Brain cells take a break.

Scientists have long wanted to know what happens inside the human brain when deep asleep. You may be unconscious, but your brain cells are busy with activity. Neurons, brain cells that conduct electricity, keep your mind humming even while your body is resting.

In a new study, a team of scientists found that neurons take breaks periodically as a person heads into deep sleep. These pauses in neuron activity help keep people asleep, even if they hear noises or are touched. Sydney Cash, a neurologist at Massachusetts General Hospital and Harvard Medical School in Boston, and his team found a way to study electricity in the brain, inside and out.

Scientists use different tools to study electrical currents in the brain. One of the most useful is the EEG, or electroencephalogram. An EEG represents the brain's activity as a graph that looks like a long series of differently shaped waves. The height, width and closeness of those waves give scientists a peek at what's happening in a person's head. Even though they can study the patterns, however, scientists don't know what causes the waves to form.

In the study led by Cash, the researchers were interested in a particular type of EEG squiggle called a K-complex. To people who don't understand EEG patterns, a K-complex just looks like a squiggle that's larger than the lines around it. To a trained scientist, a K-complex shows a significant change in the electrical activity in the brain.

A K-complex may show up on an EEG when the sleeping person hears a noise or has his or her sleep disturbed. Or these squiggles may show up for other reasons.

EEGs can't see everything, however. They only measure electric signals--including K-complexes--on the outside of the brain. In ...