New research suggests that while habitual coffee consumption may slightly reduce the total time a person sleeps, it also tends to increase the depth of their sleep. This potential trade-off indicates that the human brain might possess the ability to regulate its own sleep needs against the effects of chronic caffeine intake. The findings were published in the Journal of Psychopharmacology.
Caffeine stands as the most widely consumed psychoactive substance in the world. It is readily available in common dietary items such as coffee, tea, chocolate, and various energy drinks. People typically consume these products to enhance alertness, manage stress, or simply enjoy the taste. Wastewater analyses in Europe estimate that the average person consumes between 86 and 263 milligrams of caffeine daily. This amount is roughly equivalent to one to three cups of coffee.
Despite its popularity, questions remain regarding the long-term health consequences of caffeine, particularly concerning sleep. Good quality sleep is essential for a wide range of biological functions. These include cardiovascular health, immune system efficiency, memory consolidation, and emotional regulation.
Previous scientific investigations into caffeine and sleep have largely relied on controlled laboratory settings. These studies typically demonstrate that consuming caffeine shortly before bed impairs sleep. Common findings from these acute studies include a longer time to fall asleep and more frequent awakenings. However, such experiments often require participants to abstain from caffeine before the test.
“Most research on caffeine and sleep looks at what happens when people drink caffeine after abstaining, and find it can disrupt sleep. But this is not how most people consume coffee! Most drink it daily for years. The body and brain may adapt over time. So we asked ourselves if long-term, daily caffeine use still affects sleep? To find out, we analyzed two large datasets with objective sleep measures using statistical methods to estimate causal effects,” said study author Benjamin Stucky of the University of Zurich.
The first dataset was the UK Biobank, which provided genetic information and self-reported caffeine habits for 485,511 participants. The second was the HypnoLaus cohort from Switzerland, which included 1,702 individuals. The HypnoLaus dataset was particularly important because it included objective sleep data collected through polysomnography in the participants’ own homes.
Polysomnography is a comprehensive method for recording biophysiological changes that occur during sleep. It monitors brain waves via electroencephalography, as well as oxygen levels, heart rate, and breathing. This allowed the researchers to look beyond simple survey answers and observe the actual electrical activity of the sleeping brain.
The researchers categorized participants based on their caffeine intake. They compared individuals with high habitual intake, defined as four or more caffeinated beverages per day, against those with moderate intake of three or fewer drinks.
The study employed two advanced statistical techniques to strengthen the validity of their conclusions. The first was Mendelian Randomization. This method uses genetic variants known to influence caffeine metabolism as a proxy for caffeine consumption. Since these genetic traits are randomly inherited, using them helps minimize the influence of lifestyle factors that might otherwise skew the results.
The second method was causal matching. This involved pairing high caffeine consumers with moderate consumers who shared very similar characteristics, such as age, body mass index, and socioeconomic status.
The analysis revealed distinct differences in sleep patterns between the two groups. Models consistently indicated that high habitual caffeine consumption led to a reduction in total sleep time. The estimated reduction varied across different statistical models. While some genetic models suggested a large decrease, the observational and matching data pointed to a more modest reduction of approximately 11 to 13 minutes per night.
Despite the reduction in sleep duration, the quality of sleep appeared to change in a compensatory manner. The electrical brain recordings showed that high caffeine consumers exhibited stronger activity in the delta frequency range. Delta waves are slow, high-amplitude brain waves associated with the deepest stage of non-rapid eye movement sleep. This stage is widely considered the most restorative phase of sleep for the body and brain.
The increase in delta wave intensity suggests that the homeostatic regulation of sleep remains intact even in heavy caffeine drinkers. Homeostatic regulation refers to the body’s internal drive to compensate for lost sleep by increasing the intensity of subsequent sleep. This finding implies that the brains of daily coffee drinkers may adapt to the slightly shorter sleep duration by making the sleep they do get more efficient and restorative.
“People who consumed more than three cups of coffee per day slept slightly less,” Stucky told PsyPost. “That sounds worrying, but they also had deeper sleep, shown by increased slow-wave activity, a marker of restorative sleep. The increased sleep depth may offset the slightly shorter sleep.”
The researchers also examined subjective measures of sleep quality. Participants completed questionnaires regarding their perceived sleep satisfaction and daytime sleepiness. They also reported their preferences for morning or evening activity.
The results showed no significant differences in self-rated sleep quality between high and moderate caffeine consumers. This disconnect between the objective reduction in sleep time and the subjective feeling of restfulness aligns with the finding of increased sleep depth.
In addition to deep sleep markers, the researchers looked for changes in other sleep stages. The analysis found no significant difference in the percentage of rapid eye movement sleep between the groups. Rapid eye movement sleep is the stage most associated with dreaming and memory processing. The lack of change here suggests that habitual caffeine intake primarily affects the deep, restorative phases of sleep rather than the dreaming phases.
“We used statistical methods to infer causal effects, which rely on assumptions that are hard to fully prove,” Stucky said. “To strengthen our results, we applied multiple approaches and only reported findings that were consistent across them.”
But as with all research, there are limitations. The classification of caffeine intake relied on self-reports, which can sometimes be inaccurate. Participants might forget exactly how much they consumed or misjudge cup sizes. Additionally, the study grouped various caffeinated beverages together, potentially missing differences between coffee, tea, and soda.
The study population was also primarily of European descent. This focus limits the ability to generalize the findings to populations with different genetic backgrounds or cultural habits regarding caffeine. Future research is needed to verify these findings across more diverse groups.
“Our findings don’t tell us whether regular caffeine use is good or bad for health,” Stucky noted. “Deeper sleep might make up for the shorter sleep duration, or it could reflect an ongoing sleep debt, meaning your body is constantly catching up, which could be a strain for it. In any case, the results of our study do not support the notion that regular caffeine consumption would have very detrimental consequences for the quality of our sleep.”
The study, “Habitual caffeine consumption and sleep quality: A Mendelian randomization and causal matching study,” was authored by Benjamin Stucky, Leonard Henckel, Marloes H. Maathuis, José Haba-Rubio, Pedro Marques-Vidal, Francesca Siclari, Raphaël Heinzer, and Hans-Peter Landolt.
