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“The half-life of caffeine -- that is, the time taken for the concentration in your bloodstream to drop by 50 per cent -- is between three and seven hours, with an average of about four hours. Therefore, if you drink several cups of strong coffee during your harassed working day, your blood will still contain substantial amounts of caffeine when you go to bed several hours later.” — Paul Martin
The half-life of caffeine -- that is, the time taken for the concentration in your bloodstream to drop by 50 per cent -- is between three and seven hours, with an average of about four hours. Therefore, if you drink several cups of strong coffee during your harassed working day, your blood will still contain substantial amounts of caffeine when you go to bed several hours later.