Caffeine
Anhydrous caffeine capsules for dose precision, or coffee (~95 mg per 240 mL cup); caffeinated gum absorbs faster and needs less lead time
Strong
Several good studies agree
Both directions are strongly evidenced: an umbrella review of 21 meta-analyses for performance, and a 24-study meta-analysis for the sleep cost. The one honest gap — a 2022 meta-analysis of 5 studies found caffeine did NOT significantly alter post-exercise HRV recovery (RMSSD 95% CI −0.45 to 0.29), so any HRV drift Umbral shows should be attributed to lost sleep, not to a direct autonomic effect.
It improves performance in every kind of exercise tested. But taken too late it costs about 45 minutes of sleep, 11 minutes of deep sleep, and makes you take 9 minutes longer to fall asleep.
Ergogenic across every modality tested (aerobic endurance ES 0.22–0.63; peak power ES 0.27, 95% CI 0.08 to 0.47) but costs 45 min of total sleep (95% CI −61.5 to −29.0), 11 min of deep sleep (95% CI −18.5 to −4.3) and 9 min of extra sleep latency (95% CI 3.8 to 14.4) when taken too late.
Antagonises adenosine A1 and A2A receptors, blocking accumulated sleep pressure and lowering perceived effort while raising sympathetic drive. The same receptor blockade that delivers the performance benefit is what suppresses slow-wave sleep and sustains sympathetic tone into the night — the trade-off is one mechanism, not two.
CYP1A2 fast metabolisers get more performance benefit and clear it before bedtime; slow metabolisers pay a disproportionate sleep cost from the same dose. ADORA2A genotype predicts the anxiety and insomnia response. Habitual users retain the ergogenic effect but their sleep cost persists.
We compare each effect with how much that number normally swings from one night to the next. An effect smaller than the swing is real, but you will not see it in a week of data.
- Total sleep timeProbably visibleeffect: 45.3 minutesnormal swing: ±45 minutes
- Deep sleep (N3)Visible over weekseffect: 11.4 minutesnormal swing: ±18 minutes
- Sleep onset latencyProbably visibleeffect: 9.1 minutesnormal swing: ±9 minutes
- Post-exercise HRV recovery (RMSSD)Too small to seeeffect: 0 SMD (null)normal swing: ±9 SMD (null)
What to buy
- ✓Anhydrous caffeine capsules for dose precision, or coffee (~95 mg per 240 mL cup); caffeinated gum absorbs faster and needs less lead time
- ✓3–6 mg/kg body mass; as little as 2 mg/kg is ergogenic for some, and 9 mg/kg adds side effects without adding performance
- ✓Third-party tested (USP, NSF, Informed Sport)
- ✕Proprietary blends that hide the dose per ingredient
- ✕Unstandardised extracts with no active-compound percentage
- · Sleep disruption is the dominant documented cost: −45.3 min total sleep time, −7.0% sleep efficiency, −11.4 min deep sleep
- · Anxiety, jitteriness, tachycardia and palpitations, strongly dose-dependent and modulated by ADORA2A genotype
- · Acutely raises blood pressure and delays post-exercise heart rate and blood pressure recovery
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100 mg steps let you actually hit 3–6 mg per kilo of body weight. At 200 mg a pill, a 60 kg person can only choose between too little and too much.
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An OTC drug rather than a supplement, so the content is regulated rather than self-declared. Useful if you want the dose to be exactly what the box says.
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Eighteen thousand ratings, the most-reviewed caffeine here. Keep the last dose at least eight hours before bed or you will read the cost of it on your sleep chart.
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See all on AmazonEvery result, next to the line that means no effect
Each diamond is a study's best estimate. The thin line shows how sure the study was: the shorter the line, the more certain the result.
- Total sleep timeminutes · Gardiner et al., Sleep Medicine Reviews 2023; noise_sd = estimated within-person night-to-night SD of wearable TST−45.3−61.5 … −29
- Deep sleep (N3)minutes · Gardiner et al., Sleep Medicine Reviews 2023; noise_sd = estimated within-person night-to-night SD of wearable deep sleep−11.4−18.5 … −4.30
- Sleep onset latencyminutes · Gardiner et al., Sleep Medicine Reviews 2023; noise_sd = estimated within-person night-to-night SD of wearable sleep latency+9.10+3.80 … +14.4
- Post-exercise HRV recovery (RMSSD)SMD (null) · Porto et al., Nutr Metab Cardiovasc Dis 2022 — no significant effect; noise_sd = estimated within-person night-to-night SD of wearable RMSSD0.00−0.45 … +0.29
Each unit gets its own scale, so numbers are never mixed on one line. When a study gave no range, we leave it blank instead of inventing one.
5 studies over 6 years
The studies
- 1.Wake up and smell the coffee: caffeine supplementation and exercise performance—an umbrella review of 21 published meta-analysesBritish Journal of Sports Medicine · 2020 · systematic review · 21 experiments
Aerobic endurance ES 0.22–0.63; muscle endurance 0.28–0.38; strength 0.16–0.20; peak power 0.27 (0.08–0.47); mean power 0.18 (0.05–0.31); vertical jump 0.17 (0.00–0.34). Most common dose 6 mg/kg, 60 min pre-exercise.
- 2.The effect of caffeine on subsequent sleep: A systematic review and meta-analysisSleep Medicine Reviews · 2023 · meta-analysis · 24 experiments · n = 340
Total sleep time −45.3 min (−61.5 to −29.0), sleep efficiency −7.0% (−10.0 to −4.0), sleep onset latency +9.1 min (3.8 to 14.4), N3 deep sleep −11.4 min (−18.5 to −4.3). Cut-off: coffee (107 mg) ≥8.8 h and pre-workout (217.5 mg) ≥13.2 h before bed.
- 3.International society of sports nutrition position stand: caffeine and exercise performanceJournal of the International Society of Sports Nutrition · 2021 · systematic review
3–6 mg/kg improves performance consistently, 60 min pre-exercise most common; 9 mg/kg adds side effects without benefit. Inter-individual differences in performance and in sleep/anxiety adverse effects attributed to CYP1A2 and adenosine receptor genotype.
- 4.Caffeine intake and its influences on heart rate variability recovery in healthy active adults after exercise: A systematic review and meta-analysisNutrition, Metabolism and Cardiovascular Diseases · 2022 · meta-analysis · 12 experiments
Pooled across 5 studies, caffeine (100–400 mg or 2–6 mg/kg) did not significantly affect post-exercise HRV recovery: RMSSD 95% CI −0.45 to 0.29 (p=0.67), HF 95% CI −0.34 to 0.30 (p=0.22).
- 5.Caffeine affects autonomic control of heart rate and blood pressure recovery after aerobic exercise in young adults: a crossover studyScientific Reports · 2017 · crossover · 1 experiments · n = 32
300 mg caffeine delayed parasympathetic reactivation — RMSSD and SD1 recovered at 15–20 min vs 5–10 min with placebo; systolic BP recovery delayed from 5 to 7 min.
Umbral is a wellness product, not a medical device. It does not diagnose, treat, cure or prevent any disease. Supplement information is educational; talk to a clinician before starting anything, especially if you are pregnant, under 18, managing a condition or taking medication.
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