Nutrition | Explainer

Your Afternoon Coffee May Still Be Awake at Bedtime

Caffeine lingers in the body for hours. A study found even a dose six hours before bed cost people sleep.

By Marcus Bell, Food and nutrition writerPublished October 18, 2026 · 2 min read

Caffeine from a mid-afternoon cup can still be circulating at bedtime. Photo: Health Unfiltered

Plenty of people say coffee does not affect their sleep. They fall asleep fine, so it must be out of their system.

The research suggests otherwise. Caffeine can change the quality and length of sleep even when it does not stop you dropping off.

How long caffeine stays with you

Caffeine has a half-life of around five hours in healthy adults, meaning the body clears about half of it in that time. The range is wide, from about two hours to ten, depending on genetics, age, smoking and medications.

So a cup at 3 p.m. can leave roughly half its caffeine in your system at 8 p.m., and a quarter of it at 1 a.m.

What the study found

In a 2013 study in the Journal of Clinical Sleep Medicine, researchers gave healthy sleepers a 400 mg dose of caffeine at bedtime, three hours before bed, or six hours before bed, and compared it with a placebo.

All three timings disrupted sleep. Even the dose six hours before bed reduced total sleep time by more than an hour, measured with a sleep monitor at home. Many of the participants did not notice the difference themselves.

Four hundred milligrams is a large single dose, roughly what you might find in a few cups of brewed coffee, but the finding is clear: caffeine in the afternoon can follow you into the night.

Where the caffeine hides

The US Food and Drug Administration says up to 400 mg a day is generally not linked with harmful effects in healthy adults. Staying under that total is a different question from protecting your sleep.

An 8 oz cup of brewed coffee: about 80 to 100 mg
Black or green tea: about 30 to 50 mg a cup
Cola: about 30 to 40 mg a can
Energy drinks and pre-workout powders: often 80 to 300 mg a serving
Dark chocolate: a small amount, which adds up with everything else

A simple rule

Many sleep researchers suggest a caffeine cutoff about eight hours before bed. If you go to bed at 11 p.m., that means your last caffeinated drink by 3 p.m.

If you are sensitive to caffeine, or sleep badly and are not sure why, try moving your cutoff to noon for two weeks and see what changes.

Marcus Bell writes about food, drink and nutrition for Health Unfiltered.