L-Theanine
The amino acid in green tea that takes the edge off stress — and off caffeine — without sedation.
100-400 mg
B Good human evidenceNootropicsSleep
Nootropics · Performance
Caffeine
Also known as: 1,3,7-trimethylxanthine, coffee, caffeine anhydrous
The most-used stimulant on earth, and one of the few supplements with strong evidence for both cognitive and physical performance.
Grade AStrong human evidence
Studied dose100-400 mg
TimingMorning or early afternoon
EvidenceGrade A
CategoryNootropics
Hundreds of trials and multiple umbrella reviews support caffeine for alertness, reaction time, and endurance performance. The effects are reliable, dose-dependent, and blunted — not erased — by habitual use.
Caffeine is the rare supplement that needs no introduction and still gets underestimated. Most people treat it as a habit; the research treats it as one of the most reliable ergogenic and cognitive aids ever tested. Umbrella reviews across hundreds of trials show consistent improvements in alertness, vigilance, and reaction time, plus a 2-4% improvement in endurance performance at doses of 3-6 mg per kg of body weight.
The cognitive story is mostly about restoring and defending baseline rather than raising it. Caffeine shines when you are sleep-deprived, fatigued, or mid-afternoon — states where adenosine has accumulated and there is something to block. In a fully rested brain the margin is smaller. That distinction matters for how you use it: as a tool deployed against fatigue, not a constant drip.
Tolerance is real but partial. Habitual users keep most of the performance benefit at the same dose, though the subjective jolt fades. What does not adapt is the half-life: roughly five hours for most adults, longer for slow CYP1A2 metabolizers and anyone on oral contraceptives. A 3 PM coffee still has half its caffeine circulating at 8 PM, which is why the highest-leverage caffeine decision is not the morning dose but the afternoon cutoff.
The safety ceiling for healthy adults sits around 400 mg per day per the major regulatory reviews, with single doses up to 200 mg considered safe. Pregnant women are advised to stay under 200 mg per day. Above the ceiling the returns invert: anxiety, tremor, heart palpitations, and wrecked sleep that quietly cancels the next day’s benefit. Pairing caffeine with L-theanine smooths the edge for people sensitive to jitters without dulling the alertness.
How it works
Caffeine blocks adenosine receptors in the brain, suppressing the sleep-pressure signal that accumulates through the day. That antagonism raises alertness and perceived energy, and downstream effects on dopamine signaling and calcium handling in muscle contribute to its performance benefits.
What the evidence supports
- Increases alertness and reaction time A
- Improves endurance performance 2-4% at 3-6 mg/kg taken ~60 min before exerciseA
- Improves strength and power output smaller and less consistent than the endurance effectB
- Raises daytime energy in sleep deprivation A
Safety
Safe for healthy adults up to about 400 mg per day per EFSA and FDA reviews. It is still a stimulant: people with anxiety disorders, arrhythmias, or uncontrolled hypertension should be cautious, and pregnancy caps the advised intake at 200 mg per day.
Reported side effects
- Sleep disruption when taken too late
- Jitteriness and anxiety at higher doses
- Heart palpitations
- Withdrawal headache on abrupt stopping
Interactions
- Amplifies other stimulants — do not stack with pre-workouts that already contain caffeine.
- Some antibiotics (ciprofloxacin) and oral contraceptives slow caffeine clearance, effectively raising your dose.
This guide is educational, not medical advice. Talk to a physician or pharmacist before starting any supplement, especially alongside prescription medication or a medical condition.
Common questions
- What does Caffeine do?
- The most-used stimulant on earth, and one of the few supplements with strong evidence for both cognitive and physical performance. Its best-supported effects are increases alertness and reaction time and improves endurance performance.
- Does Caffeine actually work?
- Strong human evidence (grade A). Hundreds of trials and multiple umbrella reviews support caffeine for alertness, reaction time, and endurance performance. The effects are reliable, dose-dependent, and blunted — not erased — by habitual use.
- How much Caffeine should I take?
- The studied range is 100-400 mg, 100-400 mg total per day; for performance, a single 3-6 mg/kg dose about 60 minutes before exercise, staying within the daily total. Start at the low end — more is not better once the effective range is covered.
- When should I take Caffeine?
- Morning or early afternoon. With a ~5 hour half-life, a hard cutoff 8+ hours before bed protects sleep — the caffeine half-life calculator shows your personal decay curve.
- Is Caffeine safe?
- Safe for healthy adults up to about 400 mg per day per EFSA and FDA reviews. It is still a stimulant: people with anxiety disorders, arrhythmias, or uncontrolled hypertension should be cautious, and pregnancy caps the advised intake at 200 mg per day. The most reported side effects are sleep disruption when taken too late, jitteriness and anxiety at higher doses, heart palpitations.
- What should you not mix Caffeine with?
- Amplifies other stimulants — do not stack with pre-workouts that already contain caffeine. Some antibiotics (ciprofloxacin) and oral contraceptives slow caffeine clearance, effectively raising your dose. Ask a pharmacist or physician before combining it with prescription medication.
References
Evidence review last updated
- Grgic, J., et al. (2020). "Wake up and smell the coffee: caffeine supplementation and exercise performance — an umbrella review of 21 published meta-analyses." British Journal of Sports Medicine. Source
- McLellan, T. M., Caldwell, J. A., & Lieberman, H. R. (2016). "A review of caffeine’s effects on cognitive, physical and occupational performance." Neuroscience & Biobehavioral Reviews. Source
- EFSA Panel on Dietetic Products, Nutrition and Allergies (2015). "Scientific Opinion on the safety of caffeine." EFSA Journal. Source