The problem with coffee in the evening isn't just that you can't fall asleep - it's that even when you do, your brain doesn't actually rest the way you think. Researchers at the Medical University of Wrocław found that caffeine changes the very way the brain sleeps, not just whether it sleeps. And that's the more dangerous part, because you don't notice it.

The key concept is shallow sleep. The body can spend eight hours in bed while the brain doesn't fully recover - and the person has no idea, because there's no obvious symptom like insomnia. You sleep as long as you should by the clock, you wake up tired, and you don't know why. The answer might be in the cup you drank in the afternoon.

The researchers used electroencephalography - recording brain activity - to measure sleep quality deeper than the surface. "EEG lets us see not just whether a person is asleep, but how the brain is sleeping," explains professor Donata Kurpas. The analysis showed reduced slow-wave activity, an important indicator of the depth and restorative power of sleep.

But here the research doesn't fall into the easy trap of saying "coffee is bad." Quite the opposite. The effects depend heavily on age, metabolism, health, stress level and personal sensitivity - morning coffee for one person can be just as problematic as evening coffee for another. "Caffeine is neither good nor bad," says Kurpas, "but a biologically active substance whose effects depend on the dose, the time of day, age, lifestyle, sleep quality and personal sensitivity." In other words: don't ask whether coffee harms you, ask when and how much - the answer is different for everyone.