In one study, a team of scientists at University of Northumbria (United Kingdom) assessed the olfactory impact of rosemary and lavender essential oils on the cognitive performance and mood in a group of volunteers.
The volunteers were placed in cubicles infused with one of the two scents (or none at all) and given a series of memory- and attention-based tasks.
The study found that rosemary greatly enhanced memory and alertness while lavender impaired their working memory and their reaction times to tasks. (Not surprising, since lavender is often associated with calmness and sleep.)
If you’re wondering how simply smelling something can have that type of effect on the brain, it turns out that inhalation is one of the best ways of getting drugs into the brain.
When you eat a drug, it gets broken down in the liver (which processes everything absorbed by the gut) but when you inhale it, small molecules pass into the bloodstream and make their way to the brain without being broken down first.