How Much Caffeine Is Too Much? (FDA Limits & Sleep Science)

How much caffeine is in common drinks?

The FDA's 400mg guideline — and why it is not quite right for everyone

The FDA's 400mg daily limit is a population-level guideline for healthy adults. The European Food Safety Authority (EFSA) takes a more personalised approach: 3mg per kilogram of body weight per day as a safe daily intake, and no more than 200mg as a single dose.

What this means in practice: a 55 kg person has a safe limit of approximately 165mg/day — less than two espressos. A 90 kg person can safely consume up to 270mg before hitting EFSA's per-kg limit (though they would still be well under the FDA's 400mg flat cap).

Individual caffeine sensitivity also varies enormously due to genetics. Variants in the CYP1A2 gene affect how fast the liver metabolises caffeine. Fast metabolisers clear caffeine in 3–5 hours; slow metabolisers may take 9–12 hours. For slow metabolisers, even moderate coffee consumption can disrupt sleep and elevate blood pressure.

Caffeine and sleep: the half-life problem

Caffeine works by blocking adenosine receptors in the brain. Adenosine is the molecule that builds up over the day and makes you feel sleepy — caffeine simply masks it without eliminating it. When caffeine clears, adenosine floods the receptors at once, causing the post-caffeine crash.

Caffeine's half-life is approximately 5–6 hours. This means:

• A 200mg coffee at 2pm → ~100mg still active at 7–8pm
• A 200mg coffee at 3pm → ~100mg still active at 8–9pm
• 95% clearance takes 4–5 half-lives = 20–28 hours for a single dose

Research by Matthew Walker and others shows that even when caffeine does not prevent sleep onset, it significantly reduces deep slow-wave sleep (the most restorative phase) — even if you do not feel it consciously.

A practical guideline: avoid caffeine within 6–8 hours of your target bedtime. If you sleep at 10pm, your caffeine cut-off should be around 2–4pm.

Caffeine as a performance enhancer

Caffeine is one of the most extensively studied legal performance enhancers. At doses of 3–6mg/kg body weight (taken 45–60 minutes before exercise), it reliably:

• Improves endurance performance by 2–5% (reduced perceived effort)
• Increases muscular strength and power output by 3–7%
• Enhances reaction time, alertness, and decision-making
• Reduces muscle pain and fatigue during exercise

The International Olympic Committee recognises caffeine as a performance aid and monitors (but does not prohibit) its use. For a 70 kg person, an optimal pre-workout dose is 210–420mg — achievable with 2–4 espresso shots or a standard pre-workout supplement.

Caffeine during pregnancy

The NHS and ACOG recommend limiting caffeine to 200mg per day during pregnancy. High caffeine intake is associated with increased risk of miscarriage and low birth weight. The evidence is strongest above 300mg/day. If you are pregnant, it is sensible to reduce or eliminate caffeine wherever possible and discuss your intake with your midwife.

Caffeine also passes into breast milk in small amounts — breastfeeding mothers are typically advised to limit intake and avoid caffeine immediately before nursing.

Signs you are having too much caffeine

• Anxiety, restlessness, or racing thoughts
• Heart palpitations or rapid heartbeat
• Difficulty falling asleep even when tired
• Digestive issues (caffeine stimulates gut motility and gastric acid)
• Headaches when you do not have coffee (withdrawal)
• Increased blood pressure