So, we see three vectors of caffeine dependence:
- You aren't getting enough sleep, and your adenosine level at 6 am is increasing from day to day, requiring a higher concentration of caffeine to out-compete it.
- Your brain makes more adenosine receptors to accommodate the large concentrations of both caffeine and adenosine in your system, so you again need a higher concentration of caffeine molecules to keep your brain from getting enough adenosine signals. This theory is supported by at least one paper: https://www.sciencedirect.com/science/article/abs/pii/002432058590325X
- Your rate of adenosine production AND destruction goes up, so you need a higher dose of caffeine to compensate for the larger swings your body is generating. This is just an alternate mechanism for "building up resistance" and would have the same remedies as #2
For case #1, you just need to get more sleep. This could be a typical pattern for people who drink coffee T-F and then sleep in on the weekend - their body resets and a little boost is sufficient.
For case #2, it's about the concentration of caffeine + adenosine in the bloodstream, over time (since it takes ?hours? to form or deconstruct adenosine receptors - regardless it's not instantaneous). Taking more caffeine in the afternoon is counterproductive, since it is maintaining the same high blood concentrations of active molecules. To reduce resistance you need to give your body time at low levels of stimulator molecules so it gets rid of the excess receptor sites.
In conclusion - don't use caffeine in the afternoon if you are trying to reduce your need for it