Cortisol is a stress hormone and has a particular circadian rhythm that is regulated by the brain. According to a 2009 study, interrupting this rhythm can lead to metabolic abnormalities, fatigue, and poor quality of life.
Consuming caffeine when your cortisol levels are high can lead to interference in production of cortisol and increase your tolerance. You may then need more caffeine over time.
Cortisol levels in your body peak three times per day, one which is in the early morning.
The best times to drink coffee are between 10 in the morning and 12 noon, and between 2 and 5 in the afternoon.