Best time to drink black coffee for fat burning is 30–45 minutes before exercise, when caffeine maximally boosts adrenaline and fat mobilisation, said Dr Rajiv Kovil, diabetologist and weight loss expert at Zandra Healthcare. For non-workout days, 9-11 am is preferable, as early-morning cortisol levels are naturally high. Avoid caffeine after 4 pm to protect sleep quality, the doctor added.