Caffeine mainly works by blocking your brain’s adenosine receptors, which play a role in sleep, arousal, and cognition (1Trusted Source).
A molecule called adenosine usually binds to these receptors, inhibiting the release of brain chemicals like dopamine that increase arousal and promote wakefulness (2Trusted Source).
By blocking adenosine from binding to its receptor, caffeine increases the release of these stimulating brain chemicals that decrease fatigue and increase alertness (3Trusted Source, 4Trusted Source).
One study showed that a high caffeine dose can block up to 50% of adenosine receptors in the brain (5Trusted Source).
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 (7Trusted Source).
Caffeine tolerance occurs when the effects of caffeine decrease over time with regular consumption.
A tolerance to caffeine’s effects has been demonstrated on blood pressure, exercise performance, and mental alertness and performance.
Blood pressure and heart rate
In one 20-day study, 11 people with light caffeine use consumed a pill containing 1.4 mg of caffeine per pound (3 mg per kg) of body weight per day or a placebo (10Trusted Source).
This amount represents about 200 mg of caffeine, or two 8-ounce (240-mL) cups of coffee for a 150-pound (68-kg) person.
Compared with the placebo, caffeine significantly increased blood pressure, but the effect disappeared after 8 days. Caffeine did not affect heart rate (10Trusted Source).
Research suggests that caffeine does not lead to greater increases in blood pressure in people with high blood pressure who regularly consume caffeine (11Trusted Source).
Yet, these performance benefits may decrease with regular caffeine consumption.
In one 20-day study, 11 people with light caffeine use consumed a pill containing 1.4 mg of caffeine per pound (3 mg per kg) of body weight or a placebo daily (14Trusted Source).
Compared with the placebo, the daily intake of caffeine increased cycling power during 2 exercise tests by 4–5% for the first 15 days, but then the performance effect decreased.
The participants who received caffeine continued to experience greater performance benefits compared with the placebo after the 15 days, but the progressive decline in performance thereafter suggests a gradual but partial tolerance to caffeine’s effects.
Mental alertness and performance
Caffeine’s stimulant effect has been shown to enhance mental alertness and performance, particularly in people who don’t regularly consume it (15Trusted Source).
In regular caffeine consumers, the increase in mental alertness and performance that’s often reported is more related to a reversal of the symptoms of caffeine withdrawal rather than an enhancement above their normal state (16Trusted Source, 17Trusted Source).
You can develop a dependence on caffeine in as few as 3 days of use and from doses as low as 100 mg per day, which is the equivalent of an 8-ounce (240-mL) cup of coffee (18Trusted Source).
The symptoms of caffeine withdrawal include sleepiness, lack of concentration, and headache. They appear after 12–16 hours without caffeine and peak around 24–48 hours (19Trusted Source).