Why We Sleep: Not to Remember, But to Forget

Does sleep allow us to cull out and delete the throngs of ordinary, unimportant memories from each day?

By Anne Wootton|Tuesday, October 10, 2006

Our need for sleep is somehow tied to our ability to remember. Slumber is known to improve recall in creatures from fruit flies to humans, and the reigning theory among neuroscientists has been that the waves of brain activity during deep sleep reactivate neurons that were triggered during the day, strengthening neuronal connections and cementing them into solid memories. Now Giulio Tononi, a neuroscientist at the University of Wisconsin, says sleep scientists have it all wrong: We don't sleep to remember, we sleep to forget.

About 1,000 times a night, billions of neurons undergo a synchronous one-second burst of non-REM electrical activity. The longer a person has been sleep-deprived, the bigger the initial burst. Throughout the night the bursts become progressively smaller, until they finally disappear completely just before waking. Most researchers interpret this activity as the brain slowly reinforcing synaptic connections that already exist, but Tononi noticed that after each wave, the brain goes completely silent, which never happens when we're awake.

As we sleep, says Tononi, the brain isn't building but rather downscaling, and these silences between waves play a key role. "Going up and down, up and down, basically all the neurons fire and then all are silent—it's a wonderful way for the brain to tell the synapses to get weaker," Tononi explains. He suspects the progressive weakening allows only the strong connections to survive.

The theory is unorthodox, but it does make a certain amount of sense. Without the ability to pare away unneeded information as we sleep, our brains would face a serious energy shortage as well as a space crunch: Stronger synapses are typically bigger, and real estate in the brain is precious. By proportionally weakening synapses, the brain ensures that they retain the same strength relative to each other. So when we wake up each morning, all of our synapses are weaker, and some have vanished. With them, our smallest memories from each day may be lost forever.

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