Is Your Afternoon Coffee Wrecking Your Sleep?
See exactly how much caffeine is still active at your bedtime. Uses the 5-hour half-life formula to track caffeine clearance through your day.
A 3pm coffee still has ~50% of its caffeine active at 8pm. More than most people realise.
5–6 hrs
caffeine half-life in healthy adults on average
~95mg
caffeine in a standard 8oz brewed coffee
8–10 hrs
time to functionally clear 2 cups of coffee below 10mg
Caffeine and sleep: the science
The 6-hour rule
A commonly cited guideline: avoid caffeine within 6 hours of bedtime. Research by Dr. Christopher Drake found that caffeine consumed 6 hours before bed still reduced total sleep time by about 1 hour, even in people who felt they fell asleep without issue.
Why afternoon coffee hurts your sleep
With a 5-hour half-life, a 3pm coffee still has 50% of its caffeine active at 8pm, and 25% active at 1am. If you drink multiple cups throughout the day, the caffeine levels compound and stay elevated well into the night.
Caffeine sensitivity varies widely
Some people metabolise caffeine quickly (fast metabolisers — CYP1A2 gene variant) and can drink coffee late with little effect. Slow metabolisers may feel stimulated 8–10 hours later. Hormonal contraceptives, certain antibiotics, and liver conditions also slow caffeine clearance.
How the Caffeine Half-Life Calculator Works
Total caffeine = cups × 95mg. Remaining at bedtime = total × 0.5^((bedtime − last cup) / 5). The clearance time is calculated as: last cup hour + 5 × log(total / 10) / log(2), which gives the time when caffeine drops below 10mg.
This uses an average 5-hour half-life. Individual metabolism varies — some people clear caffeine in 3 hours, others take 9. If you find you're very caffeine-sensitive, try using 7–8 hours as your effective personal half-life.