You're fast asleep. But your brain isn't taking the night off, according to new research funded by the U.S. National Science Foundation. Researchers studied activity in the human cerebral cortex in response to music and other sounds. They observed vigorous auditory responses in the sleeping brain, similar in many ways to responses in the wakeful state but differing in a key component. While the waking state is characterized by ongoing feedback signals in the brain as it attends to and interprets incoming sounds, researchers found that those signals are greatly reduced during sleep.