How caffeine works
Caffeine blocks adenosine, the molecule that builds up during the day and makes you feel sleepy. By occupying adenosine receptors, caffeine keeps you alert — until it wears off and the backed-up adenosine hits at once.
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Understanding caffeine
Short, evidence-based answers to the questions that matter for your energy and sleep.
What caffeine half-life means
Half-life is the time for your body to clear half of a caffeine dose. With an average half-life of ~5 hours, a 200 mg coffee leaves ~100 mg after 5 hours, ~50 mg after 10 hours, and ~25 mg after 15 hours.
Why caffeine peaks after consumption
Caffeine is absorbed quickly, with blood levels typically peaking 30–60 minutes after drinking. That is why effects build shortly after your first sip and then gradually fade.
Recommended caffeine limits
Most health authorities consider up to 400 mg per day safe for healthy adults, and single doses up to 200 mg. During pregnancy, the recommended limit is 200 mg per day.
Factors affecting caffeine metabolism
Genetics (the CYP1A2 gene), smoking, pregnancy, oral contraceptives, liver health and some medications all change how fast you clear caffeine. This is why the same coffee affects people very differently.
When to stop drinking caffeine before sleep
Because caffeine lingers for many hours, a common guideline is to stop 8–10 hours before bed. Use the sleep calculator to find a cutoff time tailored to your bedtime and sensitivity.
Caffeine sensitivity profiles
Fast metabolizer
Half-life ~4h
Clears caffeine quickly. Often tolerates caffeine later in the day with less sleep disruption.
Average metabolizer
Half-life ~5h
Typical adult metabolism. About half of a dose remains after ~5 hours.
Sensitive
Half-life ~6.5h
Caffeine lingers longer. May notice effects on sleep or jitters from moderate doses.
Very sensitive
Half-life ~8h
Slow clearance (e.g. pregnancy, certain medications, genetics). Caffeine can affect sleep many hours after consumption.
CaffeineCheck provides general estimates for educational purposes and is not medical advice. Consult a healthcare professional for personal guidance.