Caffeine
The world's most-used stimulant — effective for acute alertness and athletic performance. But: the longevity benefits of coffee and green tea come from their polyphenols, not from caffeine.
How it works
Caffeine blocks the brain's adenosine receptors; adenosine builds up through the day and makes us sleepy. This raises alertness, reaction time and perceived energy, and measurably improves endurance performance. The effect blunts with habituation, and late intake disrupts sleep.
Dosage
Ergogenic from ~3 mg/kg body weight before exercise. Up to ~400 mg/day is generally considered safe; effect and tolerance vary strongly by genetics.
Considerations
An honest take: caffeine boosts alertness and short-term performance — but the long-term health benefits of coffee and green tea come from their accompanying compounds (polyphenols, catechins), not isolated caffeine. Isolated and high-dose it has downsides: disrupted sleep (half-life ~5–6 h), restlessness/anxiety, transient blood-pressure rise, tolerance and withdrawal headaches. If you want the longevity effect, coffee or green tea beats caffeine pills.
