Why do some people sleep like logs while others start at every sound? The reason may be all in the brain waves, according to new research.

Bursts of brain activity called sleep spindles occur a few times a minute during early stages of sleep. Although these spindles are short (just over a half-second each), the new study finds that the more frequent sleep spindles a person has, the less likely they are to be roused by noise.

The research was presented today (Nov. 15) at the annual meeting of the Society for Neuroscience and is detailed in the August issue of the journal Current Science.

Peace and quiet
Sleep spindles occur during stage 2 sleep, the period after dozing off and before deep sleep. To find out if these scribbles of activity influence the stability of sleep, study researcher Thien Thanh Dang-Vu, a postdoctoral researcher at Harvard Medical School and Massachusetts General Hospital, and his colleagues used electroencephalography (EEG). EEG uses electrodes on the scalp to measure electrical activity in the brain.

"The question was, 'Is there any particular feature in the brain activity that can predict how certain people react more to noise than others during sleep?'" Dang-Vu told LiveScience.

Twelve volunteers spent three nights in a sleep laboratory, a converted hotel room where the researchers monitored the sleepers' brain waves. The first night, the volunteers slept in peace wearing EEG monitors.

During the next two nights, the researchers disrupted the slumber of each volunteer with various noises, from traffic sounds to ringing phones. They began by playing the noises at 40 decibels, about the noise level of a quiet home. Next, they ramped up the sounds until the volunteer roused.

Spindles and sleep
The lightest sleepers were the ones with the fewest sleep spindles, the researchers found. The difference between the number of spindles between light and heavy sleepers wasn't huge — just a few per minute, Dang-Vu said — but the number of spindles was an important predictor of how much noise was required to wake each sleeper.

"The more spindles you have, the more resilient you are to noise during sleep," Dang-Vu said.

The spindles may dampen the brain's processing of outside noise, Dang-Vu said. The rate of spindle production is very stable within individuals, he said, which makes them good predictors of how deeply a person sleeps.

The researchers are now investigating further to work out the precise mechanism that leads from more spindles to sounder sleep. Ultimately, they hope the research will lead to methods to help light sleepers rest easier.

"In the end, we might actually want to try to enhance these spindles by using different drugs or devices," Dang-Vu said.

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