Why Does Caffeine Keep You Awake? The Adenosine Story

Oct 15, 2026 · 4 min read · Sourced and fact-checked

The short answer: Caffeine blocks the receptors that adenosine, a natural sleep-regulating substance, acts on. A 2022 sleep research review explains the evidence and why people react to caffeine differently.

A cup of coffee makes you feel less sleepy. A 2022 review in the Journal of Sleep Research, titled "Adenosine, caffeine, and sleep-wake regulation: state of the science and perspectives," lays out how.

Adenosine and sleepiness

The review says adenosine is widely accepted as an important substance that regulates sleep. Early support for the idea came from animal studies showing that adenosine levels in different brain areas of rats and cats tended to rise during wakefulness, fall during sleep, and be higher after prolonged wakefulness. This fits the idea that adenosine acts as a feedback signal for the body's need for sleep.

What caffeine does

At the amounts people usually consume, the review says caffeine, along with related compounds theophylline and paraxanthine, acts as an adenosine receptor antagonist. In plain terms, caffeine hooks onto the same receptors adenosine uses, and blocks them, which reduces sleepiness. In both rats and humans, the review says caffeine reduced the brain-wave markers of the build-up of sleep pressure after sleep deprivation.

A very old experiment

The review points to studies by Harry Levi Hollingworth in 1911 and 1912 on how caffeine affects behaviour and sleep. He used a double-blind, placebo-controlled design, still the standard approach in drug research, with 16 participants over 40 controlled study days. He concluded that caffeine doses above 6 grains disturbed sleep in most participants, though he saw large differences between people.

Why people differ

Those differences have a biological basis. The review says the trait-like individual differences in how caffeine affects sleep are linked to genetic variants in the adenosine receptors.

What is still open

The authors say there are still open questions about exactly how adenosine and caffeine regulate and influence sleep, including whether acute and chronic caffeine intake differ in their effects.

A note

If caffeine keeps you from sleeping, it is reasonable to talk to a doctor about your habits. This is general information, not medical advice.

Sources

Adenosine, caffeine, and sleep-wake regulation: state of the science and perspectives, Reichert, Deboer and Landolt, Journal of Sleep Research, 2022: https://pmc.ncbi.nlm.nih.gov/articles/PMC9541543/

Facts checked: October 15, 2026.