A new study in Science now finds that facts and experiences were better recalled from memory when subjects were cued with a scent during the memorization process and again during slow wave sleep.

Re-exposure to the odor during slow-wave sleep (SWS) improved the retention of hippocampus-dependent declarative memories but not of hippocampus-independent procedural memories. Odor re-exposure was ineffective during rapid eye movement sleep or wakefulness or when the odor had been omitted during prior learning. Concurring with these findings, functional magnetic resonance imaging revealed significant hippocampal activation in response to odor re-exposure during SWS.

I wonder how this could be used for preparing presentations, situations where a persons typically fall asleep during or after the session ;-)