Ever awakened from a dead sleep, certain that something was wrong? Something woke you up, yet you have no idea what or why? We all know that the brain can respond to the environment during sleep. If it didn’t, your alarm clock would never wake you up in the morning. But exactly how the brain responds to outside stimuli is a lot less clear. Sleep is inherently mysterious. As humans, we analyze the world using the cerebral cortex, the part of the brain which essentially disconnects during sleep. In fact, sleep scientists have found that communication between different parts of the cerebral cortex, abuzz with connection while you’re awake, stops during sleep, according to research published in 2005 in Science. Now a new study from a sleep laboratory at the University of Salzburg in Austria, “The Brain Selectively Tunes to Unfamiliar Voices During Sleep,” published Jan. 17 in The Journal …