When to have your last coffee (it's earlier than you think)

Plenty of people drink coffee in the afternoon, fall asleep fine, and conclude that caffeine doesn’t affect them. The research suggests otherwise: caffeine can shorten and lighten your sleep without stopping you from dropping off.

How long caffeine stays in your body

Caffeine works by blocking adenosine, the chemical that builds up while you’re awake and makes you feel sleepy. Your body clears caffeine slowly. Its half-life, the time it takes to clear half of it, is around 5 hours in a typical healthy adult, but it varies widely, from roughly 2 to more than 10 hours.

Pregnancy and some medications slow clearance considerably. Smoking speeds it up. Genetics plays a big part too, which is why some people really are more sensitive than others.

So a strong coffee at 3 PM can still leave a meaningful dose in your system at 11 PM.

What the study found

In a well-known 2013 study, researchers gave volunteers 400 mg of caffeine (about what’s in two to four cups of brewed coffee, depending on strength) at bedtime, 3 hours before bed, or 6 hours before bed, and measured their sleep at home.

Even the dose taken 6 hours before bed cut total sleep by more than an hour, and many participants didn’t notice the difference in how they slept. The authors suggested avoiding substantial caffeine for at least 6 hours before bedtime.

A practical cut-off

• Most people: stop caffeine at least 6 hours before bed. If you sleep at 11 PM, that’s 5 PM.
• If you’re sensitive, or sleep badly: aim for 8 to 10 hours, which means a lunchtime cut-off.
• Watch the hidden sources: tea, cola, energy drinks, pre-workout supplements, dark chocolate and some pain relievers all contain caffeine.

The US Food and Drug Administration considers up to 400 mg a day generally safe for healthy adults. Timing matters as much as the total.

If you need an afternoon lift

Try a short walk outside, a glass of water, or a 10 to 20 minute nap instead. And if you’re relying on caffeine to get through every afternoon, it’s often a sign that you need more sleep at night. The sleep calculator can help you plan a bedtime that fits.

Sources

• Drake C, Roehrs T, Shambroom J, Roth T. Caffeine effects on sleep taken 0, 3, or 6 hours before going to bed. Journal of Clinical Sleep Medicine. 2013;9(11):1195-1200.
• US Food and Drug Administration. Spilling the beans: how much caffeine is too much? Consumer update.
• Institute of Medicine. Caffeine for the Sustainment of Mental Task Performance. National Academies Press; 2001.