Caffeine increases alertness and decreases fatigue by blocking adenosine from binding to its receptor. Regularly consuming caffeine increases the number of adenosine receptors, decreasing caffeine’s effects.
Caffeine mainly works by blocking your brain’s adenosine receptors, which play a role in sleep, arousal, and cognition.
A molecule called adenosine usually binds to these receptors, inhibiting the release of brain chemicals like dopamine that increase arousal and promote wakefulness.
By blocking adenosine from binding to its receptor, caffeine increases the release of these stimulating brain chemicals that decrease fatigue and increase alertness.
One study showed that a high caffeine dose can block up to 50% of adenosine receptors in the brain.
The stimulating effects of caffeine occur within 30–60 minutes of consuming the substance and last for 3–5 hours, on average.
However, according to a seminal study from the 1980s, regularly consuming caffeine increases your body’s production of adenosine receptors and therefore the likelihood of adenosine binding to those receptors.
Consequently, this decreases caffeine’s effects, causing you to become tolerant over time.