The effectiveness of caffeine varies significantly from person to person, due to genetics and other factors in play. The average half-life of caffeine—that is, how long it takes for half of an ingested dose to wear off—is about five to six hours in a human body. Women taking oral birth control require about twice as long to process caffeine. Women between the ovulation and beginning of menstruation see a similar, if less severe, extended half-life. For regular smokers, caffeine takes half as long to process—which, in some ways, explains why smokers often drink more coffee and feel more agitated and anxious, because they’re unaware of how their bodies work without cigarettes.
To learn more, cause i sure have. lifehacker.com Thank you, Paul, for enlightening me.