Caffeine is the world's most widely used psychoactive substance. In moderate amounts it sharpens alertness, but without understanding its metabolism, it can seriously undermine sleep quality.
Caffeine's half-life in the body: Caffeine's half-life — the time for blood concentration to fall by half — is typically 5–6 hours, with significant individual variation (influenced by genetics, liver function, age, and smoking status). This means: • A 1 PM coffee (200mg) → still ~100mg in your bloodstream at 7 PM • A 3 PM coffee → still ~100mg at 9 PM
The FDA advises healthy adults to stay under 400mg of caffeine per day (roughly 4 standard cups of coffee).
How caffeine affects sleep — even when you "don't feel it": Even if you believe caffeine doesn't affect your sleep, research shows it can still: • Reduce total sleep time by an average of ~45 minutes • Suppress deep sleep, making sleep lighter overall • Delay sleep onset, even without a subjective sense of difficulty
"Caffeine tolerance" numbs your perception of its effects — it doesn't make you biologically immune.
Common caffeine content (approximate): • Drip coffee (240ml): ~100–150mg • Espresso (single shot): ~60–75mg • Black tea (240ml): ~40–70mg • Green tea (240ml): ~25–45mg • Cola (355ml): ~35mg • Energy drink (250ml): ~80–150mg
Suggested cut-off time: • General guideline: last caffeine intake no later than 2–3 PM • Sensitive individuals or those with poor sleep: consider moving the cut-off to noon