Researchers at the State Polytechnic Institute of Higher Education in Coimbra, Portugal, report that caffeine can slow the flow of blood to the brain. This finding comes from a study published on Science Direct, presenting detailed observations on how caffeine interacts with cerebral circulation.
The study recruited 45 young adults, aged 18 to 22, who rarely drank coffee. To assess their brain blood flow, researchers used transcranial Doppler ultrasound, a noninvasive imaging technique that measures blood velocity in the brain’s major arteries. Participants were randomly assigned to three groups: one received a single 45 gram caffeine capsule, another received a 120 gram caffeine capsule, and the third was given a placebo. About half an hour after ingestion, the same ultrasound assessment was repeated to capture any changes in cerebral hemodynamics.
Results indicated that caffeine led to reductions in mean cerebral blood flow velocity, as well as the peak systolic and end-diastolic velocities in participants in the caffeine groups. In this context, peak systolic velocity refers to the highest speed of blood flow during the heart’s contraction, while end-diastolic velocity denotes the speed at the heart’s relaxation phase. These findings point to a caffeine-induced constriction of cerebral vessels, which slows the movement of blood through the brain’s arterial network.
Vasoconstriction is a well-documented vascular response to caffeine. By narrowing the brain’s blood vessels, caffeine can contribute to a sense of heightened alertness and reduced perceived fatigue, effects that many people associate with a caffeine boost. Yet the same mechanism decreases the speed of blood flow within the brain. Slower cerebral perfusion has potential implications for individuals with underlying cerebrovascular concerns, and it may influence how the brain responds to stress or injury in the short term.
Beyond the direct vascular effects, caffeine also acts on the central nervous system to enhance wakefulness and cognitive arousal. This combination of stimulating properties and reduced cerebral blood flow is a reminder that caffeine’s impact is nuanced. While it can sharpen attention and transiently protect against fatigue, it also introduces a vascular dynamic that researchers continue to explore. The precise balance between these opposing effects may vary from person to person and depends on factors such as habitual caffeine use, overall cardiovascular health, and the presence of sleep deprivation.
The study adds a layer to the broader discussion about how caffeine interacts with sleep, daytime alertness, and vascular health. Sleep-disrupting foods and substances commonly cited include caffeine itself, particularly when consumed close to bedtime. Understanding caffeine’s dual role in promoting wakefulness while constraining cerebral blood flow helps clarify why some individuals may experience improved focus for a period after intake, followed by possible headaches or a sense of pressure as blood flow dynamics adjust. Health professionals emphasize moderation and timing to minimize potential adverse effects while maximizing the desired stimulatory benefits. While the Coimbra findings help illuminate the immediate vascular response to caffeine, they also invite further investigation into dose responses, individual variability, and long-term outcomes for brain health. Researchers suggest that more work is needed to map how different caffeine doses interact with age, baseline blood flow, and other physiological factors to influence cerebrovascular function over time.
Overall, the research underscores that caffeine is a potent dietary component with effects that extend beyond alertness. It exerts measurable influence on the brain’s blood vessels, a factor that should be considered by anyone evaluating caffeine use in relation to sleep quality, headaches, or vascular risk. The study contributes to a growing body of evidence about how common stimulants shape brain physiology, offering a cautious reminder that even everyday substances can alter cerebral dynamics in meaningful ways. The findings are attributed to the team at the Coimbra institute and are presented with citation to the original Science Direct publication for readers who wish to explore the full methodology and data in greater depth.