Coffee affects the brain even at night: what scientists discovered

A review of studies from Wroclaw Medical University shows that caffeine can alter brain activity during sleep, reducing slow-wave activity even if a person falls asleep without problems and does not wake up at night. Scientists explain this by its effect on adenosine receptors. The half-life of caffeine is about five hours, and a high dose (400 mg) can affect sleep even after 12 hours. Completely giving up coffee does not guarantee improved sleep and may cause temporary withdrawal symptoms.

A review of studies conducted by specialists at Wroclaw Medical University showed that caffeine can affect brain activity during sleep, even if a person has no trouble falling asleep. Professor Donata Kurpas, who participated in preparing the review, explained that falling asleep quickly does not guarantee full nighttime recovery. Caffeine can reduce the intensity of slow-wave electrical activity in the brain, characteristic of deep sleep. These changes can be detected using electroencephalography, but they do not necessarily prove a deterioration in well-being. The mechanism of action is related to adenosine, a substance involved in forming the need for sleep. Caffeine blocks the corresponding receptors, temporarily reducing the feeling of fatigue. The half-life of caffeine in an adult is about five hours. In a 2013 study, participants were given 400 milligrams of caffeine immediately before bed, as well as three and six hours before it—sleep disturbances were observed in all three cases. In a 2025 study involving 23 people, it was found that a high dose (400 mg) could affect sleep onset and nighttime sleep structure even after twelve hours. Researchers also described the mechanism of forming a vicious cycle of fatigue: a person wakes up unrested, drinks coffee, which then affects nighttime sleep, and the next morning the situation repeats. However, Professor Kurpas notes that this scenario is not typical for everyone. Completely giving up coffee does not always lead to noticeable improvement in sleep: in one study, 66 young people who regularly consumed caffeine gave it up for a week but did not find a significant increase in nighttime sleep duration. Abrupt cessation of caffeine consumption can cause temporary symptoms, including headache, drowsiness, and decreased concentration.