Long-term habitual coffee consumption has little effect on sleep quality or daytime drowsiness, according to a new study.
30,000 People Analyzed: Coffee-Sleep Relationship "Far Weaker Than Expected"
According to the Korea Food Communication Forum (KOFRUM) on Friday, a research team led by Professor Martin Neovius at Sweden's Karolinska Institute reported in the latest issue of the international journal PLoS ONE that "the relationship between coffee consumption and sleep was far weaker than expected."
The findings showed almost no difference in daytime drowsiness between habitual coffee drinkers and non-drinkers, contradicting the conventional belief that coffee fends off sleepiness.
The team analyzed approximately 30,000 adults who participated in the Swedish CArdioPulmonary bioImage Study (SCAPIS), a large-scale cohort study. The researchers compared participants' coffee consumption, sleep habits, and subjective levels of drowsiness, adjusting the results to account for genetic factors.
Contrary to the expectation that drinking more coffee would reduce daytime drowsiness, the difference in sleepiness between long-term consumers and non-consumers was not statistically significant.
"Coffee does affect sleep, but the magnitude was so small that it was virtually nonexistent," the team wrote in the paper. "It was at a level difficult to perceive in real life." In other words, coffee drinkers showed little difference in drowsiness compared with those who did not drink coffee.
"The Brain Adapts to Caffeine"... Long-Term Results Differ from Short-Term Effects
The researchers attributed the phenomenon to "physiological adaptation," in which the brain's adenosine receptor system adapts to caffeine.
The reasoning is that when coffee is consumed regularly over a long period, the brain gradually becomes desensitized to caffeine, and the drowsiness-suppressing effect may diminish as a result.
According to the analysis, this is why the alertness initially felt when first drinking coffee fades among long-term consumers. It also helps explain why the effects of caffeine observed in short-term experiments may differ from what people actually experience in daily life.
However, this study was a cross-sectional analysis examining data from a single point in time, and therefore did not directly prove causation. Because the study focused on middle-aged and older adults, the same conclusions cannot be readily applied to younger people or those with high caffeine sensitivity. The researchers added that careful interpretation is required, taking into account individual lifestyle habits and physiological differences.
