The amount of caffeine in the body of a healthy adult after drinking espresso decreases exponentially, with half-life of approximately 6 hours. that is, every 6 hours, the amount of caffeine in the body decreases by 50%. charlotte wants to go to sleep at 11 pm and does not want more than 20 milligrams (mg) of caffeine in her system at the time. what is the latest time charlotte can drink a 2-ounce shot of espresso that contains 80 mg of caffeine?