Getting through a serious amount of caffeine each day could put the brakes on the brain's ability to rewire itself, according to an analysis of two small but intriguing studies.

Researchers from the Neuromodulation Research Facility at the Butler Hospital in Providence, Rhode Island, analyzed brain signals associated with learning and lodging memories in 20 individuals, uncovering surprising details challenging assumptions that caffeine promotes plasticity.

"These preliminary data highlight a need to directly test the effects of caffeine in prospective well-powered studies, because in theory, they suggest that chronic caffeine use could limit learning or plasticity," write the researchers in their published paper.

Thanks to its ability to block adenosine, a signaling chemical in the brain which helps us feel sleepy at the right time, caffeine has a reputation for giving us an alertness boost.

Adenosine also affects a process called long-term potentiation (LTP), which in simple terms is how the neuron cells in the brain strengthen connections between themselves – thought to be crucial in the brain taking on new information and adapting over time.

A total of 16 people who drank between one and five caffeinated drinks a day and four people who barely touched caffeine were subjected to a brain stimulation process designed to mimic a readiness to learn in the brain, called repetitive transcranial magnetic stimulation or rTMS.

The team then looked for signs of electrical impulses in the nervous system as a way of measuring LTP. In those who didn't drink caffeinated drinks, these LTP effects appeared to be significantly stronger.

According to the researchers, regular stimulation from caffeine may be causing a counteracting effect in the brain, which might explain the lower levels of plasticity – but this is just a hypothesis for the time being.

There are caveats here: only 20 people took part in this research, and the caffeine/non-caffeine split was heavily weighted. The review also relied on self-reported caffeine dosages, which means the researchers can't know how different doses of caffeinated drinks actually impact long-term potentiation.

That said, the review was a pilot investigation intended to inform future hypotheses. The researchers intend to investigate further with precise and well-controlled future studies where the timing of the caffeine intake and its dose is strictly regulated before testing. This, the authors say, would lend "a better estimate of central nervous system bioavailability and correlation with plasticity responses."

The relationship between caffeine and health is already rather complicated: it's been shown to have both positive and negative effects in previous research, from potentially protecting against dementia to perhaps increasing diabetes risk.

"A better understanding of how caffeine alters the underlying mechanism of learning and memory, as well as the potential impact of caffeine on clinical rTMS effects, merits further attention," write the researchers.

The research has been published in Frontiers in Psychiatry.