In 1994 neuroscientists Matthew Wilson and Bruce McNaughton, both then at the University of Arizona, showed this effect for the first time using rats fitted with implants that monitored their brain activity. They taught these rats to circle a track to find food, recording neuronal firing patterns from the rodents’ brains all the while. Cells in the hippocampus—a brain structure critical for spatial memory—created a map of the track, with different “place cells” firing as the rats traversed each region of the track [see “The Matrix in Your Head,” by James J. Knierim; Scientific American Mind, June/July 2007]. Place cells correspond so closely to exact physical locations that the researchers could monitor the rats’ progress around the track simply by watching which place cells were firing at any given time. And here is where it gets even more interesting: when Wilson and McNaughton continued to record from these place cells as the rats slept, they saw the cells continuing to fire in the same order—as if the rats were “practicing” running around the track in their sleep.
As cited in Scientific American Mind (August 7, 2008): "Sleep on It: How Snoozing Makes You Smarter", by Robert Stickgold and Jeffrey M. Ellenbogen