How daily use blunts the effect
When caffeine is consumed regularly, the brain adapts to the chronic blockade of adenosine receptors, in part by increasing the number or sensitivity of those receptors. Over roughly one to two weeks, this upregulation means more adenosine signaling is available, so the same dose of caffeine produces a smaller felt effect. This adaptation is the basis of caffeine tolerance.
Because the receptor system has shifted, a regular user often needs caffeine partly to feel normal rather than noticeably stimulated.
Withdrawal and resetting sensitivity
When intake stops, the upregulated receptors are suddenly exposed to full adenosine signaling, which can produce headaches, fatigue, and low mood for a few days. Sensitivity tends to return after a period of reduced or paused intake, which is the rationale some people use for tolerance breaks, though the ideal length is not firmly established.
This is general educational information, not medical advice. Anyone reducing caffeine while on medication or managing a health condition should consult a qualified professional.