Tea and coffee act as short-lived nervous stimulants that temporarily mask fatigue and excite the brain.

Original passage

Tea acts as a stimulant and, to a certain extent, produces intoxication. The action of coffee and many other popular drinks is similar. The first effect is exhilarating. The nerves of the stomach are excited; these convey irritation to the brain, and this in turn is aroused to impart increased action to the heart and short-lived energy to the entire system. Fatigue is forgotten; the strength seems to be increased. The intellect is aroused, the imagination becomes more vivid.