You pour your first cup of coffee out of habit. The night went fine, the clock says eight hours, and the day can begin. For most people, that is where the story ends.
A new review of brain-wave studies suggests the story may not end there. Researchers at Wroclaw Medical University pooled 32 human studies in the journal Nutrients.
They found that caffeine can quietly cut into slow-wave sleep, the deepest and most restorative part of the night, even when total sleep duration looks completely normal.
Slow-wave sleep is the brain's heavy repair shift. The body regenerates, energy stores are rebuilt, and the brain files away what you learned during the day.
Also read: WHO seeks answers after reports of possible plague cases in Russia
An eight hour night does not mean a deep night
The review covered EEG studies published between January 1980 and January 2026. One pattern kept turning up.
Caffeine suppressed slow-wave activity in the first part of the night, especially in the delta range that marks the deepest sleep.
Total sleep time often looked normal, and people often said they slept fine.
Standard sleep stage scoring missed what quantitative EEG picked up. The clock said one thing, the brain said another.
Also read: How eating dark chocolate every day can affect your heart, brain and mood
Why losing slow-wave sleep matters
Slow-wave sleep is also when the brain consolidates memory.
During these slow oscillations, newly encoded information is replayed and transferred into long-term storage, according to a review archived by The Yale Journal of Biology and Medicine.
In other words, the hours you spend in deep sleep are doing cognitive bookkeeping you cannot feel.
If those hours go lighter without your noticing, the bookkeeping goes lighter too.
Also read: Watching horror movies can burn as many calories as a 30-minute walk
Even morning coffee may count
A common assumption is that coffee is only a problem late in the day. The review complicates that picture.
Caffeine has a long half-life. For people who metabolize it slowly, a mug finished at breakfast can still be circulating when the lights go out at night.
Sensitivity depends on genetics, age, metabolism, stress levels and chronic fatigue. Two people drinking the same latte can end up with very different EEG readings overnight.
What you can actually try
The review does not call for giving up coffee. It points to two practical levers.
Also read: How to train your concentration and regain focus
Cut back on the total amount of caffeine across the whole day, not just after lunch.
Give your body more hours between the last cup and bedtime, especially if you often feel foggy after what seemed like a full night's sleep.
If you want a simple home test, keep bedtime steady for two weeks and move your last caffeine two hours earlier.
Pay attention to whether you wake up feeling actually rested, not just technically slept.
Also read: Scientists found a way to treat gum disease without wiping out your mouth's good bacteria
This article is made and published by Ida-Marie Palm Varbæk, who may have used AI in the preparation.