Scientists have pointed to the ability of morning coffee to disrupt sleep
Caffeine can affect brain function during sleep, even if a person falls asleep without problems and spends about eight hours in bed. A systematic review has shown that the substance can reduce the activity of slow brain waves associated with deep sleep, and the changes are not always felt by the person themselves. The article "Izvestia" explains why coffee can affect the quality of a night's rest even a few hours after consumption.
Coffee can interfere with deep sleep
Caffeine is usually associated primarily with cheerfulness and the ability to maintain concentration longer. Therefore, the effect of coffee on sleep is often assessed by a simple criterion: whether a person managed to fall asleep after drinking a cup. However, this approach does not take into account what happens to the brain after sleep.
This is the conclusion reached by the authors of a systematic and mechanistic review published in the scientific journal Nutrients. They analyzed 32 studies that examined the effects of caffeine on electroencephalography (EEG) readings during sleep. The work covered various conditions: caffeine consumption before bedtime, its intake during the day, restorative sleep after prolonged wakefulness, and studies with home sleep monitoring.
The main overall result turned out to be related not so much to sleep duration as to its neurophysiological characteristics. In most of the studies reviewed, caffeine was associated with a decrease in the activity of slow waves during slow sleep, primarily in the low—frequency range. The authors of the review consider such activity as one of the sensitive indicators of sleep depth and recovery processes.
That's why a person can spend the usual eight hours in bed and still not get exactly the same sleep as without caffeine. In some cases, the usual sleep indicators may not show significant changes, while the EEG records changes in brain activity.
Donata Kurpas, specialist in family medicine at the University of Wroclaw and co-author of the study
The EEG allows us to see not only whether a person is asleep, but also how exactly the brain is sleeping. Quantitative EEG analysis reveals more subtle changes, such as a decrease in slow wave activity, which is an important indicator of the depth of sleep and its restorative nature.
What happens to the brain during sleep
Slow-wave activity during falling asleep is of particular importance. Slow waves are characteristic of the deep sleep phase and reflect the degree of synchronization of neuronal activity. In the studies reviewed, caffeine was most consistently associated with a decrease in this particular low-frequency activity.
At the same time, the changes were not limited only to a decrease in slow waves. A number of studies on the background of caffeine have noted a relative increase in faster ranges of activity, including sigma and beta ranges. Collectively, this formed a lighter and less synchronized sleep profile, which the review authors describe as more "awake."
Donata Kurpas
Caffeine can shorten sleep duration or make it harder to fall asleep; however, even if sleep duration seems normal, it can reduce slow wave activity and shift the EEG pattern towards a more "awake" state of the brain.
The researchers emphasize that quantitative EEG in some cases turns out to be more sensitive than traditional indicators of sleep structure. In other words, changes in the electrical activity of the brain can be detected when the duration of sleep or its subjective perception remains relatively familiar.
At the same time, we are not talking about the fact that any coffee automatically deprives a person of deep sleep. The review combines studies with different doses, time of use, and experimental conditions, so the severity of the effect varied. The authors separately note the dependence of the reaction on the dosage, time of administration and individual characteristics of a person.
Why Morning Coffee Can Make a Difference Too
Bedtime caffeine recommendations usually focus on a few hours before bedtime. However, the results of the studies reviewed show that for some people, not only a cup of coffee in the evening can be important, but also the total amount of caffeine received during the day.
Caffeine acts mainly through the A1 and A2A adenosine receptors. Adenosine is involved in the formation of so-called sleep pressure: as you stay awake, its effect increases, helping the body move to a state of rest. Caffeine blocks the action of these receptors and thus maintains a state of wakefulness.
At the same time, the rate at which the body processes caffeine varies from person to person. Therefore, the same cup of coffee drunk in the morning will not necessarily have the same effect on two people's nighttime sleep.
Donata Kurpas
It's not just about coffee consumed just before bedtime. For some people, the total amount of caffeine consumed during the day and whether the body has time to process it before nightfall may also matter.
The authors of the review also point to the role of habitual caffeine intake, age, circadian rhythm, and genetic features associated with the adenosine system. However, there is not enough data to establish the same effect of a certain time of coffee consumption for all people. Therefore, the very fact of a morning cup of coffee does not mean that sleep will necessarily get worse at night. It is more important to take into account the individual reaction, the amount of caffeine and how quickly the body removes it.
Why does caffeine affect people in different ways?
The researchers separately draw attention to the significant variability of the reaction to caffeine. The review says that the severity of EEG changes can be influenced by age, habitual caffeine intake, stress, time of day, and individual characteristics of the adenosine system. In particular, the role of genetic differences associated with the A2A receptor is considered.
Differences can also manifest themselves in the subjective perception of sleep. A person is able to assume that he slept normally if he did not have difficulty falling asleep and does not remember waking up at night. However, instrumental methods can show changes in the electrical activity of the brain that cannot be determined by one's own feelings.
Donata Kurpas
The subjective feeling of good sleep does not always correspond to what we observe on neurophysiological recordings. A person can fall asleep without much difficulty and not remember waking up, while the brain may show fewer signs of deep sleep.
This, according to the authors of the review, makes it more difficult to assess the effects of caffeine. If you focus only on how you feel in the morning, some of the sleep changes may remain unnoticeable. At the same time, the research results do not allow us to assert that the subjective feeling of rest is always erroneous: we are talking about a discrepancy between individual subjective and objective indicators of sleep.
In addition, a significant portion of the available studies have been conducted on healthy young adults with relatively stable sleep patterns. Women, the elderly, teenagers, people with sleep disorders, and those who regularly consume large amounts of caffeine are significantly less represented in the scientific literature. Therefore, it is not possible to transfer the results of all experiments to each population group directly.
How Caffeine can Support the Fatigue Cycle
Another possible effect is not directly related to sleep, but to how a person perceives their own fatigue. Caffeine temporarily increases alertness and can make the feeling of fatigue less noticeable. If the quality of sleep in a particular person does decrease, fatigue may persist the next day, and coffee again becomes a way to compensate for it.
Researchers describe this as a "vicious cycle": more fatigue — more need for stimulation — potentially less restorative sleep. At the same time, we are talking about a possible cycle, and not about a mandatory scenario for each person.
"The review"s authors emphasize that caffeine cannot be categorically classified as a "good" or "bad" substance. Its effect depends on the dose, time of use, age, lifestyle, sleep quality, stress level and individual sensitivity.
Thus, the main conclusion concerns not the ban on coffee, but the difference between sleep duration and its physiological characteristics. A person can sleep for the usual number of hours and not notice problems falling asleep, but an EEG can reveal changes in brain activity associated with caffeine. How significant they will be for a particular person depends on many individual factors.
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