Caffeine Has a Half-Life of 5 Hours. Do the Math.

We are not here to take your coffee. Coffee is good. Coffee is a gift. Coffee has been getting humans through mornings since the 15th century and it deserves respect.

We are here to tell you about a number: 5 to 6 hours. That’s the half-life of caffeine in the average adult — the time it takes for your body to metabolize half of a given dose. Not all of it. Half of it.

Let’s do the math together. You drink a coffee at 3pm. It contains roughly 100mg of caffeine, which is on the low end of a standard cup. By 8pm — five hours later — 50mg is still circulating. By 1am — ten hours later — 25mg remains. If you’re sensitive to caffeine, as roughly half the population is due to a common genetic variant in the CYP1A2 gene, that number is higher and the half-life is longer.

That 3pm coffee you’re convinced “doesn’t affect your sleep” is still working at midnight. It’s just doing it quietly, in the background, reducing the depth of your slow-wave sleep without necessarily preventing you from falling asleep entirely. Which is why you can fall asleep fine and still wake up tired.

What Caffeine Actually Does

Caffeine works by blocking adenosine receptors. Adenosine is the molecule that accumulates during waking hours and creates the sensation of “sleep pressure” — the increasing urge to sleep the longer you’ve been awake. Caffeine doesn’t give you energy. It masks the tiredness signal. The adenosine is still there, waiting patiently. When the caffeine wears off — usually mid-to-late afternoon for a morning coffee — that accumulated adenosine arrives all at once, which is why the afternoon crash hits so hard.

The Research

A 2013 study published in the Journal of Clinical Sleep Medicine found that caffeine consumed even 6 hours before bed significantly reduced total sleep time by more than an hour. The participants themselves did not perceive the disruption — they thought their sleep was fine. The polysomnography disagreed.

Drake C et al. Caffeine Effects on Sleep Taken 0, 3, or 6 Hours before Going to Bed. JCSM. 2013.

The Rule, Which Is Simple

The research broadly supports cutting caffeine by early afternoon — noon to 2pm depending on your sensitivity — to minimize sleep disruption. If you’re a fast metabolizer (you can have a coffee after dinner and fall asleep fine), your range is wider. If you’re a slow metabolizer (you feel a single afternoon coffee for hours), pull your cutoff earlier.

The practical version: if you’re struggling with sleep quality and you consume caffeine after noon, try moving it to before noon for two weeks and see what happens. That’s the whole experiment.

What About Decaf?

Decaf contains 15–30mg of caffeine per cup, depending on the brand and brewing method. For most people this is negligible. For sensitive metabolizers, even this amount matters after 6pm. There’s a reason the recommendation exists — know your own nervous system.

One More Thing: Alcohol

Alcohol is the evil twin of this problem, because it does the opposite in a misleading way. It makes you fall asleep faster, which feels like it’s helping. But alcohol suppresses REM sleep and causes sleep fragmentation in the second half of the night — the half where most of your REM sleep lives. You’ll fall asleep fine. You’ll wake at 3am and lie there wondering what happened. What happened was the nightcap. The research on this is extremely consistent.