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Coffee and Sleep: How Late Is Too Late?

Caffeine has a 5-6 hour half-life, so an afternoon coffee is still working at bedtime. Here's how coffee affects sleep and when to have your last cup.

6 October 20265 min read

Caffeine works by blocking adenosine, the molecule that builds sleep pressure through the day. That's great at 10 am and a problem at 10 pm - because caffeine you drank in the afternoon is often still in your system.

The Half-Life Problem

Caffeine's half-life is roughly 5-6 hours in a healthy adult (longer if you're pregnant, on certain medications, or a slow metaboliser). So if you have 200 mg at 4 pm:

~100 mg is still active at 9-10 pm
~50 mg at 3-4 am

Even if you fall asleep fine, that residual caffeine reduces deep sleep and total sleep time - studies show measurable disruption from caffeine taken 6 hours before bed.

Practical Rules

Last cup by early afternoon - around 2 pm if you're sensitive, 3-4 pm for most people, earlier if you sleep before 10.
Count all sources - tea, cola, energy drinks, dark chocolate, pre-workout, some painkillers.
Watch the total, not just timing - a big morning dose can still leave enough at night.
If you sleep badly, cut afternoon coffee first before blaming anything else.