The effects of caffeine usually take around 10-15 minutes to ‘kick in’, peaking after 45 minutes. The caffeine remains in your blood for several hours afterwards, and after 6 hours, half of the caffeine is still in your body. This is why many people are unable to sleep if they drink coffee after a certain time; if you drink coffee at 5PM, half of the caffeine content will be in your blood at 11PM.