In a French study, 18 subjects were read a list of animals and objects and told to push a button in their right hand for animals or their left hand for objects. Later, in a dark, quiet room, a similar list was read aloud—and even after subjects had dozed off, sensors showed that the same nerve cells in the brain that prompted the left/right hand movements were activated. (There was no actual movement.)
Simply put, this means that even while you’re not conscious or aware, your brain remains active to outside stimuli. Research has a ways to go, but one day we may be able to learn languages, memorize lectures or scripts, or absorb math data like multiplication tables all while catching some z’s, Matrix-style.
Source: École Normale Supérieur