Neurons in the locus coeruleus release epinephrine and signal alertness and readiness for effort.

“And the release of epinephrine from those two locations overall increases energy, it increases alertness. It does not alone increase focus, okay? So the reason I've assigned epinephrine, adrenaline as the shaft of the arrow is that if focus is the arrow, there is no focus without epinephrine. So things, whether or not they're behavioral or psychological or supplements or drugs that increase epinephrine allow focus to occur, they are necessary for focus, but they are not sufficient for focus, that is they are required but they are not enough to create focus.”