Caffeine Half-Life: the 5-Hour Rule and Its Limits

Caffeine does not leave when the lift wears off. It declines on a curve, and the number that governs that curve is the half-life: about 5 hours in the average healthy adult, according to StatPearls. That single figure explains most of what an afternoon coffee does to a night’s sleep.

What is still circulating, hour by hour

Half-life means each interval clears half of what remains — so the fall is proportional, not steady. Starting from a 100 mg dose and a 5-hour half-life:

Time since the drink	Caffeine still circulating
0 hours	100 mg (full dose)
5 hours (1 half-life)	50 mg
10 hours (2 half-lives)	25 mg
15 hours (3 half-lives)	about 12 mg
20 hours (4 half-lives)	about 6 mg

Put real numbers on it. A 95 mg cup of brewed coffee at 3 PM leaves roughly 24 mg in circulation around midnight. That is small in absolute terms and still enough to matter, because EFSA reports that single doses as low as 100 mg can affect sleep duration and pattern in some adults when taken close to bedtime.

Why a single-number calculator misleads

Any tool that turns a drink and a clock time into an exact milligram figure is quietly assuming that your half-life is five hours. For a large share of people it is not, and the deviations are not small:

• Smoking cuts the half-life by up to 50%, roughly halving how long caffeine lingers.
• Pregnancy, especially in the third trimester, stretches it to as much as 15 hours — one of the reasons the caffeine limit during pregnancy is set at half the adult figure.
• Newborns clear it at a completely different pace: up to 8 hours for full-term infants and around 100 hours for premature ones. Children older than 9 months are already close to adults.
• Liver disease and drugs that inhibit CYP1A2, the enzyme that metabolizes caffeine, prolong it in ways that vary person to person.

So treat 5 hours as the center of a wide distribution, not as your personal constant. The useful output is a cutoff time, not a milligram readout.

Setting your caffeine cutoff

StatPearls cites a systematic review supporting the sleep hygiene recommendation to avoid substantial caffeine intake at least 6 hours before bedtime, because of its significant disruptive effect on sleep. That converts to a simple rule you can actually follow:

Bedtime	Last substantial caffeine
10 PM	4 PM
11 PM	5 PM
Midnight	6 PM

Six hours is roughly one half-life plus a margin, so about 30% of the dose is still present at lights-out — which is why sensitive sleepers do better pushing the cutoff to early afternoon. If your nights are the problem rather than your total, pair this with the bedtime calculator and work backwards from the wake-up time you actually need.

The dose still matters as much as the timing

Timing only helps if the amount is reasonable to begin with. Check where your day lands against the safe daily caffeine intake calculator, and if the symptoms have already arrived, the signs of too much caffeine page covers what wears off on its own and what does not.

This content is informational and does not replace medical advice.