The Afternoon Coffee Question: How Late Is Too Late for Caffeine?
Caffeine's half-life means an afternoon coffee is still circulating at bedtime. A controlled trial put concrete numbers on how late caffeine measurably disturbs sleep.
Most people know from experience that espresso after dinner can be a mistake. The more interesting question is where the line actually runs — and it has been tested more rigorously than kitchen wisdom suggests.
The trial that timed it
In a controlled study published in the Journal of Clinical Sleep Medicine, participants took a fixed 400-milligram dose of caffeine — roughly the content of two to three cups of coffee — either at bedtime, three hours before, or six hours before, alongside placebo conditions. The result: even the dose taken a full six hours before bed measurably worsened objectively recorded sleep, cutting total sleep time by more than an hour on the monitoring devices. Tellingly, participants’ own estimates of their sleep underrated the disruption at the six-hour mark — the coffee was interfering with sleep without the sleepers fully noticing.
What the regulators add
The FDA names 400 milligrams per day as an amount not generally associated with negative effects in healthy adults, while stressing that sensitivity varies widely — some people metabolize caffeine much more slowly than others, and medications and pregnancy can shift the picture further. The agency also reminds consumers that caffeine hides beyond coffee: tea, cola, energy drinks and even some foods contribute to the daily total.
Reading the evidence for everyday life
The six-hour finding comes from a single trial with a hefty dose, so it should be read as a well-documented warning rather than a universal law. But its direction is consistent with caffeine’s pharmacology: with an average half-life of around five hours, a mid-afternoon coffee still leaves a meaningful fraction of its caffeine circulating at a typical bedtime.
The verdict
For anyone dissatisfied with their sleep, the caffeine cutoff is one of the cheapest experiments available: keep the morning coffee, move the last cup of the day to before mid-afternoon for a couple of weeks, and compare. The trial data suggest the difference may be larger than it feels — precisely because caffeine’s damage to sleep is easy to underestimate from the inside.
If the experiment changes nothing, that is a finding too: caffeine is only one of many levers on sleep, and persistent problems that survive an earlier cutoff, regular hours and a dark bedroom are a reason to talk to a doctor rather than to keep rearranging the coffee schedule.