Steven Miller said that drinking coffee between 8am and 9am would be counterproductive as the levels of stress hormone cortisol in people would be high and caffeine interacts with the hormone. He said that cortisol levels would be high in people as soon as they get up and starts to fall about an hour afterwards.
Cortisol level is also at its peak at lunchtime and between 5.30pm and 6.30pm. Drinking coffee when cortisol level is low would help boost the hormone production when the levels start to drop and they do not become resistant to its benefits.
"If we are drinking caffeine at a time when your cortisol concentration in the blood is at its peak, you probably should not be drinking it. This is because cortisol production is strongly related to your level of alertness and cortisol peaks for your 24 hour rhythm between 8 and 9am on average. One of the key principles of pharmacology is use a drug when it is needed. Otherwise, we can develop tolerance to a drug administered at the same dose", Miller wrote on his blog.