[1]Thölke, P., Arcand‑Lavigne, M., Lajnef, T., Frenette, S., Carrier, J., &Jerbi, K. (2025). Caffeine induces age‑dependent increases in brain complexity and criticality during sleep. Communications Biology, 8(1), 685.
Every weekday afternoon, there is always a magical 'sleep inducing magic'. The brain seems to have rusted, no matter how hard you try, it can't move. I tried my best to move the bricks seriously, but unfortunately, my heart was full of energy but my strength was insufficient. My thoughts had already drifted somewhere. Looking at the pile of work, the thought of "slacking off" with peace of mind dissipated halfway.
So, the 'cows and horses' had no other choice. Anyway, as the saying goes, 'sharpening a knife doesn't make a mistake for chopping wood.' It's better to have a cup of coffee before resuming work. It has to be said that this trick often works. After drinking coffee, the drowsy mind quickly regained its vitality, and the previously stuck train of thought slowly came back in.
A cup of coffee, sleep 'suffers'
Brain enters' working mode 'overnight
After caffeine pulls people out of the afternoon drowsiness, will this "mental energy" disappear as night falls?
A research team from the University of Montreal and other institutions published a study in Communications Biology [1] that found that after "showing off" coffee, even if the brain has entered a sleep state, it has not completely calmed down. On the contrary, the complexity and activity of EEG signals have significantly increased, and the brain exhibits an information processing mode closer to wakefulness, weakening the repair function that deep sleep should have undertaken.
During nighttime sleep, researchers recorded the volunteers' EEG signals using electroencephalography, and then divided the data into non rapid eye movement sleep (NREM, deep sleep phase) and rapid eye movement sleep (REM, dream phase).
After the data collection was completed, they calculated two core metrics, sample entropy and Lempel Ziv complexity (LZc). Sample entropy mainly measures the "unpredictability" of EEG signals. The more complex and irregular the signal, the higher the entropy value. LZc evaluates the amount of "new information" contained in the signal, and the richer and less repetitive the signal, the higher the complexity.
The results showed that after consuming caffeine, both sample entropy and LZc indicators showed significant increases. This also means that even if a person is already asleep, the brain's electrical signals are still more complex and active than normal.
Not only that, caffeine not only makes the brain "lively", but also pushes it towards the "critical state" direction.
Simply put, the "critical state" is the "optimal working range" of the brain between excessive silence and excessive chaos. Of course, for daytime activities that require concentration and quick response, the brain being in a "critical state" is a good thing. At this point, the brain is able to flexibly respond to external changes without getting too excited and falling into disorder, resulting in the highest efficiency of information processing.
The honey of A, the arsenic of B ", at night, the brain that has been" baptized "with caffeine does not rest steadily, approaching the" critical state "and still maintaining an active state that should belong to the daytime.
The researchers also used two indicators, the "DFA scaling index" and the "EEG 1/f non periodic activity slope," to verify this judgment. The former measures the persistence of brain signals over time, with higher indices indicating more sluggish signals and less willingness to switch states; If the index decreases, it means that the signal becomes more flexible. The latter reflects the steepness of EEG background noise, with a flatter slope indicating higher brain excitability.
And both indicators showed a significant decrease under the action of caffeine. It can be seen that caffeine is pushing the sleeping brain from a state of "unresponsive" to the outside world during deep rest to a more excited, flexible, and ready to respond mode.
However, caffeine has different effects on people of different ages. Especially during REM sleep, caffeine has a significantly stronger impact on the brain of young people than middle-aged people.
The researchers compared the original EEG states of two groups of people who did not consume caffeine and only took a placebo. As a result, it was found that the EEG of middle-aged individuals already exhibited higher complexity, lower DFA scaling index, and smoother 1/f slope. But similar changes only occur after young people consume caffeine.
So researchers speculate that as age increases, the brain "simulates" the effects of caffeine in advance during natural aging, gradually forming an activity pattern similar to that of caffeine. Since the brain itself is already in such a state, if caffeine is pushed again, there is naturally less room for it to be pushed.
Why does caffeine disrupt sleep?
Researchers point out that the direct target of caffeine is actually adenosine, the "drowsiness signal" in the brain. The longer you stay awake during the day, the more adenosine accumulates, causing the brain to gradually feel tired. At night, the drowsiness becomes stronger. But when caffeine comes, it first "seizes" the receptors that adenosine was supposed to bind to, temporarily preventing the signal that "it's time to sleep" from being transmitted. So, after drinking coffee, people are less likely to feel sleepy, and at night, their brains may still maintain a sense of clarity.
Moreover, caffeine can also affect multiple neurotransmitter systems in the brain, affecting the activity of substances such as dopamine, norepinephrine, and acetylcholine, while disrupting the balance between GABA and glutamate. The deep sleep stage should be the golden time when brain inhibitory activity dominates. Only when the brain quiets down can it properly clear metabolic waste and consolidate daytime memories. The action of caffeine pulls the brain, which should have been dormant, back to a more excited state.
Caffeine, or disrupting hormone balance
In fact, the trouble of consuming caffeine is not just affecting sleep.
A study published in Frontiers in Nutrition analyzed the relationship between caffeine metabolism and sex hormone levels using data from the National Health and Nutrition Examination Survey (NHANES) in the United States. The research team found that caffeine actually affects the balance of sex hormones in the body.
Specifically, the caffeine metabolism pathway represented by PC2 is significantly negatively correlated with male estradiol (E2) levels. For men, E2 is involved in regulating bone density and cardiovascular function, and low levels may increase the risk of fractures and cardiovascular problems.
After analyzing both males and females together, the overall caffeine metabolites showed a significant negative correlation with sex hormone binding globulin (SHBG) and total testosterone (TT). The decrease in SHBG levels may further alter the availability of testosterone and E2 in the blood; A decrease in TT levels may affect male muscle mass and sexual function, while for females it may affect bone health and overall metabolism.
Overall, the above research reveals that consuming caffeine significantly increases the complexity and activity of the brain during sleep, pushing it towards a highly efficient operating mode that should belong to the "critical state" of the daytime, weakening the repair function that deep sleep should have, and this interference is more pronounced in young people. Not only that, caffeine metabolites can also secretly disrupt hormone balance, laying hidden dangers in bone density, cardiovascular health, and muscle mass.
Balzac once said that coffee played a crucial role in his life, and its significance can be discussed on an epic scale. For the busy "cattle and horses" nowadays, this statement is not exaggerated at all. To maximize work efficiency and overcome the drowsiness of weekday afternoons, a cup of coffee is indeed the best choice. But if we find that after drinking coffee, we tend to toss and turn at night and can't sleep soundly, and wake up the next day with a layer of fog in our minds, then this cup of "life-saving water" needs to be re evaluated.
Note: The sample size of study [1] is limited, and 200mg caffeine intake is an acute intervention and cannot be directly equated with long-term chronic caffeine intake; Research [2] is based on cross-sectional analysis and cannot directly infer causal relationships.
reference material:
[1]Thö lke, P., Arcand‑Lavigne, M., Lajnef, T., Frenette, S., Carrier, J., &Jerbi, K. (2025). Caffeine induces age‑dependent increases in brain complexity and criticality during sleep. Communications Biology, 8(1), 685.
[2]Zhou J, Qin L. Associations of urinary caffeine metabolites with sex hormones: comparison of three statistical models[J]. Frontiers in Nutrition, 2025, 11:1497483. Writing | Mu Bai
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