire during the daytime—say, when a rat is learning to navigate a maze—are reactivated during the REM stage of sleep.

"Basically, the brain is reviewing its recently stored data," he says. Eventually the brain consolidates those patterns into permanent connections—or, as neuroscientists like to say, "neurons that fire together, wire together." Interestingly, says McNaughton, that process appears to happen not just during sleep but during restful states throughout the day as well.

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Slow-Wave Learning
Better equipment has also given researchers a new respect for what can be accomplished during slow-wave sleep. In a study published in July in Nature, Wisconsin's Tononi and others showed that a specific part of the brain that had been busy learning a new skill while awake needed much more slow-wave sleep in order to improve performance.

The scientists had 11 volunteers play a simple video game that required them to reach for objects on the screen with a mouse-controlled cursor. What the volunteers didn't realize was that the game sometimes introduced a slight bias to the cursor's motion, forcing them to adjust their movements. Half the group slept between sessions and the other half did not. Among the sleepers, the part of the brain that was learning to compensate for the bias while awake turned out to have the largest slow waves during sleep. "The bigger the slow waves were in that part of the brain, the better they performed the next day," Tononi says.

So far, so good. But what does it mean? Tononi speculates that instead of strengthening neural connections responsible for a given task, as appears to happen during the day or in REM sleep, slow-wave sleep actually indiscriminately weakens the connections among all nerves. The idea sounds counterintuitive, but it may simply be a matter of self-preservation. "Normally the brain takes up 20% of the energy of the entire body," Tononi explains. Most of that energy goes into sustaining the connecting points, or synapses, between neurons.

The more you learn, the greater the number of synapses. "So by the end of the day, if you have synapses that are much stronger, the cost of running the brain is much higher," he says—perhaps another 20%.

It doesn't take a neuroscientist to figure out where that leads.

After a few days, the number of new synapses in the brain would require more energy than the body could possibly supply. So some of those connections must be weakened—and the best guess is that it happens during slow-wave sleep.

That explanation is still hypothetical, but Tononi thinks he has evidence to back it up. "In slow-wave activity, all the neurons fire for half a second," he explains. "Then they're totally silent for half a second." For complex bioelectrical reasons, that turns out to be a perfect way for the brain to lower the strength of the connections between its neurons. Intermittent firing makes the connections leaner and more efficient and may even allow the weakest ones to drop out, clearing the mind so that it can learn something new in the morning.

A Theory of Sleep
Perhaps that's what sleep really is—a series of repeated cycles of pruning and strengthening of neural connections that enables you to learn new tricks without forgetting old ones. Of course, none of that explains why you have to be unconscious for all the pruning and strengthening to occur. Maybe it's just easier to be asleep than awake while the work is going on. "When you fall asleep, it's like you're leaving your house and the workmen come in to renovate," suggests Terry Sejnowski, a computational neurobiologist at the Salk Institute in La Jolla, Calif. "You don't want to live in the house while the construction's going on because it's a mess."

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