The internal clock responds best to coffee in the morning around nine o’clock , confirmed the study of the American University Uniformed Services University of the Health Sciences . It is best to drink between half past nine and half past eleven o’clock , or between half past five and half past six in the afternoon , because then the levels of cortisol , a hormone that increases alertness , the lowest , the Daily Mail reports . Cortisol interacts with caffeine , so be careful at the time of drinking this beverage black . People who drink coffee between eight and nine o’clock in the morning when cortisol is highest develop a tolerance to caffeine , which decreases its efficiency . What is the level of cortisol increased , the higher the risk of resistance to caffeine . Coffee does not work , so the body no time seeking additional cup or stronger drink to wake them up , according to research . – Just as you should not overdo it with medication , so you should not overdo it with coffee . It should be well- timed , and not to drink when the body najbudnije because there will be a counter-effect – said the head of research , neuroscientist Steven Miller .