Has the enigma of how late to have the last cup of coffee been solved?

Coffee is a potent psychostimulant with the power to reduce fatigue and boost our alertness, mood, memory, and cognitive performance. Plus, it tastes so great!

There are many reasons why a big mug of coffee in the morning is a must for me —and millions, even billions of people. Many of us know that having another cup after dinner is a bad idea. We’d love to, but then sleep becomes impossible.

Why is that? And when is the latest time in the day to have a cup of coffee without disrupting sleep? Now, scientists have conducted a study to help answer these questions.

How Caffeine Works
The regulation of sleep is a complex process in the brain, with one key player being a small molecule called adenosine. Adenosine accumulates throughout the day while you're awake. The longer you're awake, the more adenosine builds up, making you more likely to feel tired and fall asleep at bedtime.

Caffeine blocks the effects of adenosine on the brain, delaying the natural sleepiness that comes on gradually at the end of the day.

This can be useful when you need to stay alert for a deadline and work late. That said, drinking coffee every evening can create a cycle of poor sleep—you’re tired in the morning and need coffee to stay awake the next day.

To enjoy coffee and still get good sleep, timing your caffeine intake is crucial. Sleep experts recommend avoiding caffeine close to bedtime. But should we stop drinking coffee even earlier in the day?

The Study
The study investigated the effects of different caffeine doses consumed in the morning, afternoon, or evening on subsequent sleep quality.

The participants were men who regularly drank coffee but were not heavy coffee drinkers. They went through several scenarios, consuming either a standard dose of caffeine (100 mg, roughly one cup of coffee), a high dose (400 mg, equivalent to 4-5 espressos), or a placebo (no caffeine). The doses were ingested in the morning, 8 hours, or 4 hours before bedtime.

During the night, their sleep was measured using polysomnography, a method that records brain waves during wakefulness and various sleep stages. The participants also kept sleep diaries, noting their subjective experience of the night.

The results are good news for coffee lovers: a normal cup of coffee (100 mg of caffeine) up to 4 hours before bedtime does not disrupt sleep. There was no measurable difference in sleep quality between one cup and a placebo.

But—there's always a but—having a strong coffee, like a ++ double espresso, at any time of the day did disturb sleep. The scientists could detect adverse effects on sleep no matter what time the caffeine was ingested. It took participants significantly longer to fall asleep, and their sleep quality (sleep architecture) changed on the polysomnograph.

Regardless of when the high caffeine dose was consumed, sleep was affected. The negative impact increased when the high dose was ingested closer to bedtime, but even having it in the morning (12 hours before bed) had a significant impact on sleep quality.

Interestingly, participants themselves only noticed the negative impact when they consumed a high dose near bedtime. So, a high dose of caffeine in one go could affect your sleep—and consequently your overall health—even if it doesn't feel like it.

Bottom Line
Enjoy your coffee - a final cup of coffee 4 hours before bedtime is fine. But a very strong or large coffee at any time of the day may disrupt your sleep.

About the scientific paper:

First author: Carissa Gardiner, Australia
Published: Sleep. October 2024.
Link to paper: https://academic.oup.com/sleep/advance-article/doi/10.1093/sleep/zsae230/7815486?login=false