You may get a full night’s sleep without obvious interruptions. Yet your brain may not be resting as deeply as it should. A common afternoon habit – drinking coffee – could be the reason.

A new systematic review published in the journal Nutrients suggests that caffeine affects not only how long we sleep, but also how well we sleep. Even when people complete eight hours of rest, the brain may not pass through the deeper restorative stages to the same extent.

The findings

Researchers analysed 32 human studies that examined how caffeine influences sleep using electroencephalography (EEG), a method that records brain activity.

The most consistent finding was a reduction in low-frequency brain activity during non‑REM sleep, particularly so‑called slow‑wave activity. These waves are widely considered a key indicator of deep, restorative sleep and are closely linked to physical recovery and brain function.

At the same time, several studies observed an increase in faster brain rhythms, such as sigma and beta frequencies. This pattern suggests a lighter, more activated form of sleep, closer to wakefulness than true rest.

“Classical sleep assessment measures duration and stages, whereas quantitative EEG analysis reveals more subtle changes, such as reduced slow‑wave activity,” explained Professor Donata Kurpas of Wroclaw Medical University.

Why hours alone are not enough

Traditional sleep assessment focuses on duration, awakenings and basic stages. EEG, however, provides a deeper perspective, showing how the brain functions while the body rests.

This distinction matters because caffeine does not always cause obvious insomnia. Many people fall asleep easily and do not wake frequently during the night.

However, the quality of their sleep may still decline. Caffeine can make sleep more superficial, which helps explain why someone may sleep “normally” but wake without feeling refreshed.

“The subjective feeling of having slept well does not always correspond to what researchers observe in neurophysiological recordings,” Kurpas noted.

How caffeine works

Caffeine acts primarily by blocking adenosine receptors. Adenosine is a substance that builds up while we are awake and contributes to the feeling of sleepiness.

By interfering with this process, caffeine temporarily reduces fatigue and increases alertness. At the same time, it may alter how the brain expresses its need for sleep during the night, keeping it in a more activated state.

Do effects vary between individuals?

The response to caffeine differs widely between individuals. Factors such as dose, timing, age, habitual intake, stress levels and genetic sensitivity all play a role. [medicalxpress.com]

For some, a single late‑afternoon coffee is enough to disrupt night‑time recovery. For others, the issue may be cumulative – the total caffeine consumed throughout the day.

The findings are particularly relevant for people who rely on caffeine to cope with demanding schedules, intense mental work, study or prolonged fatigue.

The fatigue cycle

Caffeine can help in the short term by boosting alertness and reducing the feeling of exhaustion.

However, when it begins to interfere with deep sleep, it may create a cycle: more fatigue leads to increased caffeine intake; higher intake leads to poorer sleep; and inadequate recovery increases the need for stimulation the following day.

What this means in practice

Researchers do not describe caffeine as inherently harmful. It is a biologically active substance whose effects depend on dose, timing, lifestyle and individual sensitivity.

What they emphasise is that sleep is not measured solely in hours. Depth, quality and the brain’s ability to recover are equally important.

If you sleep enough but still wake up tired, it may be worth looking not only at when you go to bed, but also at what you consume during the day. For many people, the answer may lie in the afternoon coffee.

Source: protothema.gr

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