Sound sleepers share a surprising secret: a bustling brain. A new study reports that people who can sleep through anything show more frequent bursts of brain activity called sleep spindles than do their light-sleeping counterparts. Researchers say the discovery could lead to spindle-enhancing techniques that offer lighter sleepers a chance at dead-to-the-world rest.

Sleep spindles happen only during sleep when brain waves slow. Scientists first spotted them in the 1930s, but they didn’t suspect they were involved in how deeply people sleep. For decades, researchers instead chalked up the vast variability between light and heavy sleepers to differences in sleep stage; sound sleepers were thought to spend more of their repose in the deeper stages of sleep.

Then in the 1990s, scientists tracked down the spindle’s source: the thalamus, a brain region that regulates sleep and also processes and relays sensory information to the cerebral cortex. The spindle-thalamus link made it “logical that the sleep spindle would play a role in regulating sensory input while we sleep,” says Jeffrey Ellenbogen, a sleep researcher at Harvard Medical School and Massachusetts General Hospital in Boston. “But no one had actually shown this.”…

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