We know adenosine receptors A1 & A2 subtypes physically increase in the basal ganglia with prolonged caffeine exposure over the period of about 1-2 weeks, but at what rate do they return to normal (downregulate)? Id est, what is the extinction rate of upregulated adenosine receptors?
If you take coffee daily, you no longer get a boost from it after about a week, but rather, you're just staving off the harsh withdrawal symptoms, not to mention over time it will physically change your brain to have more adenosine receptors i.e., "sleepiness receptors". (Ramkumar V et al, J Clin Invest 82:242-247)
"A 1995 study suggests that humans become tolerant to their daily dose of caffeine—whether a single soda or a serious espresso habit—somewhere between a week and 12 days. And that tolerance is pretty strong. One test of regular caffeine pill use had some participants getting an astronomical 900 milligrams (9 cups of coffee) per day, others placebos—found that the two groups were nearly identical in mood, energy, and alertness after 18 days. The folks taking the equivalent of nine stiff coffee pours every day weren't really feeling it anymore. They would feel it, though, when they stopped.
"You start to feel caffeine withdrawal anywhere from 12 to 24 hours after your last use. That's a big part of why that first cup or can in the morning is so important—it's staving off the early effects of withdrawal."
Caffeine, the natural pesticide of bright red coffee berries, creates an emergency response in your body to fight it off (pooping, peeing, and adrenaline). It inherently doesn't give you energy or focus, but the adrenal side effect does. Studies show that people who have coffee 7 days in a row no longer receive any energy or focus benefits. At that point their brain has been sufficiently altered with new adenosine receptors that they are just at their baseline behavior. They found this was true with any caffeine amount, from 1 to 10 cups of coffee. The exception is after 10 cups is it just starts frying your brain.
"You might think all of this probably takes a while, but it takes about seven days to become addicted to caffeine. Once addicted, you need more and more coffee to get buzzed as your brain gets covered in receptor sites. "
Life Sciences Volume 36, Issue 24, 17 June 1985, Pages 2347-2358 Caffeine tolerance: Behavioral, electrophysiological and neurochemical evidence Dorothy T.Chou Sukur Khan Jesse Forde Kenneth R. Hirsh https://www.sciencedirect.com/science/article/pii/002432058590325X
Robertson D, Wade D, Workman R, Woosley RL, Oates JA. Tolerance to the humoral and hemodynamic effects of caffeine in man. Journal of Clinical Investigation. 1981;67(4):1111-1117. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC370671/
"Multiple components of the A1 Adenosine Receptor-Adenylate Cyclase System are Regulated in Rat Cerebral Cortex by Chronic Caffeine Ingestion." (Ramkumar V et al, J Clin Invest 82:242-247.) https://www.ncbi.nlm.nih.gov/pubmed/3392208?dopt=Abstract
"Effects of chronic caffeine on brain adenosine receptors: Regional and ontogenetic studies" (Paul J.Marangos, Jean-Philippe Boulenger, Jitendra Patel) https://www.sciencedirect.com/science/article/pii/0024320584902078