The reason people who regularly drink caffeine wake up feeling groggy, confused or with a headache is that they are experiencing withdrawal symptoms from yesterday's coffee, Lane said. This makes it easier for people to develop a dependency on caffeine because it makes people want to continue to drink it the next day, Lane said. Pregnancy can extend caffeine's half-life even more, lengthening it to 10.5 hours during the final four weeks of pregnancy, according to one study.īecause it can take up to 12 hours to clear caffeine from the body, the drug often wears off when a person is almost ready to go to bed. In contrast, women who take birth control pills may keep caffeine in their systems for up to 4 hours longer than women who are not on the pill, according to one study. But that doesn't mean that all of the caffeine is gone after 8 hours, in fact, it may take 12 hours to completely eliminate the caffeine in a morning cup of coffee, said Lane, who has studied the health effects of the drug.Ĭaffeine's half-life may be shortened to about 3 hours in people who smoke, Lane said. The half-life of caffeine, or the time it takes to eliminate one-half of the caffeine people have in their bodies, is about 4 hours, said James Lane, an emeritus professor of psychiatry at Duke University School of Medicine in Durham, North Carolina. But it takes much longer than that for the body to break it down and clear it from a person's system. Caffeine is absorbed into the blood and tissues within about 45 minutes of being consumed.
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