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."
So Ellenbogen and colleagues invited 12 people to spend 3 nights in his lab's cushy digs. Presented with comfy beds and soundproof rooms, the subjects slept peacefully through the first night while the researchers measured their baseline brain waves. During the next 2 nights, the team played an assortment of 14 different sounds, including flushing toilets, loud conversations, ringing phones, and car traffic, 40 to 50 times throughout the night, gradually raising the volume of each sound until each sleeper stirred.
When the researchers matched the sleepers' spindle production—which ranged from three to six spindles per minute and remained consistent for each sleeper across the nights—to the loudness required to rouse them, they found that sleepers with higher spindle rates were harder to wake up. The spindles seem to indicate when the thalamus is blocking noise from reaching the cortex and disrupting sleep, the team reports  in the 10 August issue of Current Biology.
"This is a very elegant study," says Mathias Basner, a sleep researcher at the University of Pennsylvania School of Medicine. "We see huge variability in noise sensitivity, and this gives us a marker to predict that sensitivity." That marker could be used to gauge sleep quality in problem sleepers and assess how well sleep therapies are working for them, adds neuroscientist Matthew Walker of the University of California, Berkeley.
Knowing more about spindles could also help researchers design drugs or behavioral techniques that deepen sleep, says Ellenbogen. In the meantime, some questions linger. Researchers don't yet know why some people produce more spindles than others, or how exactly the thalamus shields stable sleepers from sound. Ellenbogen plans future studies to lay these mysteries to rest.