Caffeine has been found to increase alertness via several biochemical pathways, primarily by directly inhibiting phosphodiesterases and increasing cyclic AMP. In addition, caffeine inhibits adenosine receptors and stimulates adrenaline release. Adenosine is primarily an inhibitory chemical; caffeine blocks the actions of adenosine. A recent study on cats—renowned sleepers—pin-pointed one arousal network and confirmed that adenosine is a natural sleep inducer. The scientists found that natural concentrations of adenosine built up in parts of the brain system during the cats’ waking periods and caused them to fall sleep. Caffeine works by binding to these adenosine receptors, thereby keeping adenosine from binding with its receptor. Adenosine also inhibits the release of most brain excitatory neurotransmitters—particularly dopamine— and may reduce dopamine synthesis. Decreases in dopamine have been linked to fatigue during exercise.

Caffeine increases performance in the gym—but what’s the optimal dosage? Scientists had volunteers do exercises against four incremental loads (25%, 50%, 75%, and 90% 1RM) and participate in a test in which cycling peak-power output was measured using a four-second inertial-load test in a randomized, double-blind, crossover experiment. Subjects ingested either a placebo or caffeine at dosages of 3mg/kg body weight, 6mg/kg body weight, or 9mg/ kg body weight (for the record, 9mg/per kg of body weight is the equivalent of five to six regular-size cups of drip-percolated cofee, or about 693mg of caffeine).

The good news is that caffeine produced ergogenic effects at all dosages. All the subjects’ perceptions of performance and vigor increased five to seven times above placebo during the 3mg and 6mg caffeine trials (38% and 54%, respectively). Those in the 9mg/kg body-weight trial had the highest increase in perception of performance and vigor or “activeness” (62% and 54%). The bad news is that the highest dose of caffeine (9mg/kg of body weight) was associated with many side effects, including increased heart rate, gastrointestinal distress, inability to sleep, and anxiety.

Also, remember that higher doses of caffeine can cause increased levels of cortisol, so 3–6mg/kg of body weight seems to be the reasonable amount. So, take small doses of caffeine, around the 200mg mark, and find a dosage you can tolerate without having the unwanted side effects.