It’s a paradox that intrigues researchers. Can we really sleep less while having, in certain aspects, more intense sleep? An international team has just provided new elements of response in the Journal of Psychopharmacology.
Four caffeinated drinks a day: this is where the difference appears
The researchers compared two groups: people consuming at least four caffeinated drinks per day and those who drink it three or less. Coffee, tea, sodas and energy drinks were taken into account.
Result: heavy consumers sleep on average 11 to 13 minutes less per night.
The difference may seem minimal. But it appears consistently in the analyzes considered the most reliable by researchers. Some statistical methods produced much larger discrepancies, up to 229 minutes. However, the authors consider these extreme estimates unrealistic and instead adopt a reduction of around 11 to 13 minutes.
So far, nothing very surprising: caffeine is known to promote alertness.
But researchers discovered another phenomenon.
Their brains show more signs of deep sleep
To understand what really happens during the night, the team relied on data from 1,702 volunteers from the Swiss HypnoLaus studywhose sleep was recorded by home polysomnography.
This technique makes it possible to measure the electrical activity of the brain during the different phases of sleep.
And this is where the result becomes unexpected.
Among heavy caffeine consumers, researchers observed a increase in delta powerin the frequency band between 1 and 4 Hzduring non-REM sleep (NREM).
These slow waves constitute one of the characteristic markers of
deep sleep.
In summary: Heavy caffeine users sleep a little less, but their brains produce more of the sluggish activity associated with deep sleep.
A result which obviously does not mean that coffee improves sleep. It could rather reflect the brain’s ability to adapt to regular caffeine consumption.
Nearly half a million people trying to sort out fact from fiction
To study this phenomenon, researchers used data from the UK Biobankwhich includes
485,511 participants.
They used certain genetic variants associated with habitual caffeine consumption in a method called Mendelian randomization. This approach allows us to more robustly explore the possibility of a causal link, beyond a simple statistical association.
The researchers also used matching methods between participants with comparable characteristics.
Another element is particularly interesting: questionnaires assessing sleep quality, such as the Pittsburgh Sleep Quality Index, do not show a clear difference between heavy consumers and others.
In other words, the changes observed in sleep are not necessarily felt by the people concerned.
The brain therefore seems to be able to modify its functioning without the sleeper necessarily having the impression that something has changed.
An adaptation of the brain… but not a blank check to multiply the number of coffees
The authors remain cautious. Their study presents several limitations: caffeine consumption is notably
self-reportedthe different sources of caffeine are not detailed with precision and the polysomnography measurements only concernjust one night.
It is therefore impossible to say that four coffees a day have no consequences on health or that deep sleep systematically compensates for the lost minutes.
Researchers nevertheless put forward the hypothesis of a progressive adaptation of the brain.
“Data show that habitually high caffeine consumption alters sleep characteristics in the general population, while preserving key physiological principles of sleep-wake regulation, perhaps through adaptation.”conclude Benjamin Stucky and his colleagues from the Institute of Pharmacology and Toxicology at the University of Zurich.
The result is ultimately less simple than the traditional argument that “coffee prevents you from sleeping”.
Among habitual heavy consumers, sleep appears both shorter and, on certain brain markers, deeper. It remains to be seen whether this adaptation really constitutes a beneficial compensation or simply a response of the brain to chronic exposure to caffeine. A question that this study does not yet answer.