Brain cells take time out while you sleep

By Tina Hesman Saey

Even neurons need quiet time. A new study shows the brain cells take time out while you sleep, preventing you from waking up at the drop of a hat or other nonthreatening object.

For decades, scientists have been measuring electrical activity in the brain during sleep with electroencephalograms, or EEGs. Researchers easily recognize the hallmark dips and blips of each stage of sleep, but what brain cells are doing to produce the signals hasn’t been apparent.

Now, a new study in the May 22 Science shows that a prominent electrical signal of slow-wave sleep, called the K-complex, indicates downtime for neurons. The quiet periods could help people ignore distractions, such as sounds and touches, and stay asleep, the researchers report.

K-complexes appear as sharp dips in EEG tracings. The events happen shortly after a person falls asleep, during a period of slow-wave sleep when people are transitioning from light sleep into the heaviest periods of deep sleep. Such dips correspond to a quieting of brain cell activity in animals. Though the traditional electrodes used in EEG measure activity over large areas on the surface of the outer layer of the human brain — the cortex — no one really knew what the signals indicated about fine-scale brain activity in the cortex’s deeper layers.

In the new study, researchers in the United States and Hungary recorded sleep patterns in eight people with epilepsy. The eight people had previously had surgery to implant a series of 24 microelectrodes in each of their brains so doctors could pinpoint the source of the epileptic seizures. The participants had normal sleep patterns.

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