Caffeine
A single shot of espresso contains 90-150mg of caffeine, while a 8oz cup of coffee contains 184mg of caffeine. This is because the coffee brewing process absorbs nearly 100% of the caffeine, while the espresso process only extracts about 2/3.
As a point of reference a can of Coke contains about 45mg of caffeine.
Research has indicates a 30% increase in alertness after only 30mg of caffeine is consumed. Within 30 to 60 minutes of consuming coffee, caffeine levels in the bloodstream peak to produce the highest levels of alertness and concentration.
Studies also tell us that the human body will only absorb about 300mg of caffeine at a given time; anything additional is cast off and provides no additional stimulation.
The body also dissipates 20% of the caffeine in the system each hour.
Milk coats the stomach lining which in turn slows the absorption of caffeine into the system.
Caffeine can squelch headaches by constricting blood vessels in the head, which is why it is added to some pain relievers. In fact, caffeine extracted from coffee to make de-caf is often sold to the pharmaceutical companies. In its pure form, caffeine is a pure white powder.