Obviously, it's probably not a good idea to drink your daily dose of caffeine in the afternoon if you know it keeps you up all night. But is drinking your morning coffee at 8 a.m. really the best time? The answer actually depends on your circadian rhythm of cortisol production.

This all has to do with 'chronopharmacology', which is defined as the study of the interaction of biological rhythms and drug action. This can not only affect your physiology and behavior in variety of ways, but can also alter the effects of certain drugs (like caffeine).

Taking medicine at the wrong time causes the body to develop a tolerance, which then requires the drug to be administered at a larger dose to achieve the same effect. So in theory, having the same cup of coffee at random intervals could also make it less effective.

Since cortisol levels in humans usually peak between 8 a.m. and 9 a.m., between noon and 1 p.m., and between 5:30 p.m. and 6:30 p.m, the most effective to drink coffee should be between 9:30 a.m. and 11:30 a.m., when your cortisol levels are dropping before the next spike.

[Brainfacts.org]
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