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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."
It all sounds plausible enough, but that doesn't mean everyone is convinced. "It may not sound exciting, but I think sleep is essentially for rest," says Robert Vertes, a neuroscientist at Florida Atlantic University in Boca Raton. Vertes thinks most sleep scientists are overinterpreting their data because they find it so hard to believe that our brains just need to shut down for eight hours or so every night. As for what's being done during that time, the short answer, he says, is "We don't know."
Perhaps the brain just needs to restore itself. "We've all had the experience of going to bed with a problem, getting a good night's sleep and waking up in the morning, and there's a solution," says Dr. Gregory Belenky, who recently retired as head of sleep research at the Walter Reed Army Institute of Research in Silver Spring, Md., and is now at Washington State University at Spokane. But instead of thinking that extra information processing is going on during sleep, he says it makes as much sense to suggest that depleted circuits are just being rejuvenated.