Indeed,  during odor delivery in sleep the researchers noted that the two hemispheres of the subjects’ brains exhibited the kind of electrical activity thought to be involved in memory consolidation, with the half that was “smelling the roses” showing the more synchronized activity that is important for the consolidation process. When the subjects woke up and retook the test, they were better at remembering the locations of words that had been presented to the side of the brain that had been stimulated with the smell while they slept.

— Read more in One-Sided Memories and Sleep: A unique sleep experiment to uncover brain activity comes up smelling of roses at Weizman Institute.