Four or More Caffeinated Drinks a Day Shorten Sleep but Make It Slightly Deeper, Study Finds

People who drink four or more servings of caffeinated beverages a day sleep an average of 11 to 13 minutes less per night than those who limit themselves to one to three cups. At the same time, monitoring devices record slightly higher delta brainwave activity during slow-wave sleep—a metric traditionally linked to sleep depth. These are the findings of an international team of researchers from Switzerland, Ireland, and the Netherlands, published in the Journal of Psychopharmacology.

Polysomnography Instead of Subjective Questionnaires

The study gained traction after being covered by PsyPost, largely due to its rare combination of methodologies for this research area. Most conclusions about caffeine’s impact on the body rely solely on self-reported survey data, but this time researchers paired genetic analysis with objective hardware measurements.

The study was based on two datasets:

• Anthropometric and genetic profiles from more than 485,000 participants in the UK Biobank, which tracked overall consumption of tea, coffee, and energy drinks.
• The Swiss HypnoLaus cohort, in which roughly 1,700 people underwent comprehensive at-home polysomnography with continuous recording of electroencephalography (EEG), respiration, and heart rate.

Why “Deeper” Does Not Mean “More Efficient”

On social media, the study’s conclusions quickly spurred claims that four cups of espresso somehow “optimize” rest and extend the recovery phase. The authors’ actual findings, however, do not support the idea of neurohacking.

The difference in slow-wave activity between the groups was only about 0.8%. This shift is microscopic and, according to the researchers, reflects a basic compensatory response: when the brain gets less sleep, it tries to intensify restorative processes within the available window. Meanwhile, neither morning alertness nor REM sleep duration improved among heavy caffeine consumers.

The authors also tested the relationship using Mendelian randomization, a genetic method designed to identify causal links. Some mathematical models indicated an extreme sleep reduction of two to three hours, but the researchers considered those estimates skewed by the weak specificity of caffeine metabolism genes. The statistically verified effect was limited strictly to 11–13 minutes of lost sleep.

Caffeine should not be viewed as a way to “get more sleep in less time”: the study shows the nervous system adapting to constant stimulation, rather than providing a magic shortcut for saving time in bed.