I love it when research comes out "proving" what we already know to be true.
I just recently came across some research on the cognitive effects of both essential oils, as well as their effects on mood. Unsurprisingly, the researchers found that the smell of peppermint enhanced memory, while Ylangylang impaired memory and seemed to slow mental processing. At the same time, tests of mood before and after the cognitive tests showed that subjectively, the peppermint increased alertness, while the ylangylang decreased awareness, but significantly increased calmness.
The full paper is available at Pubmed as well as other online sources.