Brains disconnect as people sleep.

Consciousness fades during sleep not because the brain shuts down but because it loses its capacity to integrate information via networks of interconnected areas, a new study suggests.

Giulio Tononi of the University of Wisconsin-Madison and his colleagues studied six adults who sat in reclining chairs with their eyes closed and gradually fell asleep. The researchers used a technique called transcranial magnetic stimulation to generate mild magnetic pulses that briefly activated the right-premotor area of each participant's brain. The team targeted that area because it connects to many other parts of the brain.

In volunteers who were awake, magnetic prodding of the premotor area elicited a rise in electrical activity within a fraction of a second, as measured by sensors in a cap worn by each volunteer. Over the next several seconds, brief waves of electrical activity appeared in four other brain regions, the scientists report in the Sept. 30 Science.

During bouts of deep sleep early in the night, the magnetic stimulation induced a stronger initial surge of premotor-electrical activity than had occurred during wakefulness. However, that activity rapidly vanished, and brain regions with premotor connections showed no subsequent signs of arousal.

When the researchers magnetically stimulated tissue in another brain network, neural activity again spread only while participants were awake.

Tononi's group plans to investigate whether communication across brain regions partly recovers during late-night sleep and especially during periods of rapid eye movement, when dreaming is common.--B.B.