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For many people, the question of whether evening coffee disrupts sleep seems simple: does caffeine keep you awake or not? But researchers are increasingly looking beyond how quickly people fall asleep and how long they remain in bed, focusing instead on what happens inside the brain during sleep.
Scientists are using electroencephalography (EEG) to examine how caffeine may alter brain activity during the night. Unlike conventional sleep assessments, which mainly measure sleep duration and stages, EEG can reveal subtler changes in brain activity that may indicate whether sleep is deep and restorative.
“EEG allows us to see not only whether a person is sleeping, but also how the brain is sleeping,” said Prof Donata Kurpas of the Department of Nursing at Wroclaw Medical University.
She said quantitative EEG analysis can detect changes such as reduced slow-wave activity, an important marker of deep sleep and its restorative function.
You may sleep normally but recover less
Caffeine does not necessarily cause obvious sleep problems. Some people may fall asleep at their usual time and sleep for a normal number of hours while still experiencing changes in the quality of their sleep.
“Caffeine may shorten sleep or make it more difficult to fall asleep; however, even when sleep duration appears normal, it may reduce slow-wave activity and shift the EEG pattern toward a more ‘wakeful’ brain,” Kurpas said.
Slow waves are a major feature of deep sleep, which plays an important role in physical recovery, energy restoration and healthy brain function.
This means someone could spend eight hours in bed but potentially receive less of the deep, restorative sleep their body and brain need.
The effect can also go unnoticed because it may not involve frequent awakenings.
“A person may fall asleep without major difficulty and not remember awakenings, while the brain may display fewer features of deep sleep,” Kurpas said.
Why coffee affects people differently
Caffeine’s impact varies widely among individuals. Genetics, metabolism, age, stress levels and accumulated fatigue can all influence how long caffeine remains active in the body and how strongly it affects sleep.
For some people, caffeine consumed well before bedtime may still have an effect at night.
“It is not only about coffee consumed just before bedtime,” Kurpas said. “For some people, the total amount of caffeine consumed during the day and whether the body has enough time to metabolize it before nightfall may also be important.”
The issue can be particularly relevant for people who regularly depend on coffee or other caffeinated products to maintain concentration and energy.
Could caffeine create a fatigue cycle?
Caffeine can temporarily increase alertness and make tiredness feel less noticeable. But if it also reduces the quality of nighttime recovery, people may feel more fatigued the next day and respond by consuming more caffeine.
“If caffeine helps a person function during the day while simultaneously worsening the quality of nighttime recovery, a vicious circle may develop: greater fatigue, greater need for stimulation, and poorer sleep,” Kurpas said.
The findings highlight why sleep researchers are increasingly looking beyond the number of hours people spend asleep. The brain’s activity during those hours may provide a better indication of whether sleep is genuinely restorative.
“Caffeine is neither ‘good’ nor ‘bad’,” Kurpas said. “It is a biologically active substance whose effects depend on dose, time of day, age, lifestyle, sleep quality, stress burden, and individual sensitivity.”