Neurons take a break during stage 2 sleep

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 stage 2 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 what’s called non-rapid eye movement sleep when people are transitioning from light sleep into the heaviest periods of deep sleep. This period of stage 2 sleep is one of four ever-deepening stages of non-REM sleep. People spend most of the night in stage 2 sleep, which is characterized by K-complexes as well as the distinctive brain wave signal known as spindles. 

The K-complex 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. 

Sydney Cash, of Massachusetts General Hospital and Harvard Medical School in Boston, and his colleagues realized that the microelectrodes could provide a valuable picture of what happens during sleep in the cortex’s deeper layers while scalp electrodes monitor what happens on the surface. So while the patients slept, the researchers observed what happened to the brain’s surface and deeper cortical layers during spontaneously generated K-complexes. The researchers also observed activity after playing a quiet sound, known to trigger K-complexes.

In each case, wherever a surface electrode recorded a K-complex, the researchers saw a corresponding dip in activity in the implanted electrode, “all together and all at once,” Cash says. For any particular K-complex, the researchers saw a local effect, but also found that K-complexes can happen in any part of the brain.

Cash and his colleagues speculate that K-complexes represent an instantaneous quieting of neuron activity that keeps a person asleep when there is an outside stimulus that the brain decides is harmless and not worth waking up for.

Some scientists had thought that K-complexes are part of the waves of activity that ripple through the brain in the early stages of sleep, but the new study shows that the two are separate, Cash says. Unlike other waves, K-complexes don’t spread throughout the whole brain. Instead, they quiet neurons in only part of the brain at a time. The finding may help scientists better understand the brain’s circuitry, he says.

The study shows “a great relation between what we see in humans to results seen in animals,” says Maxim Volgushev, a neuroscientist at the University of Connecticut in Storrs. “It’s a very nice parallel that shows activity of neurons is decreased during K-complexes.” 

But, he adds, “It’s a long way between seeing that a K-complex can be induced by a stimulus and saying that the brain is rejecting that stimulus.” K-complexes could represent not active rejection of a stimulus, but rather “a small push toward silence,” he says. That push might be generated not by the brain making a decision about the wake-worthiness of a stimulus but by fluctuations in brain activity.