What does caffeine do to the sleeping brain?

For many of us, a cup of coffee is the perfect kickstart to the morning. We know that caffeine can boost alertness and cognitive performance. Most of us also know that, especially when it's consumed close to bedtime, caffeine can make it harder to fall asleep, and shorten sleep. Much less is known, though, about what a late-day coffee does to the sleeping brain. Now, a paper in Communications Biology suggests some worrying effects, with younger people more vulnerable than older adults.  

Philipp Thölke at the University of Montreal and colleagues collected EEG data on the brain activity of 40 participants across the course of two separate nights. On both study days, the participants stopped consuming any caffeinated products at noon. On one of the days, they were given a capsule containing 100mg of caffeine three hours before their usual bedtime, and another identical capsule one hour before. (A standard shot of espresso contains around 70mg of caffeine.) On the other study day, they received placebos at these times.

The team then used artificial intelligence to look for any differences in patterns of neuronal activity between the two nights, and during the different stages of sleep. During NREM (non rapid-eye-movement sleep), our memories are consolidated, and physical repair takes place. REM, which is associated more with intense dreams, helps with the processing of emotions, as well as memories.

The researchers found that during NREM sleep, caffeine altered typical electrical rhythms, causing a decrease in delta, theta, and alpha brainwaves — which are generally associated with deep, restorative sleep — and an increase in beta waves, which are more common during wakefulness.  

They also report, for the first time, that especially during NREM sleep, caffeine led to a broad increase in the 'complexity' of brain activity. That is, the neuronal activity was less predictable and more diverse — a state typical of wakefulness and, to a lesser extent, REM sleep.

Another major finding relates to what's known as brain 'criticality'. "Crucially, we also discovered that, compared to a placebo, caffeine shifts the brain closer to a state known as a critical regime," the team writes. In this state, they explain, the brain is most sensitive to inputs, most adaptable, and able to process information most efficiently. "While this is useful during the day for concentration, this state could interfere with rest at night: the brain would neither relax nor recover properly," commented co-author Julie Carrier of the University of Montreal in a press release.

The researchers also separately analysed the data from 22 young participants, aged between 20 and 27, and the other 18 middle-aged adults, aged between 41 and 58. They found that during REM, caffeine did not change brain activity in the older group — but in the younger group, it did. In fact, during REM, the brain activity of the older adults on a placebo looked similar to the brain activity of the young adults after consuming caffeine. This "supports the hypothesis that aging alters sleep EEG dynamics in a manner similar to caffeine, potentially diminishing its impact in the middle-aged group," the team writes.

The cause of this age difference needs further investigation. But with age, we lose brain receptors for adenosine, the molecule that accumulates during the day and makes us feel sleepy. Caffeine blocks adenosine receptors, reducing feelings of fatigue. The fact that older adults generally have fewer of the adenosine receptors on which caffeine acts might explain the different impacts on REM sleep for younger vs older adults. But whatever the exact cause, this finding provides another reason for students to beware of using caffeine-containing 'energy drinks' or caffeine pills to stay alert into the night.

The team would now like to see further research on the effects of caffeine on our brains, and not just during sleep. "Considering the widespread consumption of caffeine, gaining a comprehensive understanding of its effects on the brain both during wakefulness and sleep could have far-reaching implications for society and public health," they conclude.

Read the paper in full:
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. https://doi.org/10.1038/s42003-025-08090-z

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