Scientists studying caffeine's effects on sleep are increasingly using electroencephalography (EEG), which records the brain's electrical activity and can detect changes in sleep quality that are not visible through conventional sleep measurements.

"EEG allows to see not only whether a person is sleeping, but also how the brain is sleeping. Conventional sleep assessment shows the length of sleep and its stages, while quantitative EEG analysis reveals more subtle changes, e.g. weakening of slow-wave activity, which is one of the important markers of the depth and regenerative nature of sleep," explains Professor Donata Kurpas from the Department of Nursing, Medical University of Wrocław.

Slow-wave activity is a hallmark of deep sleep, the phase responsible for physical recovery, restoring energy reserves and supporting healthy brain function. According to the researchers, caffeine can make sleep more superficial even when it does not noticeably reduce sleep duration.

According to Kurpas, caffeine can shorten sleep or make it more difficult to fall asleep, but even with apparently normal sleep duration, it can reduce slow-wave activity and shift the EEG image towards a more "wakeful" brain.

The findings suggest people may spend eight hours in bed without achieving full neurological recovery. The expert says many people are unaware of the difference between how well they think they slept and what brain activity reveals.

"The subjective sense of good sleep is not always consistent with what we see in the neurophysiological recording. A person may fall asleep without any major problems and not remember waking up during the night, but their brain may show fewer features of deep sleep," the expert adds.

Researchers also found that people's responses to caffeine vary widely, with genetics, metabolism, age, stress levels and chronic fatigue all influencing how long its effects persist. As a result, even coffee consumed early in the day may affect sleep in some individuals.

"It is not just about coffee consumed right before going to bed. For some people, the total amount of caffeine during the day, and whether the body is able to metabolise it sufficiently before night, may also be important," Kurpas emphasises.

While caffeine improves alertness and reduces fatigue, Kurpas says it may do so at the expense of overnight recovery.

"If caffeine helps you function during the day and at the same time impairs the quality of night regeneration, it may lead to a vicious circle: greater fatigue, greater need for stimulation and poorer sleep," Kurpas says.

The findings reflect a broader shift in sleep research away from simply measuring how long people sleep and toward assessing how effectively the brain recovers during the night.

"Caffeine is not good or bad. It is a biologically active substance, the effect of which depends on the dose, time of day, age, lifestyle, sleep quality, stress and individual sensitivity," Kurpas says. (PAP)

PAP - Science in Poland

ros/ bar/

tr. RL