Rosemary for remembrance, Shakespeare wrote in Hamlet. Now we might add, Rosemary for cerebral blood flow. The monoterpenes from rosemary include cineole, borneol, camphor, and pinenes, which give it its pungent, piney smell. One study using brain magnetic resonance imaging show that after people inhale cineole, cerebral blood flow increases. Rats sniffing borneol had increased hypothalamic levels of serotonin and histamine. Histamine plays a major role in wakefulness in the brain. (That's why certain antihistamines can make you sleepy.) So the common subjective perception of rosemary as "stimulating" could have a biochemical basis after all.