From Scientific American:
MONTREAL—A good night's sleep, or even just a nap, can be an aid to memory. Psychologists have known for years that sleep solidifies what we've learned during the day, transforming tenuous associations into stable ones. Learning while you snooze seems supremely efficient, and so people have long dreamed of co-opting this process so that their dozing brain shores up what matters to them—say, material they've studied for a test or a talk, or verbiage in a foreign language they want to master. But until now there has been little support for the notion that studying in your sleep is useful. Psychology graduate student John Rudoy at Northwestern University in Illinois reported findings here on Monday at the Cognitive Neuroscience Society 2010 annual meeting that hint at a way to do that.
Rudoy, who works in neuroscientist Ken Paller's group, and his colleagues showed study participants 50 photographs and asked them to memorize where each one appeared on a computer screen. To help the participants remember the locations, the researchers asked them to practice moving each picture to where they thought it had appeared, and after they’d made their move, showed them the picture's correct location. In addition, the participants were taught to associate each photograph with a distinct sound—say, a chirp, ring, buzz or tone—that was related to the image. For example, the sound of an object hitting the water accompanied a picture of a splash. The participants then took a nap lasting for up to 90 minutes in an easy chair in the laboratory. As they dozed, the investigators exposed the subjects to 25 distinct sounds—the ones they had associated with half of the photographs. When the nappers woke up, they again tried to move each of the 50 photographs to its previously assigned spot on a screen.
More here.